Image from Reichel et al 2009Imagine you found this fossil. At first glance you might think it was some kind of ornamented cranial horn but a closer look would reveal that one end bore two articulation facets on a vaguely hockey-stick shaped head. These features identify the bone as the rib of a tetrapod. But the rib of what, exactly? The row of knobs along the dorsal surface is very unusual and does not bear a close resemblance to any other known tetrapod.
These questions were raised when some of these unusual ribs were unearthed at a locality in middle Triassic sediments of southern Brazil by Father Cargnin, a preist with an interest in palaeontology. Since these ribs were isolated in an assemblage of mixed vertebrates, little could be said about their affinities. About all one could surmise is that they belonged to some exceptionally weird tetrapod. The answer came in 1989 when Father Cargnin found a partial articulated skeleton, with a skull, of the beast with the knobbed ribs at a second locality. It turned out to be a traversodontid cynodont, one of a group of plant-eating close mammal relatives that were common across the globe in the Middle and Late Triassic. Ranging in size between something that was roughly rabbit-sized up to something slightly larger but lower slung than a wolf. As advanced cynodonts they had much of the mammalian adaptive toolkit,probably including endothermy and a furry pelt, but lacked the defining dentary-squamosal jaw joint.
A reconstruction of Exaeretodon a typical traversodontid. From Wikipedia
Considering the unusual nature of the ribs, it is actually remarkable how unremarkable the rest of the animal is. The skull is much like that of any other traversodontid (there are some minor differences in skull proportions and the thickness of the skull roof).
The skull of Protuberum, above (taken from Reichel et al. 2009) in comparison to another traversodontid Massetognathus , below (taken from Romer 1967).
Anyway it has taken some time but Father Cargnin’s lumpy traversodontid has finally been published. It was given the name Protuberum cabralensis by Reichel et al. in the most recent issue of Palaeontology. The entire ribcage of Protuberum from the neck to the hips (and the dorsal edges of the hips themselves) was covered in these lumpy excrescences. The lumps are protrusions of the bone itself, not fused on dermal bones as one would expect since no cynodont is known to be armoured with dermal bones. Nonetheless these lumps would have protruded abone the level of the skin during life and were probably covered in tough, leathery or even keratinized skin. It would have made for an unusual looking animal, particularly if these lumps protruded above a layer of fur. The purpose of the knobs seems to have been defensive, perhaps against predators (they shared the earth with big crocodile relatives called rauisuchians ) or perhaps against each other.
Hat tip to Bill at Chinleana for alerting me to this publication.
References
Romer, A.S. (1967) The Chanares (Argentina) Triassic Reptil Fauna. III. Two new gomphodonts, Massetognathus pascuali and M. teruggii. Breviora 264: 1-25.
MÍRIAM REICHEL, CESAR LEANDRO SCHULTZ, MARINA BENTO SOARES (2009). A NEW TRAVERSODONTID CYNODONT (THERAPSIDA, EUCYNODONTIA) FROM THE MIDDLE TRIASSIC SANTA MARIA FORMATION OF RIO GRANDE DO SUL, BRAZIL Palaeontology, 52 (1), 229-250 DOI: 10.1111/j.1475-4983.2008.00824.x
6 comments:
This leaves me with two questions.
First, whether or not this might be the result of some pathological state to which these animals were prone.
(Sorry for that last sentence- "There are some unsplit infinitives up with which I will not put.")
That seems unlikely, but when I see something that freaky I have to wonder.
I'm also curious about the ears on the reconstruction -- are there any skeletal characteristics that would indicate ears? And if so, do we have any idea when and where ears started to show up?
I have to confess, I find it frustrating that there's so little popular science work done with synapsids. I find them fascinating.
The knobs are regular and present in all known individuals of this taxon--they are not pathological.
There is currently no evidence for external ears in non-mammalian cynodonts.
Also, because Protuberum is a neuter name, this species name will have to be emended to Protuberum cabralense.
Okay, so this means that mammal-like reptiles were very abundant in the form of cynodontids in the Triassic. I guess that means that the split either occured in the Permian or in the Jurassic. Hmmm...I still wonder where that evolutionary split occured. Could cynodonts be classified as mammals because they're more mammal than reptilian? This is similar to the bird-dino issue.
O.k., my memory isn't COMPLETELY shot. Thrinaxodon, Cynognathus and Diademodon all had "costal plates". (Source I found since last posting: the Castorocauda article by Luo et al. in "Science" 2006, comparing C. to them.) I've never seen any very detailed hypothesis about their function. If, as you suggest with regard to Protuberum's protuberances, it is for armor, perhaps -- given the particular nature of the threat -- bumps were a lighter-weight but nearly as effective variant? Sort of replacing plate armor with mail? ... In any event, given that Diademodon and Protuberum are closely related (am I right about that?), it seems likely that whatever the function was, it was something bumps or plates could both serve!
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I am very glad Anwen's surgery was successful: scary business!
My initial comment seems to have gone missing: no loss, just saying there were other non-mammalian synapsids with very strange ribs and I couldn't remember details.
Allen,
My comment disappeared as well! Don't know what is going on. Anyway your idea about protuberum replacing the costal plates with tubercles doesn't work because Protuberum has costal plates as well. A bunch of NM cynodonts have costal plates but these are almost certainly not defensive as they are only developed in the lumbar region. NM cynodonts were at a cross-roads, with old-style costal inhalation giving way to diaphragmatic breathing (hence the strong division between thoracic and lumbar regions) and the abandonment of the the old sinous wriggling of the body during locomotion (like modern lizards still do)which was being replaced with stiffly held straight bodies and parasagittal limb movements. Perhaps during the early years extra bracing was required in the lumbar region which was later lost as mammals got better at their new ways of breathing and walking.
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