New discoveries - Heterodontosaurus

Have a look at the cover of the latest newsletter of the PSSA (Palaeontological Society of South Africa). This is one of two new skulls of Heterodontosaurus that Billy De Klerk has found in the Elliot Formation of South Africa. It seems Billy is O.K. with showing them off to the world before he publishes on them, so I don't think there should be any problem with me posting this picture here. Things to note: the premaxilla fails to contact the lacrimal (a lacrimal-premaxilla contact was one of the proposed characters linking Heterodontosauridae to Ornithopoda)and the angular (ventral edge of the lower jaw) has a Yinlong-like rugose boss. There is a complete postcranium to go with this skull - so we can expect even more information on these rather wierd, early ornithischians in the nearish future.

Spongebob is a child of snowball Earth

Image from commons.wikimedia.org

Although looking a little like plants, sponges (Phylum Porifera) are animals. Admittedly, they area very different kind of animal from the ones we see around us in everyday life. Unlike most others they lack bodies constructed from properly organized tissues, they are instead more like giant colonies of single cells. Indeed a famous experiment demonstrated that sponges that have been completely disaggregated into its constituent cells (by forcing them through a fine-mesh screen*) will begin to reconstitute themselves. Sponges lack any sort of gut, they instead live by sieving fine organic particles out of seawater. The food is absorbed directly by specialized ‘collar cells’ (more formally choanocytes) that line the canals and chambers that run through the sponges body. Sponges are also remarkable for their unique skeletons, which are generally made of tiny to microscopic spicules of silica or calcite or a lacy network of elastic organic material known as spongin (as in the original bath sponges). As you might expect with animals that lack a gut, nervous system or indeed organs of any sort, sponges are a very early branch of the animal tree. Indeed the tiny little disc of cells known as Trichoplax adhaerens may be the only living animal that branched away before sponges and all other animals split. Actually I’m oversimplifying here, because the evidence is looking good for sponge paraphyly. That means some of the different sponge groups are more closely related to tissue-grade animals (a clade called Eumetazoa) than to other sponges, so it wasn’t a single sponge-eumetazoan split. The remarkable conclusion that leads to is that we are directly descended from an animal that we would call a sponge.
Given their early divergence and simple construction, one would expect sponges to have a venerable fossil record. Indeed they do, but not quite as long as one might expect. Sponge spicules seemed to appear in the fossil record at the beginning of the Cambrian Period along with a great number of eumetazoan groups (now dated to 542 million years). Some spicules have been reported from older strata but these are not without controversy. In contrast eumetazoans clearly had an older fossil record (e.g. the 545-565 million year old Ediacaran fauna). The mid 1990’s saw the publication of Palaeophragmodictya, the first probable whole body fossils of sponges from the Ediacaran fauna. Nevertheless without spicular preservation (like other ediacaran macrofossils Palaeophragmodictya are preserved as impressions on the base of sandstone beds) there may always be some doubt as to its identity. What remains unusual is that it took so long for such Ediacaran sponges to be found and that they remain a very rare component of Ediacaran faunas.



Small individuals of the putative Ediacaran sponge Palaeophragmodictya. From Gehling and Rigby (1996)



Close up of a large Palaeophragmodictya showing what might be the impression of a spicular mesh.From Gehling and Rigby (1996)

Furthermore divergence dates calculated using molecular clock methods suggested a sponge- eumetazoan divergence of 650 million years. If the common ancestor was itself a sponge-grade organism we should expect the record of sponges and sponge-grade animals extending back to pre-Ediacaran times. This is well illustrated in the following diagram that teases apart the so-called ‘Cambrian explosion’(image from www.snowball.org). An interesting aspect of this molecular date is that it extends the range of animals back into a Period known as the Cryogenian, or just after it.



What is special about the Cryogenian? It was period in Earth history lasting from 750-620 million years where the Earth went through several severe glaciations events known as ‘snowball earths’. Although the exact severity of the snowball-earth glaciations is a contentious topic, there is convincing evidence that the Earth was cold enough to support sea-level glaciers in the equatorial belt. So an important question is: had animal divergence begun in the Cryogenian as the molecular divergence dates suggest? That question has been conclusively answered this year in a Nature paper by Gordon Love and colleagues.
Love et al. found convincing sponge fossils in sediments securely dated to the age of the Marinoan glaciation, the last snowball earth glaciations event of the Cryogenian. As a small aside the Marinoan is named after the seaside suburb of Marino, on the southern coast of Adelaide, my home town. My first ever geological field trip for my degree was looking at the Marinoan rocks of Marino. Anyway enough reminiscing, onto the Cryogenian sponges fossils. What were they? Spicules? Whole body impressions? No, Love et al. found molecular fossils, in particular 24-isopropylcholestanes. These particular hydrocarbons are, according to the authors, the degraded products of C30 sterols, a class of molecules only produced by members of the sponge class Demospongiae (here I have to accept the author’s word, I know far too little about organic chemistry to have any way of assessing the veracity of this statement). An interesting feature of snowball Earth Glaciations is that they are usually covered by a thin but continuous layer of carbonate rock: the 'cap carbonates'. These are thought to have precipitated out under extraordinarily hot conditions bought on by the retreat of the glaciers leaving behind an atmosphere dense in CO2 (which would accumulate while terrestrial weathering was essentially shut down underneath the ice-cover). Anyway these molecular sponge fossils are found in rocks below the cap carbonates, that is during the glacial period itself.
The implications are pretty huge. It means that multicellar animals arose and first diversified in frigid seas largely covered by ice, and not during the flush of warmth that suffused the planet immediately after the glaciers lost their grip. Where could animals exist in such a sea? There were probably numerous little oases of light where cracks in the relatively thin equatorial sea ice would allow local blooms of bacteria and algae. These little patches may well have been the birthplace of multicellular animal life.


