Tuesday, December 23, 2008

More temnospondyls: old big eyes from the Moenkopi

Temnospondyls are among the several groups I’ve dabbled in, and they certainly deserve more attention than they get. They are the most speciose and long-ranging of the ancient tetrapod groups. I don’t intend to open the can of worms surrounding what exactly a ‘tetrapod’ is – I’ll just use it in the Clackian sense (sarcopterygians with limbs and digits or descended from ancestors that had limbs and digits). We don’t even know whether or not temnospondyls are stem amphibians, although I wouldsay that the case is looking very good right now. In anycase my interests lie with temnospondyls that lived in the Triassic, after the main clades of modern amphibians had already diverged. One of these groups are the enigmatic brachyopids.


Batrachosuchus, a classic brachyopid from the Early Triassic of South Africa.

Brachyopids present a classic case of the difficulty in disentangling convergence from relationship in an extinct group. Their shortened parabolic skulls bear more than a passing resemblance to another group of temnospondyls, the dvinosaurs (dinosaurs are good but a dvinosaur is devine – sorry, couldn’t resist) and indeed the consensus opinion is that brachyopids are the derived descendants of earlier dvinosaurs.



Dvinosaurus, a dvinosaur from the Late Permian of Russia.

However brachyopids share some unusual derived characters with other derived temnospondyls from the Triassic known as stereospondyls. Some of these characters include: a double occipital condyle; the pterygoid bone in the palate forms a long broad suture with braincase rather than a narrow synovial contact; and lack of exposure of the opisthotic in the occiput. I took this to mean that brachyopids really were stereospondyls and share a more recent common ancestor with long snouted stereospondyls like Paracyclotosaurus than they do with short snouted dvinosaurs like Dvinosaurus , or rather that is what I found in my cladistic analysis that I performed for my PhD thesis, later published with my supervisor, Dr Anne Warren. This kind of ecophenotypic convergence seems to have happened multiple times in the evolution of crocodile snout shape (though maybe a little less than previously thought if false gavials and gavials really are sister taxa) and seeing it in temnospondyls was almost to be expected. Of course the situation isn’t quite so simple, for instance some late surviving incontrovertible dvinosaurs DO develop some of the stereospondyl synapomorphies convergently (even more disconcerting is that they develop them at the same time that the streospondyl lineage does!). So it really maybe the case that it is the unusual and apparently unrelated features of stereospondyls that are the convergences while the broad trophic adaptations such as snout shape are a true indication of relationship. It’s a wonderfull and truly juicy puzzle that I once wanted to tackle myself, but I’m so thoroughly bogged down in dinosaur projects now that I can’t see myself getting to it anytime soon. Furthermore the travel involved in unraveling this tangle is pretty daunting. Significant fossils are scattered all over the globe, with important specimens in many parts of the US, England, Argentina, South Africa, Australia and Russia. So for now I am happy to sit back and watch the progress from the sidelines. One researcher who has really picked up where I left off is Marcello Ruta. Marcello hasn’t solved the problem yet but he has started really squeezing more phylogenetic information out of temnospondyl fossils than I ever did.
One little step on the road to understanding brachyopids has just been published by Marcello together with John Bolt of the Field Museum. They looked at Hadrokkosaurus bradyi a large brachyopid from the Moenkopi Formation of the US and one of the better known brachyopid names. The name Hadrokkosaurus means ‘big eyed lizard’ and the skull disseminated around the world in the form of casts truly does have big goggling orbits.


The famous skull, referred to Hadrokkosaurus.

It comes as a surprise to many, myself included, that this famous skull isn’t the holotype. Indeed the holotype doesn’t even preserve orbits at all,for it is an isolated lower jaw ramus. Furthermore this lower jaw was found over 100km away from the skull. With a name like Hadrokkosaurus it is hard to dispute that the Welles had the skull in mind when he erected the name. However the jaw was found first and originally named Taphrognathus, which was unfortunately preoccupied by a conodont (NOT an arthropod for once!). Thus Hadrokkosaurus was created to replace Taphrognathus, leaving the lower jaw as the holotype specimen. This is a pity and it has created a messy taxonomic situation. Jupp and Warren suggested way back in 1986 that the lower jaw might not belong to the skull, and might not even be a temnospondyl at all! They cited the presence of an external mandibular fenestra, teeth that are partially sunk into sockets, weak surface ornamentation, a splenial bone that does not participate in the symphysis of the jaw and a surangular-prearticular contact behind the jaw joint, to suggest that the jaw is infact an archosaur (archosauriform in recent classifications). I need not remind you that a pair of roundned lower jaws fitting onto something roughly the size and shape of a dustbin lid makes for a pretty unusual archosauriform, particularly one of proterosuchian grade with subthecodont teeth. Because of the uncertainty surrounding the identity of the type jaw Anne Warren and Caudia Marsicano decided to bestow a new name upon the well-known skull, they called it Vigilius wellesi. The genus name means 'watchfull' or 'vigilant' and sort of echoes the original 'big eyed lizard'. The species name obviously honours Samuel Welles, the original describer of Hadrokkosaurus.
So is Hadrokkosaurus a weird-ass archosauriform? Definately not: Ruta and Bolt demolish any chance that these jaws belong to an archosauriform. The jaw shows some additional primitive bones (three coronoid bones, and two splenial bones) that are not found in any crown group amniotes, let alone in archosauriforms.
So its not an archosauriform what is it? Well it is without doubt a brachyopid after all. The so called un-temnospondyl like features are either artefacts of damage or misinterpretation (e.g. the so-called external mandibular fenestra) or are derived characteristics that are present in other temnospondyls (e.g. reduced ornamentation of the bone surface, subthecodont teeth and failure of the splenial to reach the symphysis). Furthermore a number of other characteristics, most obviously the honking big retroarticular process, are fairly convincing synapomorphies of Brachyopidae.
So are Vigilius and Hadrokkosaurus the same thing after all? I think they probably are, although Ruta and Bolt suggest that they may be two different brachypoids on the basis of non-matching jaw curvature. However we are dealing with different individuals of different sizes in a taxon that did not have precise occlusion in anycaseso slight differences in jaw curvature not convince me that they are distinct. Indeed both the lower and upper jaws seem to me to have slightly squared-off tips and angular margins that differ ever so slightly from the typical parabolic jawlines of most other brachyopids. This observation coupled with the highly reduced ornamentation of both Vigilius and Hadrokkosaurus (extreme even for brachyopids) and their occurence in the same formation leads me to suspect that the two taxa are indeed the same. We'll just have to wait to find a skull with jaws included to prove it.



The skull of Vigilius (left) and the jaws of Hadrokkosaurus (with the right side mirrored) on the right. Taken from Ruta and Bolt (2008).

Apart from clearing up the identity of Hadrokkosaurus, Ruta and Bolt's paper is important because it demonstrates that a great deal of phylogenetic information can be gleaned from the lower jaws. They analyse lower jaw characters alone and recover a topology that has much in common with my own (there is some weirdness but what do you expect from analysing just one organ system?). In contrast my analysis included a paltry 14 lower jaw characters and probably would only be able to resolve a couple of nodes, if any at all, if run by themselves. Thats a whole lot of information that shouldn't be ignored.

references

Jupp R, Warren AA (1986) The mandibles of the Triassic temno−
spondyl amphibians. Alcheringa 10: 99–124.

