Tuesday, 16 June 2009

Tyrannosaurids (By the Numbers)


In his excellent article "Oh no, not another Wealden theropod!", Darren Naish briefly describes the remains of MIWG 6350 as belonging to a basal tetanuran theropod. Although fragmentary and non-diagnostic at the taxa level, they are certainly good enough to confirm the presence of yet another big theropod from the Wessex Formation.

Quite rightly he points out that there are several examples of late Jurassic and early Cretaceous palaeo-faunas that have at least three big theropods. Generally, they are from different clades as well and, when you first take that into account, it appears that there was obviously some form of resource partitioning - but does it? And also, as Darren states, why was the late Maastrichtian apparently dominated by only the one big theropod?

Sometime it takes an article such as this to slap you around the face and you ask yourself "What on earth have you been thinking about?". For some time now, I've been looking at the tyrannosaurid fauna of the Dinosaur Park formation (DPF) and particularly the fact there were two big contemporary tyrannosaurids in the form of Gorgosaurus and Daspletosaurus. It appeared to be nice and simple at first. Gorgosaurus appears to be a quick, agile, lightly built albertosaur ideal for tackling hadrosaurs. Daspletosaurus, a much more robust and powerful tyrannosaur, was obviously designed to take on the much tougher ceratopsians. Resource partitioning at its best, right? Maybe not.

Suddenly it appeared that there could be multiple species of tyrannosaurids resident in the DPF alone. Certainly there appears to be another species of Daspletosaurus which (hopefully) will eventually be described by Currie et al. There may be another tyrannosaur in the form of Aublysodon, known from D-shaped non-serrated teeth and (possibly) very fragmentary skeletal remains, but this seems unlikely now since Currie (2005) attributes these teeth to being juvenile premaxillary teeth from the previously mentioned Daspletosaurus Sp.


So, Aublysodon aside, we appear to have, again, three big theropods in the DPF. The only marked difference is the fact that these animals all belong to the one clade, the Tyrannosauridae. So where am I all leading with this? Bear with me and I'll explain my thoughts.

There is no doubt that tyrannosaurids suffer from being over-split at the generic level. However, when you look into tyrannosaurid remains throughout the Campanian and into the Maastrichtian, there is, for me, pretty good fossil evidence (albeit sometimes scrappy), that there are multiple tyrannosaurs present throughout the Upper Cretaceous (for the sake of argument I will use the term "multiple" to describe formations where there are at least two big tyrannosaurids present).

And yet, or so it appears, the Hell Creek formation (HCF) has only the one big predator in the form of Tyrannosaurus. And yet this animal may have a greater stratigraphic range than was initially realised for there is certainly circumstantial evidence that this may be the case and, if not, the evidence again proves the existance of multiple tyrannosaurids right through the Cretaceous until the uppermost Maastrichtian.

By way of examples then. There are elements of a big tyrannosaur from the De-na-zin member of the Kirtland formation as well as the underlying Fruitland formation. Urban and Lamanna (2007) reported on the possible existance of a rex-sized tyrannosaurid in the latest Campanian of the Judith River formation in Montana based on an isolated lacrimal. Although, at the time, it was tentatively referred to Tyrannosaurus, further comparative studies have now rendered this specimen as indeterminate. But what it does show is that the Judith River fauna also had multiple big tyrannosaurs.

There are also large tyrannosaurs known from the Naashoibito member of the Ojo Alamo formation in New Mexico, known mainly from teeth and postcranial skeletal elements. Also from New Mexico, there is material from the McCrae formation. And the same again - large tyrannosaurids are known from both the Javelina and North Horn formations. Interestingly now, the Javelina is dated at being around 69 million years old so this is right on the verge of the "official" T.rex timeline.

This being the case then, why does it appear that there is only the one large theropod in the Maastrichtian of the HCF? Well there are a few theories about this and, to be fair, each of them have a little merit. Firstly the non -predatory fauna of the HCF includes hadrosaurs, pachycephalosaurs, ankylosaurs and ceratopsians and this is a lot prey animals for one single species of tyrannosaur to "control", especially considering all the previously mentioned formations where multiple large theropods are present.

This may suggest an age specific partitioning of Tyrannosaurus to enable an efficient hunting strategy. Alternatively there may be other large theropods which are as yet unknown but this seems unlikely since Hell Creek is one of the most sampled formations in the world and it appears that the collected biomass is close to the actual biomass.

There were other predators present. Theropods such as dromaeosaurs, troodontids, ornithomimosaurs and oviraptorosaurs have all been found in the HCF but these are small animals and could hardly be classed as even mid-sized theropods. Even if the dromaeosaurs and troodontids hunted in packs, and could bring down fair sized prey, there would still be a vast amount of prey animals for adult tyrannosaurs and their juvenile forms to predate on.

This brings us nicely to Nanotyrannus. Currently, it would appear that the general concensus regarding the taxanomic status of this animal is that it is a juvenile Tyrannosaurus. But this is not really an issue since, as mentioned earlier, the probable over-splitting of tyrannosaurids means that these theropods are so closely related to one another as to make no difference. Of course this may mean that sampling is still an issue or, on the other hand, may confirm that resource partitioning by age is relevant.

Maybe it wasn't necessary to have two or three big theropods in the HCF. Perhaps juvenile and half grown Tyrannosaurus were the mid-sized theropods of this time. Of course others may point out that, if this was the case, why was it necessary to have multiple tyrannosaurids in the Campanian? And also, and this particularly frustrates, why are the remains of juvenile tyrannosaurids/Nanotyrannus so rare in relation to adults? Even a half grown rex could be twenty feet long and you would think that there would more found of them than there ever has been.

So where does that leave us? Darrens' question at the beginning remains valid and there appears no definitive answer. Despite my earlier protestation that Hell Creek is extremely well sampled, it is obvious that more sampling will need to be done and, maybe then, a combination of this and a review of tyrannosaurid systematics will provide an answer. Until then, this particular issue will rumble on and on. Don't you just love palaeontology?


References

Currie, P.J. 2005. Theropods, including birds; pp. 384-388 in P.J. Currie and E.B. Koppelhus (eds), Dinosaur Provincial Park, a Spectacular Ancient Ecosystem Revealed. Indiana University Press, Bloomington, Indiana.

Urban, M.A., and Lamanna, M.C. 2007. Evidence of a giant tyrannosaurid (Dinosauria: Theropoda) from the Upper Cretaceous (?Campanian) of Montana. Annals of the Carnegie Museum 75(4):231-235.

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