The hot news on the block right now is the description of the new sauropod Abydosaurus mcintoshi, which, amazingly, is known from four more or less complete skulls (Chure et al. 2010).  This is unheard of — absolutely unprecedented.  There are few enough sauropods for which a skull is known at all; but four of them, all in decent nick, is breathtaking.

And here is one of them, the holotype:

Abydosaurus mcintoshi holotype skull DINO 16488, from Chure et al. (2010:fig. 3)

It’s a real shame that, presumably due to space limitations, this is the only one of the skulls that’s figured in the paper; but the good news is that some of the referred material is illustrated in the supplementary information, which — like the paper itself — is freely available, thanks to the wonder of open-access publishing.

According to the phylogenetic analysis in the paper, Abydosaurus is a brachiosaurid — it is recovered in all MPTs as the sister taxon to Chure et al.’s “Brachiosaurus” OTU (on which, see below).  Since it’s from the mid Cretaceous (Cenomanian-Albian, from the Mussentuchit Member of the Cedar Mountain Formation), it’s likely about 105 million years old, which means it lived the best part of 50 million years after the better known brachiosaurs Brachiosaurus and Giraffatitan.  It was evidently attracted by the Giraffatitan component of the compound OTU, since the skull and neck are effectively unknown in Brachiosaurus (see Taylor 2009 for a review of the holotype and referred material).  Because it lived in pretty much the same time and place as Sauroposeidon, there is the tantalising possibility that it is actually the skull of that animal; on the other hand, the four recovered skulls are all too small to fit the Sauroposeidon holotype, so unless they were all subadult, that appealing idea is probably wrong.

Unlike Giraffatitan — the only other brachiosaur with decent cranial material, so far as I recall — Abydosaurus has narrow teeth , superficially similar to those of diplodocids and titanosaurs.  Chure et al. show that this seems to be part of a general trend of sauropods evolving progressively narrower tooth crowns through time, perhaps because narrow teeth can be replaced more quickly and turnover rate is more important than robustness.

Abydosaurus mcintoshi, reconstruction of skull and anterior neck based on holotype and referred specimens (from Chure et al. 2010:fig. 4). Note your weekly helping of sauropod-vertebra goodness in the upper-right corner, in the form of a transverse slice though cervical 3 just behind the diapophyses.

One aspect of this paper particularly pleases me, and that is that the new species is named after John McIntosh.  For anyone out there who doesn’t know who McIntosh is, he’s been working on sauropods since forever: he’s produced a stream of important papers on the skulls of diplodocids, among many other things, and wrote the Sauropoda chapter in the original The Dinosauria (McIntosh 1990).  All of this in his spare time, mind you, because as his day-job he was a professor of theoretical physics at Yale and Princeton.  He’s probably seen more sauropod material than anyone else alive.  And on top of all that, he is one of the good guys.  I drew the long straw at the Austin SVP in 2007, and got to sit next to him at the informally convened sauropod-workers’ lunch, and it was a revelation to see his face light up as I tried to describe the weird morphology of the as-yet-unpublished vertebra that we now know as Xenoposeidon.  At an advanced age — I don’t know exactly how old he is, but you can get some idea from the fact that he flew over Hiroshima and Nagasaki less than a week after the bombs were dropped — his enthusiasm remains undimmed, and he is truly an inspiring example to every avocational palaeontologist.

So it’s sort of scandalous that it took so long before McIntosh got a sauropod of his own.  (Jensen did name Ultrasaurus after him, but as has been much discussed, that ended up synoymised with Supersaurus).  I know there’s at least one more new sauropod in the works that’s slated to be named after him, and I’m in favour.

Brooks Britt (a co-author on the paper) with one of the skulls of Abydosaurus. Stolen from Science Daily.

A note on brachiosaur taxonomy

I suppose I ought to mention this, only because if I don’t, everyone will just ask me about it.  Chure et al. (2010) refer to Giraffatitan by the old name “Brachiosaurusbrancai throughout, and explain why they do so on page 2:

Taylor (2009) recently suggested that the North American species Brachiosaurus altithorax is generically distinct from the African species Brachiosaurus brancai, which is known from abundant material including a complete skull and many craniodental elements. Based on numerous differences between overlapping parts of both holotypes, Taylor (2009) proposed that the African species should be known as Giraffatitan brancai. While we are open to this possibility, we do not believe that it is sufficiently justified at present because the identified differences have not been defended as separating genera, rather than species, populations, or individuals. The sister-taxon relationship between the two species recovered in the phylogenetic analysis performed by Taylor (2009) neither supports nor refutes their generic-level separation. At this point, we consider the decision to recognize the African species as a genus apart to be arbitrary. We choose to retain the original nomenclature in this contribution, distinguishing between the two species where appropriate.

I am sort of nonplused by this.  I’m certainly not saying that my 2009 paper is unassailable: as soon anyone comes along with evidence that Brachiosaurus and Giraffatitan should after all be considered congeneric, I’ll be first in line to hear them out.  But I do feel that now 26 osteological differences have been described between the species, the null hypothesis has shifted, and the burden of proof is now on those who wish to synonymise the genera.  “We choose to retain the original nomenclature” is not an argument, and doesn’t really advance understanding.  So I’m afraid I think this was a regrettable misstep.

Anyway — I don’t want to end on that note!  The big deal here is that we now have four fantastic new brachiosaur skulls, no doubt to be described in more detail hereafter, and John McIntosh has a beautiful sauropod named after him.  Happy days!

References