Ruta M, Bolt JR (2008)The brachyopoid Hadrokkosaurus bradyi from the early Middle Triassic of Arizona, and a phylogenetic analysis of lower jaw characters in temnospondyl amphibians. Acta Paleontologia Polonica 53: 579-592

Warren AA, Marsicano C (2000) A phylogeny of the Brachyopoidea
(Temnospondyli, Stereospondyli). Journal of Vertebrate Paleontology
20: 462–483.

Yates AM, Warren AA (2000) The phylogeny of the “higher”
temnospondyls (Vertebrata: Choanata) and its implications for the
monophyly and origins of the Stereospondyli. Zooogical Journal of the
Linnean Society
128: 77–121.

Friday, December 19, 2008

Predictions for 2009

I enjoy reading John Hawks' blog for a change of pace away from matters archosaurian. I quite enjoy reading his predictions and seeing how they pan out. So I'm giving it a go here. These are my predictions for the big stories in dinosaurs and dino-related science in 2009. (Some obvious ones are ommited because I have insider knowledge and that would be cheating).
Arranged from most likely to most far out.

1) A new genus in the Tyrannosauridae will be named. There are several contenders floating around, lets hope they will see the light of publication next year.

2) An Early Jurassic tetanuran will be found (several have been claimed but none stand up to scrutiny).

3) The first incontrovertible evidence (e.g. a neck column)for a mamenchisaur outside of Asia.

4) A definitive non-avian dinosaur parasite will be found (probably will be a louse or a mite amongst the feathers or protofeathers of something from Liaoning).

5)A major descriptive monograph from the Sereno stable. Hopefully Eoraptor or Jobaria.

6) A complete well-preserved non-dinosaurian dinosauromorph skull will be found.

7) A Late Norian-Rhaetian herrerasaur will be announced.

8)An incontrovertible proto-pterosaur will be announced (I know there are some supposed contenders but I don't think they qualify as 'incontrovertible').

9) Good evidence that air sac systems are basal to crown group archosaurs will be published.

10) A site spanning the Late Pliensbachian to Aalenian will be found with evidence for a mass extinction.

Feel free to suggest your own or let me know if one or more of these is indeed a reality in the pipeline!

Wednesday, December 17, 2008

Picture of the Day: a Temnospondyl


This is a drawing I did for a book that never appeared. A pity, since I don't even have the original, just this bromide. It is one of my better works and portrays the Australian Middle Triassic Paracyclotosaurus davidi, a member of the Mastodonsauridae (but thats another story).

Sunday, December 14, 2008

Dracovenator by request: the age of the Elliot

Randy and Bill asked an apparently simple question that will require an involved answer. What age is the Elliot Formation? What follows is a detailed account of my thoughts that will probably get to technical for those without a geological bent.
Still here? Great I’ll try to make it worth your while. As I mentioned in the last post the Elliot Formation occurs near the top of the vast sedimentary pile that fills the famous Karoo Basin of South Africa. It underlies the Clarens Formation and overlies the Molteno Formation. Dating the formation is difficult as there is little to constrain it, there are no known ash or lava beds within it that have been radiometrically dated, and the formation lacks any marine microfossils or palynomorphs (spores and pollen) that could be used to date it. The upper constraint on its age comes from the Drakensburg lavas that overlie the Clarens Formation. These represent a rapid pulse of volcanism that is precisely dated to 183 million years ago, which places it at the Pliensbachian-Toarcian boundary of the Early Jurassic in the timescale of Gradstein et al. (2004). Thus the Elliot Formation cannot be any younger the Toarcian-Pliensbachian boundary. At the other end, the constraint on the maximum age of the Elliot Formation is much woollier. The Molteno Formation is said to have palynomorphs of Carnian age, but this is a thick unit and it is not known how much younger the upper parts of the Molteno Formation gets. Furthermore it is now know that terrestrial ‘Carnian’ deposits (e.g. the famous Ischigualasto Formation of South America) actually correlate with the latest Carnian to early Norian stages of the marine sequence. A Carnian/Norian age for the Molteno Formation is also supported by the vertebrate fauna of the Pebbly Arkose Formation of Zimbabwe which is presumed to be a lateral equivalent of the Molteno Formation. This fauna consists of a rhynchosaur and a primitive dinosaur that resembles Saturnalia (which comes from the ‘Ischigualastian’ of Brazil). Thus the Elliot Formation is unlikely to be any older than the Early Norian (about 225 million years old). However this is a huge spread of time, about 42 million years in fact, can we narrow it down any further? Before we examine this question we need to dispel a common model for the deposition of the Elliot Formation which is often portrayed in the literature. This is the model of continuous deposition. This figure (taken from Holzforster 2007) is typical.

In this diagram the Elliot Formation is portrayed as filling the entire block of time between the Molteno and Clarens Formation. However this is not realistic. Deposition in the Karoo Basin was controlled by subsidence in response to tectonic activity in the Cape Fold Belt. Tectonic activity is rarely, if ever, continuously sustained over tens of millions of years. Further the sediments themselves show evidence being deposited in discontinuous pulses. The lower and upper members of the Elliot Formation represent two such pulses. Although no angular unconformity or extensive erosional surface separates these two members there is evidence that there was a non-depositional gap between the deposition of these two units. Firstly we have the wholesale changeover in fauna between the two members. There is no known vertebrate species or genus that occurs in both units. There are also lithological differences that indicate that the style of rivers crossing the floodplain had changed, from sinuous, deep permanent streams to shallow emphemeral braided streams. The latter seems to be coupled with a more arid climate which is also betrayed by other indicators of aridity such as the development of extensive calcareous paleosols, deeply muckcracked overbank horizons, and evidence of floodplain denudation during flash-flood events. Other features such as palaeocurrent indicators suggest that the overall direction of drainage also changed as did the source of the sediment. These features could have suddenly ‘switched’ but a time gap allowing these features to change more gradually seems far more likely.
So if we accept two pulses of deposition what age constraints can we put on them? There is little doubt now that the first pulse was Late Triassic in age. I’m currently preparing a paper describing rauisuchians from the Elliot Formation. They have been reported before but this will be the first time the identification will be based on diagnostic derived characteristics. If we accept that rauisuchians went extinct at the end of the Triassic then these occurrences certainly place the lower Elliot in the Triassic. But where in the Triassic? Geology can help us a little here. According to the model of Catuneanu et al. 1998 and Bordy et al. 2004 the deposition in the part of the Karoo basin where the Elliot Formation crops out occurred during an offloading phase when the Cape Fold mountains were shedding sediment after a mountain building event. During the offloading phase the more distal part of the basin (where the Elliot Formation lies) sags after bulging upward, creating accommodation space for the sediment to collect in.


The reciporical flexural model for basin development, as applied to the deposition of the lower Elliot Formation. Part of a figure from Bordy et al. 2004.