A modern day equivalent of the oases that was the birthplace of animal life? Image from www.snowball.org.

References
Gehling, J.G. and Rigby, J.K. (1996) Long Expected Sponges from the Neoproterozoic Ediacara Fauna of South Australia. Journal of Paleontology 70: 185-195.

Love, G., Grosjean, E., Stalvies, C., Fike, D., Grotzinger, J., Bradley, A., Kelly, A., Bhatia, M., Meredith, W., Snape, C., Bowring, S., Condon, D., & Summons, R. (2009). Fossil steroids record the appearance of Demospongiae during the Cryogenian period Nature, 457 (7230), 718-721 DOI: 10.1038/nature07673

*They were the luckiest of all

Now is the winter of our fish content

once again the dreaded lurgy has struck our family this winter with Anwen needing a stay in hospital. So this post and the next few to follow were supposed to be posted a month ago.

Teleost fish outnumber all other modern vertebrates two to one. Despite this staggering diversity it is accurate to call fish palaeontology the poor cousin of amniote palaeontology, particularly when it comes to grabbing publicity. Nevertheless with such a huge diversity it is not surprising that the clade has thrown up more than a few subgroups that do grab public attention. What is surprising is that the austral winter’s edition of Journal of Vertebrate Paleontology (volume 29, number 2) carries articles on no less than three of these attention grabbing teleost groups.
One is the giant ocean-going sunfish (Molidae). These giant jellyfish-suckers include the largest living bony fish but due to their poorly developed skeletons and pelagic habits have left a scrappy fossil record. Thus the finding of three articulated skeletons that exceed modern sunfish in size (one skeleton reaches 4 metres from fin tip to fin tip) is a big deal.
Another curious fossil fish reported in this issue is a deep-sea anglerfish (Ceratoidea). These bizarre fish are well known for their disproportionately large mouths lined with needle-like fangs and the ability attract prey with a luminescent lure. Finding a fossil of one of these is also quite unusual for the ceratoid clade is not all that old by geological standards. There simply aren’t that many places where such recent sediments have been laid down at great depth but have since been brought to a position above sea-level where someone might find any fossils that they might contain.
Both of these discoveries are blogworthy finds, however it is the fossil piranha that I want to highlight in this post (Cione et al. 2009). Piranhas are, of course, the fabled freshwater fish of South America that are said to be able to skeletonise a cow in a matter of minutes when a school is whipped up into a feeding frenzy.
The fossil piranha was found late Miocene (about 5 to 11 million years old) river sediments in northeastern Argentina. It has a concave dorsal margin of the premaxilla and tall, sharp triangular teeth that indicate that its affinities lie with the piranhas among the serrasalmids. Aptly named Megapiranha the fish is immediately striking for its great size. Known from a single jaw bone (the premaxilla) and some isolated teeth it is about two and a half times larger than the premaxilla of an average modern piranha. Assuming similar proportions to a modern piranha may have approached a meter in length.

A modern piranha against a silhouette scaled up to fit the size of the Megapiranha premaxilla.

Now the thought of a school of those getting into a feeding frenzy is worthy of any Hollywood B-grade creature feature. However it is far from certain that Megapiranha would have indulged in such hypercarnivorous behavior. It should be noted that piranhas form a clade of closely related species amongst a broader family of fish known as Serrasalmidae. Most serrasalmids (for example pacus) and even some piranhas are vegetarian, indicating that the herbivory is the ancestral diet of serrasalmids. Given the primitive position of Megapiranha (the sister group of all other known piranhas) it is quite likely that Megapiranha was at least partly vegetarian.
What makes Megapiranha interesting, other than its size, is that it gives us some idea how the piranhas evolved their famous dentition. Primitive herbivorous serrasalmids have seven rounded, flat-topped teeth arranged in two rows. In contrast piranhas have a single row of six double-cusped, blade-like teeth. One would expect that the single rowed condition evolved from the double rowed condition by simple suppression of one of the rows, most likely the inner row which contains just two teeth. An alternative, proposed by Gosline (1951) is that the two rows integrated to become one. Megapiranha provides evidence that supports Gosline’s hypothesis, for it shows just a single tooth row but with the teeth placed in a staggered arrangement as if two rows were merging.

The premaxillae of a pacu (top), Megapiranha (middle), and a modern piranha (bottom)in lateral (left) and ventral (right) views. Scale bars equal 1 cm. Images from Cione et al. 2009.

The teeth themselves are intermediate as well for although they bear tall, sharp-edged triangular cusps like modern piranhas, there is no secondary cusp and the bases are broad, perhaps supporting the idea that Megapiranha was not a hypercarnivore. It is a pity really, I find the idea of a giant ground sloth or an astrapothere being stripped to its bones by a school of meter-long piranhas somehow appealing.

References

Cione, A.L., Dahdul, W.M., Lundberg, J.G. & Machado-Allison, A.(2009) Megapiranha paranensis, a new genus and species of Serrasalmidae (Characiformes, Teleostei) from the upper Miocene of Argentina. Journal of Vertebrate Paleontology 29: 350-358.

Gosline, W. (1951) Notes on the Characid fishes of the subfamily Serrasalminae.
Proceedings of the California Academy of Sciences 27: 17–64.