Structural and metamorphic geologists date the end of the mountain building event that is believed to have immediately preceded the deposition of the lower Elliot Formation to 215 million years, give or take 3 million years. This puts the lower Elliot in the mid to late Norian, maybe even extendig into the Rhaetian (depending how long the offloading phase lasted after the mountain building finished) as Randy suggested. Whatever its age I’m willing to bet that the lower Elliot Formation is more or less equivalent to the vertebrate-bearing horizons of the Los Colorados Formation based on similarities of their sauropodomorph faunas. These similarities include Lessemsaurus and Antetonitrus which are very, very similar (though a few telling differences do keep them as separate taxa, e.g. the proportions of metatarsal I). Eucnemesaurus (ex Aliwalia) is also extremely similar to one of the taxa lurking under the label ‘Riojasaurus’.
OK so that’s the lower Elliot, what about the upper Elliot? Here we don’t have ahelping hand from geology. A pulse of mountain building after 215 million years has not been detected (indicating it was a minor event).
Faunally the upper Elliot Formation contains taxa that seem to indicate an Early Jurassic age (e.g. the crocodyliform Protosuchus and a diversity of ornithischians) but this is rather weak reasoning. Nevertheless I think an age younger than the Hettangian one usually assigned to the unit can be supported on the following grounds:
1. The same biozone (the Massospondylus Range Zone)can be found from the beginning of the upper Elliot through to the top of the Clarens. Fossils are extremely rare near the top of the Clarens but Billy De Klerk of the Albany museum has collected a couple of good Massospondylus skeletons, that if I recall correctly, come from near the top of the Clarens. Basically the fauna of the Clarens is a depleted subset of what you find in the upper Elliot (easily explained by the limited sample from the Clarens). The only possible faunal change is that the little trithelodontid cynodont, Pachygenelus, might be replaced by a different trithelodontid, Diarthrognathus, in the Clarens. This suggests to me that we aren't dealing with a large span of time. Dinosaur genera seem to turn over every five million years or so, thus we are probably dealing with a duration in this vicinity for the entire upper Elliot to Clarens sequence.
2. The deposition of the Clarens is terminated by a sudden and precisely dated volcanic event - the eruption of the Drakensburg lavas which is dated to 183 million years.
3. There is no evidence of a time gap between the Clarens Formation and the volcanic eruptions. Indeed there is evidence that one followed the other without a hiatus. For instance in some localities lava flows can be seen to have filled the interdunal spaces in the Clarens dune desert. Clarens deposition seems to have continued after the eruption of the first few flows in some places. A great example can be seen on the road between Barkly East and Rhodes in the Eastern Cape. Lastly there is evidence that the volcanism was begining during the deposition of the Clarens - as is shown by the Clarens filled crater reported by Holzforster (cited in my last post). It would be great to get a date for the pyroclastic flows that form the basal layers of this crater but sadly no-one seems to have done it yet - any young geochronologist looking for a project?
4. So if we accept that the end of the Clarens can be precisely dated to 183 million years AND we only allow a duration of 5 million years or so for the Massospondylus Range Zone then the begining of the upper Elliot Formatio dates to the Early Pliensbachian (about 188 million years). Perhaps we could allow a few million years to have elapsed right at the end of the Clarens, before the volcanic lava flows began spilling out over the basin, and allow a rather long duration for the Massospondylus and its cohabitants but that would still only take us down into the late part of the Sinemurian.

There you have it. My guess is that the lower Elliot is mid-late Norian, there is a hiatus of about 15 million years before the upper Elliot and Clarens which probably span most of the Pliensbachian. How can this be tested? Detrital zircons would be a wonderfull source of data - once again any young geochronologists out there looking for a project?

references

Bordy EM, Hancox PJ and Rubidge BS (2004) Basin development during the deposition of the Elliot Formation (Late Triassic - Early Jurassic), Karoo Supergroup, South Africa. South African Journal of Geology, 107: 395-410.

Catuneanu O, Hancox PJ, Rubidge BS (1998) Reciporical flexural behaviour and contrasting stratigraphies: a new basin development model for the Karoo retroarc foreland system, South Africa. Basin Research 10: 417-439

Holzforster F (2007) Lithology and depositional environments of the Lower Jurassic Clarens Formation in the eastern Cape, South Africa.South African Journal of Geology, 110: 543-560.

Wednesday, December 10, 2008

Thoughts from the field: Welcome to Lake Drumbo

I want to float some ideas based on some observations I've made in the field this year. So what have I been up to? Well firstly I've been poking my nose further south than I usually do. Most of the sites I've been working over the past half a decade are in the north end of the main Karoo Basin of South Africa. Here the dinosaur bearing Elliot Formation is relatively thin, probably because it lies over the solid Kaapval Craton, a big ancient continental block. This block probably prevented the basin floor from sagging too deeply during mountain building events off to the south (Tectonics is a VERY important factor controlling of deposition in the Karoo Basin). But down south you get off the Craton and the Elliot Formatin becomes thicker...over four times thicker. So with some money from Germany, and a collaborative team from Germany (led by Ollie Rauhut) and Great Britain, we began looking down south (mostly in the Eastern Cape Province). Despite the great thickness of sediment good outcrop is hard to find as the rainfall is quite high and vegetation rather thick.
Another rucurrent problem is figuring out where you are in the stratigraphy, these rocks don't come with labels!
So first a quick primer of the stratigraphy of the dinosaur bearing part of the Karoo.
The top of the sedimentary pile is usually referred to as the 'Stormberg Group' although this is not an officially recognised stratigraphic unit. The Stormberg Group consists of three formations: the Molteno, the Elliot and the Clarens. The Molteno is a series of coarse grained to conglomeratic sandstones, with fine grained siltstones, mudstones and coals in the south. It is rich in plant fossils (and some insects) but has not produced a single piece of bone - although there a some dinosaur tracks reputed to have come from this formation. The Elliot Formation marks the begining of the dinosaur body fossil record in South Africa. It is divided into two units: the upper and lower. At times it can be very difficult to determine in which unit you are in. The Elliot Formation is predominately made of red overbank muds and silts deposited on a humid (lower) to semi-arid (upper) floodplain. It would appear that the Elliot Formation covers quite a time range, with the lower member almost certainly being of Late Triassic age while the fauna of the upper member is very much Early Jurassic in aspect. The Clarens Formation consists of pale cream to white massive cliff-forming sandstones that are aeolian (wind blown) in origin. It records further aridification and the onset of a dune desert. Strangely the fauna doesn't seem to change much, if at all between the upper Elliot and the Clarens. You get pretty much the same taxa in the Clarens, just fewer specimens.
Now here is a parorama of the main valley wall on Upper Drumbo near Barkly East.

We excavated two dinosaur skeletons (Nelly and Charlie) from near the bottom of the Valley. You can see the cap of massive sanstone at the top of the peak in the centre (Castle Rock. This is clearly Clarens Formation. But where is the top of the Elliot Formation and where in the Elliot Formation do our dinosaurs come from?

At first glance the change in slope at the top of the valley seems to mirror the outcrop pattern of the boundary between the lower and upper Elliot Formations.

A colourised diagram of the outcrop of te Elliot and Clarens Formations. Note the change in slope between the two members of the the Elliot Formation. Modified from Bordy et al. 2004.
You can see this even better from a photograph taken from a higher vantage point looking across at Castle Rock, rather than up at it from the valley floor.

In this photo only the thick sandstone bench at the top of the valley can be seen. The smooth ramp-like slope leading up to Castle Rock is obvious. The mountains in the distance are made of the 2km thick pile of basalt that was extruded toward the end of the Liassic. You can read more about them here.
If this was the case then the upper Elliot Formation would form the smoother upper slopes leading to Castle Rock and our dinosaurs would be from the lower Elliot (hence Triassic). BUT one of them is almost certainly a Massospondylus (typical of the upper Elliot) and the sediments that enclose them are also typically upper Elliot in aspect. Perhaps everything from the base of Castle Rock to the valley floor is upper Elliot. Thats a verticle height of 210 m. The upper Elliot reaches thicknesses of 150 m down in the south, but 210m is a bit much. A few observations point me towards what I think is the answer. Firstly you will notice a thin bed of narrowly banded siltstone just below the big sandstones of the valley rim.

I've see a similar bed at the very base of the Clarens Formation in a number of locations (but not all). These are probably a series of playa lake deposits (about the only fossils you will find in them are little mussel shrimps or conchostracans) and they form a pretty good marker for the end of the Elliot.the smooth slope above the valley is not made of red overbank fines like the Elliot Formation. Further evidence that the upper slopes are actually within the Clarens Formation can be found if you actually climb up to them to take a look. The slope isn't made of red fluviatile mudstones, instead you will find thinly laminated pale creamy-grey shales. I've seen this lithology before - in small localised lenses of the Clarens Formation. They are interpreted as small interdune emphemaral pond, or playa lake deposits. However the thickness here (about 80 m) is, as far as I can find out, unprecedented. It seems to me that far from being a dune desert, this part of the world was host to a pretty large long lasting lake. Even more cool is that the same big package of shale can be found in near Blikana and at Rhodes. This may indicate a lake about 100 km across. Strange that I can find no mention of such an obvious feature in the literature. So what lived in or or around it (assuming my interpretation is correct? Bugger all I'm afraid. You won't find even scattered fish bones or scales. My guess is that it was very shallow, hypersaline and prone to frequent drying out. Much like Lake Eyre in the Tirari Desert of South Australia. Indeed given that it is surrounded by a dune desert and the northern section of the Lake is about the same size as the lake I'm suggesting, Lake Eyre makes a pretty good analogue.

Lake Eyre

However there IS a report of a large Lake in the Clarens Formation off to the South West of this deposit. However this lake is an altogether different kind of thing. Holzforster (2007) reports on lacustrine deposits in the Clarens Formation in the Exterem South West end of the outcrop of the Formation. However here the Clarens is incised down into the Elliot Formation, and the lake deposits are underlain be thick pyroclastic deposits. In other words this south-western lake was developed in a volcanic crater. Sadly no fossils have come from here either, - its a pity because a late Early Jurassic Liaoning-style lagerstatte would be sooo cool.

references

Bordy EM, Hancox PJ and Rubidge BS (2004) Basin development during the deposition of the Elliot Formation (Late Triassic - Early Jurassic), Karoo Supergroup, South Africa. South African Journal of Geology, 107: 395-410.

Holzforster F (2007) Lithology and depositional environments of the Lower Jurassic Clarens Formation in the eastern Cape, South Africa.South African Journal of Geology, 110: 543-560.

Tuesday, December 9, 2008

Field trip photos

I'm currently writing up a report on the lightening quick field trip I took at the end of November for this blog but am getting delayed by all sorts of real-world duties. So for now I'll just show-case some of the pictures.



Matthew Yates assists Charleton Dube and Sifelani Jirah in excavating a dinosaur tail.



The tail of 'Charlie' the sauropodomorph (yes another one!) emerges



Upper Drumbo, type locality of Dracovenator regenti, and site of 'Charlie'. Casle Rock is in the background but where is the contact between the Elliot Formation and the Clarens Formation?.

Monday, December 1, 2008

Comments

Mea Culpa, I didn't realise the comments were only letting google users have their say. I'm sure when I set this up I allowed comments from everyone. Anyway the problem is fixed - I invite comments from anyone.

Two new Fossil Cowries

ResearchBlogging.org
A small paper has just been published on two new fossil cowries from the Miocene of South Australia (Yates 2008). Although it is unlikely to set the palaeontological world on fire it is a personally satisfying paper as it represents my first published foray into a subject area that has actually been close to me for most of my life. As I have mentioned before growing up in South Australia provided next to nothing in the way of actual dinosaur digs or even museum displays of dinosaur bones. If you wanted to get out and dig for your own fossils then the marine limestones and marls of the River Murray cliffs was about the only game in town. Most of these sediments are rather coarse grained calcarenites that unfortunately offered no protection ravages of groundwater which dissolves shells made from aragonite (the form of calcium carbonate that the majority of molluscs use). As a consequence nearly all mollusc fossils are present only as moulds surrounding the void where the shell once was. There is a gleaming exception: a silty marl unit called the Cadell Formation (formerly the Cadell Marl Lens of the Morgan Limestone).


The Cadell Formation in outcrop (note the house boat on the river channel in the background).



The Cadell Formation in context. The creamy coloured beds (largely grassed over) are the Cadell Formation while the strongly banded orange limestones above it belong to the Bryant Creek Formation.

This formation is packed with aragonitic mollusc shells, sometimes so well preserved they look as if they have freshly come off the beach. The extent of the shelly facies of the Cadell Formation is quite limited, the main exposure stretches for just over 10 km between the towns of Murbko and Morgan, however for most of this length the exposures form sheer cliffs that plunge straight into the river. Only one decent access point exists, at the type locality for the formation, about 6km south of Morgan. This site is well-known and collectors have visited it for over a century. The first thorough documentation of the fossils of the Cadell Formation were published by Ralph Tate, a British born geologist, palaeontologist and botanist who emigrated to South Australia, and became the head of the Department of Geology at the University of Adelaide. Incidentally the Tate medal is still awarded each year to the best honours research project in the department for that year. In 1994, yours truly was the recipient of this award, definitely one of the proudest moments of my life, not least because Tate with his extremely broad knowledge of natural history was a personal hero of mine. I was very surprised to learn, while googling around for details of the man’s life I found that he and I share the same birthday.
Anyway back to the Cadell Formation, one would think that with such a long and venerable history of study and collection, there would be little left to discover. Not so; the mollusc fauna has not received a comprehensive survey since Tate’s pioneering work and the paleoenvironment of the Formation remains an enigma. I first visited this site when I was just 13 years old and fell in love with the site. I visited the site several times a year until I finally left Australia when I was 28. Over the years I’ve amassed a collection that includes more than 200 species of mollusc. Many of these are new records for the formation, and several represent new species. However as my academic career took me into vertebrate paleontology and dinosaur research, I left my interest in these mollusc fossils lie dormant but not forgotten. Late last year I finally got my chance to produce my first publication in this field. I hope many more of greater significance will follow. The paper outlines two new species of cowry from the Cadell Formation that were formerly thought to belong to middle Miocene species from the mollusc-rich basins to the east in Victoria. The first of these is Umbilia caepa, an extraordinarily fragile member of the basal cowire genus Umbilia.


Umbilia caepa

Umbilia was featured on this blog here and here. U. caepa is quite similar to the Victorian contemporary species U. leptorhyncha but consistently differs from it in a number of respects including the weaker apertural dentition, the development of a plate-like posterior columellar callus bordering the posterior canal and broad plate-like flanges on each side of the anterior rostrum. It also has a more strongly developed pyriform shape which resembles the bulb of an onion (hence the name). Of course with palaeontological samples it would have been impossible to demonstrate that U. caepa was a reproductively isolated from the eastern U. leptorhyncha or was simply the western end of a clinally variable species. However much to my surprise that when sorting through the various fragments from the Cadell Formation I found a small thin piece of Umbilia that does indeed have strong apertural dentition and weak lateral ridges on each side of the anterior rostrum (as opposed to broad flanges) amongst other features that indicate it was actually a true U. leptorhyncha. No intermediate specimens could be found indicating that the two species were sympatric in the Murray Basin but only U. leptorhyncha extended east into Victoria.
The next species I described belongs to the endemic southern Australian clade Austrocypraea, which is now regarded as cold-water adapted subgenus of the large tropical Indo West Pacific genus Lyncina (this includes the famous ‘golden cowry’ Lyncina aurantium) based on molecular evidence (Meyer 2003).


Lyncina aurantium image from en.wikipedia.org/wiki/Cypraea_aurantium

The species, which I called L. (A.) cadella, is abundant at the site and many specimens had been found and examined by previous researchers but had not received its own name due to a particularly bad tangle of taxonomic confusion surrounding the species.
Tate had found this species but had regarded as a mere variant of the Victorian species L. (A.) contusa. In a similar case to U. caepa, L. (A.) cadella is close to L. (A.) contusa but consistently differs from it in a number of respects relating to size, dentition and shape of the fossula.

Lyncina (Austrocypraea) cadella

As there are consistent differences between the two populations I think that the South Australian population is deserving of separate species status. Frank Schilder thought so too, when he revised the Australian fossil cowries in 1935. Schilder was a dedicated cowry researcher, and it is a testimony to his deep knowledge of the group that much of his generic classification of these extremely conservative and homoplastic shells was upheld by recent molecular phylogenic work. Sadly his work on Australian fossil cowries was not among his better efforts. The main problem was that he was working from collections held in Europe that were rife with poor locality data, leading to all sorts of confusion. To cut a very long story short Schilder described L. (A.) cadella -twice! – using two different names, neither of which are available. In the first instance he confused his own specimen of L. (A.) cadella with an Eocene species named by Tate – ‘Cypraea’ ovulatella and referred it that species using the combination ‘Austrocypraea ovulatella’. But ‘C’ ovulatella (now Willungia ovulatella) clearly isn’t the same thing as L. (A.) cadella, it isn’t even a cowry! (the confusion was the result of relying only on illustrations and an icorrect locality label). Secondly he described a second sample of L. (A.) cadella that was obtained directly from Tate himself by the French malacologist Alexandre Cossmann as a new species ‘Austrocypraea subcontusa’. So the species from the Cadell Formation should be called L. (A.) subcontusa right? Wrong. In an inexplicable move after describing the Cossmann’s sample Schilder selected an aberrant dwarfed Victorian specimen as the holotype of his new species. After looking at the Victorian specimens I’m convinced that the holotype of Austrocypraea subcontusa is just an extreme variant of true L. (A.) contusa. It still differs from L. (A.) cadella in a number of respects and can be connected to typical L. (A.) contusa by a number of intermediates. The upshot of all this is that the common species of Lyncina (Austrocypraea) from the Cadell Formation has never received a valid scientific name despite being known for well over a century.
So what is the significance of all this arcane taxonomy? The main significance is that these species are more evidence of faunal differentiation between the various Miocene epicontinental basins. This is in contrast to the modern molluscsn faunas of southern Australia where most species have broad ranges stretching across the entire southern Australian seaboard. Although there certainly were many widespread middle Miocene species in southern Australia there does appear to have been higher levels of endemicity, perhaps fueled by the presence of restricted epicontinental basins and the propensity for many southern Australian molluscs to abandon the planktonic dispersal stage of their development.

Adam Yates (2008). Two new cowries (Gastropoda: Cypraeidae) from the middle Miocene of South Australia Alcheringa: An Australasian Journal of Palaeontology, 32 (4), 353-364 DOI: 10.1080/03115510802417927

C.P. Meyer (2003) Molecular systematics of cowries (Gastropoda: Cypraeidae) and diversification patterns in the tropics. Biological Journal of the Linnean Society 79: 401-459.

Thursday, November 27, 2008

Closing in on turtle origins


ResearchBlogging.org
I wrote in this blog in October that we can expect a more complete prototurtle. Never would I have dreamed that it would appear so quickly and that it would be even more primitive than Proganochelys or Chinlechelys. Finally something that hasn’t progressed so far down the road to turtlehood that its ancestry has been all but erased. Named Odontochelys semitestacea, it came as quite a shock to me – why? Because of all the competing hypotheses of turtle origins this guy seems to support the one I found the least convincing – that is turtles are the sister group of sauropterygians (an aquatic group of diapsid reptiles including placodonts, nothosaurs and plesiosaurs). My own musings that aetosaurs might be related to turtles now seems very unlikely indeed. It will take time for the remains of Odontochelys to be hashed over (the announcement paper is somewhat light on anatomical detail) to really determine what origin theory it supports. However I think we can now confidently rule out a the pariesaurian hypothesis. Of the synapomorphies linking pareiasaurs, or derived subclades within pareiasaurs, to turtles a good many of them are missing in Odonotochelys. These include: basal tuberae (ventral swelling of the braincase) midway between the occipital condyle and the basipterygoid processes (where the palate attaches to the braincase); acromial process on the scapula, closure of the spaces between the ribs (it is debatable wether or not any turtle actually has this condition); fewer than twenty caudasl vertebrae; and body covered in united osteoderms. Note that although it appears to support a diapsid origin for turtles the skull of Odontochelys lacks any trace of temporal openings so perhaps we can't quite rule out other anapsid sister groups just yet.

Not a part of the turtle sister group, the pareiasaur Bradysaurus. Image from wikipedia commons

Odonotchelys is yet another gem from the palaeontologically rich nation of China, this time from the marine Triassic deposits of the Guizhou Province, which are famous for their diverse Marine reptile fauna. One last note on the dating. The age of Odontochelys is given as about 220 ma based on biostratigraphy which places the unit it comes from in the Lower Carnian. As discussed recently by Bill Parker over at Chinleana the dates for the Triassic have been substantially revised of late, and if a lower Carnian age is to be upheld for Odontochelys then its absolute age is probably closer to 235 ma.

Chun Li, Xiao-Chun Wu, Olivier Rieppel, Li-Ting Wang, Li-Jun Zhao (2008). An ancestral turtle from the Late Triassic of southwestern China Nature, 456 (7221), 497-501 DOI: 10.1038/nature07533

Thursday, November 20, 2008

The giant pineal 'eye' of Platycyclops


The answer to the the little photo quiz was as the title suggests the relatively enormous pineal foramen of the oudenodontid dicynodont Platycyclops - so kudos to Matt. Quite why these dicynodonts have such enormous pineal openings is an unanswered question. Modern mammalian pineal glands are buried deep in our grossly inflated brains but are still used to regulate day/night cycles and seasonal cycles. In mammals it is influenced indirectly by exposure to light via signal that originate from the retina. In other vertebrates with a pineal foramen direct exposure of the pineal itself triggers the pineal gland to secrete its regulatory hormones. Pinealocytes have a strong resemblance to retinal cells and it is certainly tempting to speculate that the pineal organ of Platycyclops and related dicynodonts had crude image forming abilities. The forward tilt of the opening certainly gives the impression of a third 'eye'.
Googling around for information turned up one other little factoid, the name Platycyclops Broom 1932(the dicynodont) is a junior homonym of Platycyclops Sars 1914 (a copepod crustacean). As far as I know no-one has proposed a replacement name for the dicynodont.

Sunday, November 16, 2008

Picture of the Day: long tailed widow bird


Just a quick post while I'm on leave - I'll be revealing the mystery orifice soon, so keep guessing - one of you is very close. We took a family outing to the rhino and lion park in ‘The cradle of humankind’ a world heritage area that includes the famous Sterkfontein and Swartkrans caves where several australopithecines have been found. While there I managed to get this shot of a breeding male long-tailed widow bird (Euplectes progne), the epitome of an elaborate sexual display that is a major handicap to its owner. Non-breeding males are drab, brownish, sparrow-like birds with tails of normal length.

Tuesday, November 11, 2008

What is it?


Yes, another round of 'what is it?' I'm off on leave for a week. I hope I can post something in the during this time but in case I don't have a go at guessing the identity of this osteal orifice. Clue: it has nothing to do with any sort of sauropodomorph (for once). Oh the scale bar represents 2 cm.

Friday, November 7, 2008

Defending the indefensible


A couple of weeks back there was a fresh round of bashing on poor little Pantydraco. Bashing the name, not the dinosaur that is. It is a matter of some embarassment to me that I should be tied to what is widely regarded as the worst dinosaur name ever. I take pride in the names I craft, and I don't think I'm too bad at it (Dracovenator, Antetonitrus, Nanalania are some faves) even if I do say so myself.
So here is some of the backstory behind the name. Firstly: it was not my invention! That particular dubious honour goes to Peter Galton. When I first named the species caducus I found some characters that linked it with what I was calling (and still call) Thecodontosaurus antiquus, so thought that the wisest course of action was to name a new species in that genus. However as I found out more about Thecodontosaurus antiquus (and I regard all of the English cave fill sauropodomorphs as one taxon) more and more differences with the Welsh ‘Thecodontosaurus caducus started to show up. For instance the ischial shafts of T. antiquus are an unusal flattened ovoid cross-sectional shape whereas those of ‘T.’ caducus have the classic triangular cross section seen in most other basal sauropodomorphs. Significantly support for a monophyletic Thecodontosaurus had diminished to the point that it was no longer recovered in all of the most parsimonious trees of my improved cladistic analyses (as more characters were added and more scorings were based on first hand observations) . So I had come to the conclusion that it was time to erect a new genus for ‘T.’ caducus. I even had a tentative name thought up – Cambrambulus – the Welsh wanderer. However I was not quick enough and at the 2005 SVP Peter Galton told me that he was close to submitting an MS giving caducus that infamous generic name.
Naturally I felt rather attached to the first dinosaur species that I named and wanted to remain associated with the generic name (which after all is the most frequently cited name in dinosaur circles). So I told Peter about my plans and we agreed to write the paper together, even though our reasons for naming the new genus were different. Peter felt that Thecodontosaurus was a dubious name because he thought that the type specimen of T. antiquus (the type species of the genus) was indeterminate. I agree that it isn’t the best and that it has no single derived character that cannot be seen in other sauropodomorph taxa. However I am still unconvinced that there are two morphs in the original Durdham Downs quarry, or that Asylosaurus is all that distinct. Taken collectively the Thecodontosaurus antiquus sample is unique and diagnosable. The utility of bone bed taxa is itself the subject for another post but to put it simply I’m all for using them in the right circumstances. In this case the surviving scanty sample from Durdham Downs is backed up by a large sample from Tytherington . This is another set of sauropodomorph bones from a fissure fill in the English south-west. Here there is absolutely no sign of two morphs and all of the recovered bones are virtually identical with those from Durdham Downs.
So there we are, the Pantydraco paper is very much a compromise - melding two different viewpoints but agreeing that little caducus needed a new generic title. Perhaps I should have pushed Peter to change the name, but being my meek and mild self, didn't do so. In anycase I didn't think it was quite that bad at the time,although yes, I was quite aware of the conotations (I guess thats a little bit of my juvenile sense of humor showing though).
Lastly for all those who absolutely hate the name - remember it is correctly pronounced 'Pant - uh - dray - co' which is not quite so bad as 'panty'.

Wednesday, November 5, 2008

From the galleries of the BPI: The Cape Giant Zebra

This specimen is a partial set of jaws of Equus capensis, the so called cape giant zebra, from Makapansgat, the most northerly australopithecine site in South Africa. Actually, although robust these equids are not so giant, being about the size of a big modern horse. Fossils of this robust equuid are widespread throughout South Africa, with the type coming from close to Cape Town, way down in the southwest.
It is often said that Africa escaped the megafaunal extinctions of the late Pleistocene but there is a definite set of large African mammal species that clearly did not make it through to the present. These include the giant buffalo Pelorovis, other bovids like Megalotragus, and supposedly Equus capensis. But if Charles Churcher is right reports of E. capensis' demise are greatly exaggerated. It s apparently alive and well in the form of.... Grevy's Zebra.


Image from wikimedia commons

Apparently the teeth (which are what most extinct Equus taxonomy is based on)of E. capensis do not differ in significant ways from those of E. grevyi and a bunch of east and northern african fossil equiids (e.g. E. oldowayensis).
Grevy's zebra is now restricted to East Africa and cannot be found anywhere near Suth Africa. So if it doesn't represent an actual extinction it does represent a drammatic range contraction.

reference

Churcher CS (2006)Distribution and history of the Cape zebra (Equus capensis) in the Quaternary of Africa. Transactions of the Royal Society of South Africa 61:89-95

Tuesday, November 4, 2008

What was it?



That ugly mystery bone is out now. What a disaster! when lifting it from its jacket, an undetected puddle of glue had soaked through the sepparator and firmly welded the underside to the jacket (lesson for the future - make sure there are several layers of sepparator between the bone and the jacket). The remarkable thing about the bones from this quarry is that although the compact outer bone is well-preserved the interior is like compressed flour. Once a break is started the whole bone tends to crumble to powder, much like the unravelling of a woollen jumper once the stiching comes undone in one place. Consequently the finished product is not one of our lab's proudest moments and I won't be showcasing it here. Nonetheless two things are apparent 1) it is a busted ischial peduncle from an ilium - exposed in medial view in the photo. 2) this part alone is about the same size as a middling Massospondylus, so although Rutger was halfway right he doesn't get full marks. The diagram below indicates which part of the ilium the bone is, and should give the astute clues to where I think its relationships lie (incidentally it is from the same site as the sauropod caudal I described in the South African Journal of Science (2004, vol. 100. 504-506)

Wednesday, October 29, 2008

End of year blues

Hi guys,

This blog has become far more quiescent than I ever intended it to be. I am currently feeling exhausted and swamped by marking at the end of the year, including honours projects, exam papers and essays - urgh. Of all my duties I hate marking the most. Time is of the essence but I want to be scrupulously fair to all, a desire that used to compell me to read and re-read each essay two or three times before finally settling on a mark. When not marking I am tidying up various other bits of end of year business - and occasionally writing up some of my research for publication. I wish I could tell you guys about it (it is one of the most enjoyable aspects of my job) but sadly this work has to go through the long drawn out process of peer-review and publication first. Maybe I'll be able to say something in 2010!
Anyway I've dragged up a picture of an old field trip from my vaults. You can see that the weather was kind of .... damp. This happened a lot on my field trips (and not always in rainy season either) so much so that one of the dinosaurs we happened to find in between rainstorms received the nickname 'Rainmaker'.
Anyway the load on shoulders should be lifting soon. I'll have a visit from my mum soon and we'll be doing a few trips round Gauteng. Then I'll be off on a short field trip to retrieve a dinosaur we left behind earlier in the year. So by December my batteries will be recharged and blogging will begin with renewed vigour (I hope!)

Tuesday, October 21, 2008

What is it?


...... The answer is - I don't know. This is one of those weird 'head scratchers' or GOKs (God only knows) that we get from time to time. Celeste is prepping it up right now. Its been sitting down in the lab for over two years and various volunteer preparators have had a go at it and quickly left it for easier more exciting projects. It comes from the upper Elliot Formation in a quarry that has produced more than one type of sauropodomorph as well as a protosuchian crocodile and some honkin' big theropod teeth. Any suggestions?

Friday, October 10, 2008

Good News Everyone!

ResearchBlogging.orgAfter spending the whole week in hospital (it wasn't just tonsilitis after all) Matthew will probably be coming home tomorrow ..... and one of palaeontology's holy grails - a basal stem turtle has been found and published.

I imagine most of my readers know that pinning down the closest relatives of turtles and the origins of their bizarre morphology has been one of the most recalcitrant problems in tetrapod evolution.
While the lack of holes in the cheek region of the skull suggests that they are an offshoot of the anapsid reptiles, with procolophonoids or pareiasaurs being the main contenders, more than one large scale morphological cladistic analysis has found that turtles are members of the Diapsida (derived reptiles with two pairs of holes in the temporal region of the skull). Molecular work also supports a diapsid origin for turtles but tends to place them close to, or within, the Archosauria (crocodilians, dinosaurs, birds and their kin)whereas the morphological work tends to align them with the Lepidosauromorpha (lizards, snakes and kin).

Sadly the new turtle described by Walter Joyce and colleagues in Proceedings of the Royal Society is too fragmentary to really speak to this vexatious issue. But it does tell us quite a bit about how their most distinctive feature - their shells - evolved and lets us know that the answers are out there (given the recent spate of excellent discoveries in the Chinle and its equivalents I'm sure a more complete proto-turtle won't be be a long time coming).

But first a little of the who, what where and when. The new fossil is called Chinlechelys tenertesta (the thin shelled turtle from the Chinle)and the known remains consist of fragments of shell with attached underlying parts of the vertebrae and ribs and isolated osteoderms found in the Bull Canyon Formation of New Mexico. The name is taken from the Chinle Formation, which is a little odd since most stratigraphers and palaeontologists would not to place the Bull Canyon Formation in the Chinle Formation (or Group according to some), prefering instead to place it in the Dockum Group. The age of this part of the Dockum is probably mid-late Norian of the Late Triassic, somewhere between 210 and 219 million years old.
There are other equally old turtles in Europe, Greenland and South America but none so primitive in shell design as Chinlechelys. For starters its shell is unusually thin but more importantly it still has rather individualised ribs, that although joined to the inside of the shell, are not fully subsumed into it.
The telltale fragment - a piece of shell with an individualised rib beneath it.Modified from Joyce et al. 2008

This is important because it helps falsify the model that the shell is a modified ribcage alone and supports the hypothesis that the shell is a composite structure formed from the agglomeration of trunk vertebrae, ribs and several rows of osteoderms (bony armour in the skin). Intriguingly modern developmental studies have suggested the former hypothesis because it appears that the shell grows as part of the endoskeleton (that is it is made of replacement bones that are preformed in cartilage and derived from scleritomic mesodermal cells. Developmental studies can yeild powerfull evolutionary data but I think the takehome message here is that developmental pathways can and do evolve. Palaeontology definately still has a place at the high table of evolutionary studies!
Hypothetical steps in the evolution of the turtle shell following the osteoderm hypothesis. From Joyce et al. 2008

Bonus musing: If the molecular data are correct it is perhaps no co-incidence that some pseudosuchian, or crurotarsan - whatever you preference, archosaurs have rather turtle-like carapaces. Aetosaurs even have a plastron of plate-like osteoderms.
Paratypothorax, an aetosaur. Compare to hypothetical stage II above. Image from www.dinotime.de

WG Joyce, SG Lucas, TM Scheyer, AB Heckert, AP Hunt (2008). A thin-shelled reptile from the Late Triassic of North America and the origin of the turtle shell Proceedings of the Royal Society B DOI: 10.1098/rspb.2008.1196

Monday, October 6, 2008

Tonsilitis again!

My little guy, Matthew, who is one month shy of his second birthday, has come down with serious tonsilitis for the second time in as many months. It would appear to be a law of nature that kids always fall sick on a weekend, usually a Sunday evening when medical services tend to be a little thin on the ground (especially for those not wishing to sit in a casualty waiting room for hours). At the moment this is my main blog-writing time. So sadly I'm empty handed - once again. I don't feel to bad about this however because I managed to get hours of research writing in, before Matthew got sick.
In lieu of my own work I would like to point folks to Matt Wedel's thorough dissection of the recent Aerosteon paper.

Matthew in better health doing some computer work of his own.

Monday, September 29, 2008

Umbilia gazing - part II

We last left off our survey of Umbilia in the middle Miocene where we looked at U. eximia the most abundant and widespread species.
Two of the remaining four described middle Miocene species are some of the weirdest of all crown-group cowries (I say crown group because there were some truly bizarre looking stem-group cowries, e.g. Gisortia).

U. siphonata (above) is one of these. It is a very large cowry, attaining a length of almost 17 cm, which is not far behind the biggest specimens of Macrocypraea cervus, the largest extant cowry, which used to reach sizes of 19 cm in length. However U. siphonata is cheating a little since the anterior and posterior rostra of this species are produced into great upwardly curving ‘horns’. The rudimentary flanges that support the bases of each rostrum of U. eximia are much better developed in this species. Even stranger is U. gastroplax, the ‘flanged cowry’ (specimen on the left is from Darragh 2002). This species also has elongated horn like rostra, although they are not as long as in U. siphonata. However the flanges have expanded outwards, merging together and making a continuous brim that encircles the entire base of the shell. The result looks much like a snow-shoe. Indeed it has been suggested that this is exactly what its function was and that it was an adaptation to living on soft, ‘soupy’ bottoms. One wonders then if U. siphonata was adapted to the same conditions but simply didn’t bother to keep itself on top of the sediment surface, and used its long rostra to carry its siphons up into clear water.
The fourth middle Miocene Umbilia we will look at is little U. leptorhyncha (below). Although common and widespread, good specimens are rare due to the thin-shelled fragility of this species. This species is a departure from the other mid Miocene species in its small size, globose shape and poorly developed rostra. In these respects it most closely resembles U. prosila and may be closely related to it.

All of these species can be found in both South Australia and Victoria (U. gastroplax has not been officially recorded from South Australia but I have personally collected two specimens from the Cadell Formation on the banks of the River Murray)
Darragh recorded the extant U. hesitata as a fifth middle Miocene species, albeit one that only appears at the end of the stage, with little to no time overlap with the previously mentioned species. The taxonomy of these rare later middle Miocene Umbilia is a complicated issue. Two species have been named Umbilia tatei and Umbilia cera. Both are short , with weakly developed beaks and heavily calluses surrounding the basal margins and probably represent the same species whatever they are. Problematically if these really are small specimens of the extant ‘wonder cowry’ (as U. hesitata is sometimes called) then we have the problem that U. tatei would have priority over U. hesitata. I’m sure all the avid cowry collectors would object to replacing the entrenched U. hesitata with U. tatei. Fortunately I don’t think they have to. Although U. hesitata does display a range of adult sizes which overlaps with the small shells of U. tatei, small modern U. hesitata resemble typical large specimens more than they do U. tatei. In particular no modern U. hesitata has such thick marginal calluses as U. tatei, nor do they develop the elongate coarse dentition seen on the holotype of U. cera (these are not present in the types of U. tatei but the dentition of these specimens appears to be underdeveloped due to immaturity).

A late Miocene U. 'hesitata', probably belonging to U. tatei.

From the late Miocene and Pliocene (there is no Pleistocene record of Umbilia at all) there is just a single species, the extant U. hesitata (although some of these are a little different from modern U. hesitata while others probably belong to U. tatei). Then in our modern seas we find five species: U. hesitata, U. armeniaca, U. capricornica, U. orriettae and U. petillirostris.
However unlike the middle Miocene where you can find up to four species at the same locality almost all of the modern species have mutually exclusive ranges (only U. capricornica and U. petillirostris overlap in the deep Capricorn Channel of the Great Barrier Reef. Moving anticlockwise around the Australian coast we find U. armeniaca (Western Australia to Kangaroo Island, South Australia), U. hesitata (south eastern South Australia to Southern Queensland), U. orriettae (Moreton Bay, Queensland) and U. capricornica/U. petillirostris on the Great Barrier Reef. It is interesting to note that this pattern matches the phylogenetic pattern recovered in a comprehensive molecular phylogenetic analysis of modern cowries (Meyer 2004). In this analysis U. armeniaca was the sister group to all other living species and U. hesitata was the sister group to U. capricornica + U. petillirostris (U. orriettae was not included but morphologically it appears to be intermediate between U. hesitata and U. capricornica). Thus the modern forms appear to be the result of a radiation that proceeded from west to east. All of these living species are rather similar to one another and have a rather generalised shell structure compared to the excesses of the middle Miocene.

Umbilia hesitata, the most abundant extant species.

Most specimens display a moderately well-developed posterior rostrum and have highly reduced anterior tubercles that are separated by an oblique sulcus. These characters suggest that the modern taxa are more closely related to U. eximia and U. tatei than any of the other extinct species. However the type specimens of U. petillirostris stand out as something unusual. Unlike other living Umbilia the types of U. petillirostris are globose and thin-shelled with a very short posterior rostrum. In these respects U. petillirostris resembles the smaller middle Miocene species, U. leptorhyncha and the late Oligocene U. prosila. Darragh (2002) suggested that these three species represented a lineage that had been separate since the Oligocene. But using the molecular phylogeny this would suggest that all of the modern species have been separate since at least the late Oligocene, despite sharing a similar hesitata-like morphology that does not show up in the fossil record until the late Miocene. However I strongly doubt that U. petillirostris is closely related to U. prosila and U. leptorhyncha. Despite its globose shape and thin shell it displays characters typical of the modern clade such as large size, a moderately produced posterior beak, and weak anterior tubercles. A greater sample of specimens shows that U. petillirostris and U. capricornica are quite variable and that individuals of each species can be found that approach the other in morphology. Indeed some have suggested that the two species are not distinct at all (Wilson and Clarkson 2004). Nevertheless limited genetic sampling does indicate that U. petillirostris does maintain a distinct haplotype (Meyer 2004). Excellent photographs of all the living forms can be seen here.

Next week: the origin of Umbilia.

References

Darragh, TA (2002) A revision of the Australian genus Umbilia (Gastropoda: Cypraeidae). Memoirs of the Museum of Victoria 59: 355-392.

Meyer CP (2004) Toward comprehensiveness: increased molecular sampling within Cypraeidae and its phylogenetic implications. Malacologia 46: 127-156.

Wilson B, Clarkson P (2004) Australia's Spectacular Cowries: A Review and Field Study of Two Endemic Genera-Zoila and Umbilia. Odyssey: El Cajon, 369 pp.

Wednesday, September 24, 2008

Lookin' out my back door


Well out my back window actually. We've been watching this industrious little guy for almost three years now. He is a masked weaver (Ploceus velatus), and in order to attract a mate he has to construct a nest of sufficient quality to encourage a female to lay her eggs in it. Sadly he's a bit of a loser. Although several females have checked him out he has never been able to seal the deal. After every rejection he would demolish the nest and start again. Now however it appears he is so riddled with frustration and self-doubt that he just builds and destroys nest after nest without even getting it looked at first. I kind of empathize with him, my early years (15 through to 26) weren't too dissimilar.

Monday, September 22, 2008

More sauropod vertebrae/ ceratopsian frill convergence


I was reminded by Mike Taylor's recent post, noting that a Camarasaurus vertebrae seems to have a ceratopsian frill growing out of it, that I had had the exact same thought when I saw 'Max'. Max is a diplodocid (identified as Apatosaurus but I have my doubts) found by the crew at the Saurier Museum in Aathal. However this time the 'frilloid' process is composed of the two postzygapophyses and the perforate interpostzygapophyseal lamina. Incidentally the interpostzyg laminae of most of Max's cervicals are similarly perforate. It is a real feature, not caused by damage - weird huh?

Thursday, September 18, 2008

The last dicynodont



With so much going on I've had little time for blogging. Recently there was some discussion of the supposed Australian Cretaceous dicynodont (maxillary fragment of the specimen is pictured on the left) over at Chinleana. I'll add my two cents here rather than commenting there just to keep something ticking over on my blog. Randy Irmis made a startling comment that the consensus was that it was indeed a dicynodont. This is news to me, I had always thought that the identity of the specimen was always the weak part of the claim. I have to add that I've never seen the specimen myself. Randy goes on to add that because it was surface float it is the provenance of the specimen that is suspect. Here I have to add my voice in support of Thulborn's original assesment, whatever it was there can be little doubt that it came from the Cretaceous. As has been noted Australia is flat and rather geologically quiescent. The nearest Triassic rocks are many hundreds of kilometres away. Nor do these Triassic rocks have much in the way of dicynodonts in them anyway - just one beat-up quadrate from more than 20 years of intensive collecting in the Arcadia formation (the main fossil-bearing Triassic formation of south-eastern Queensland). When you are out prospecting in most parts of the world you almost never find fossils more than a few tens of metres from the formation that bore them (unless there is a transport mechanism such as a river). Australia certainly never had post-Triassic glaciations that can randomly transport objects over large distances. So if the morphology is definately saying dicynodont then hey, I'm prepare to accept this extraordinary claim. Indeed recently another clade thought to have died out before the end of the Triassic has been found to have survived until the Cretaceous (I can say no more) so survival of the Dicynodonts may not be so weird after all.