The Aussie Tyrant – Additional Remarks

In my last post, I talked about the possibility of an Australian tyrannosauroid [1]. I even touched on one of the issues that often plagues zoology, and that’s biogeography. This philosophy, often with good reason, tends to hold that species or identifications can be made on the basis of where they are found, or that identifications can be excluded. Usually due to the fact that a particular group or taxon is present, or that there are no close relatives found nearby, one may demonstrate a likelihood between 0-100% of saying “Taxon X is found here.”

In many cases, zoology has it right: We can argue that with lemurs found exclusively on Madagaskar, they are biogeographically contained therein; thus, primates found on Madagaskar are more likely to be lemurs than “anything else.” While this does not exclude “anything else,” it means you should take claims of “anything else” with a grain of salt, and attempt to demonstrate this identity. Fossil lemurs are found in Africa proper, and their biogeography is noted in the paleontological realm, thus linking the two, although previous claims of when the lineages crossed the intervening strait (Mozambique Channel) have greatly shortened to when the two landmasses had fully separated, as no fossils were found to link the two when they were conjoined. Now, this data shows they were capable of a sort of one-way crossover from mainland Africa to Madagaskar, and includes mroe than just lemurs, but various other Malagasy fauna [2].

But paleobiogeography is more than just extending biogeography into the fossil record; unlike the current models, we have less data available, and must make certain extensions of our premises, and much of that in turn is informed by phylogeny itself, rather than just reconstruction of general appearance records of known or closely-related taxa. This becomes much more difficult when the material, rather unlike mammalian teeth, is very sparse.

So take the Aussie “tyrant:” Herne et al. make a marked claim about the biogeography issue, based almost solely on their interpretation of the material’s location and the dearth of definitive features for which to ascribe to Tyrannosauroidea [3:1310c pg.2]:

Despite more than 100 years of collecting, there is no record of tyrannosauroids on any of the other southern continents.

And this is true. Two of the authors were involved on research, along with the lead author on the original paper ( see [1]) in which they describe taxa that are almost certainly self-contained within Gondwana. The likelihood of a Laurasian invasion or Gondwanan origin for tyrannosauroids is weak, which means that such extraordinary claims must derive from extraordinary evidence. Does the pubis discussed by these two groups qualify?

Yet, Benson et al. reply, making a sharp series of points [4:1310d]:

[B]ecause a long history of collecting has not uncovered any Gondwanan tyrannosauroids, their presence is unlikely. There are three problems with this claim. First, any hypothesis of tyrannosauroid absence should be based on evidence, not expectation. We have made the case, here and previously, that the anatomical evidence for an Australian tyrannosauroid is clear. Second, the Gondwanan dinosaur record is poorly sampled in comparison with that from Laurasia. New discoveries and more critical paleobiogeographic analyses have shown that the hypothesis of distinct “Laurasian” and “Gondwanan” dinosaur faunas during the Early Cretaceous is too simplistic.Many “endemic” clades are now known from both hemispheres [e.g., “Gondwanan” abelisauroids in Europe and “Laurasian” dromaeosaurids in South America]. Third, of the Australian theropod clades cited by Herne et al., only the carcharodontosaurian Australovenator is represented by substantially complete remains. These are of similar age to NMV P186046 and indicate that it is the sister taxon of Fukuiraptor from Japan. This demonstrates that closely related theropod taxa can be present in both Laurasian and Gondwanan faunas during the middle Cretaceous. Thus, the presence of Australian tyrannosauroids is not implausible and should not be rejected a priori. [JAH:Citations ommited]

What is the substantive margin of completeness which we can use to assess likelihood for biogeography? The first of these is not just phylogenetic; as Andrea Cau remarked in the reply to my last post, the pubis comes out as a tyrannosauroid in his analysis (although I do not have the data) agreeing with Benson et al. [1] in part, but differing in a more basal placement (which agrees more in line with the Early Cretaceous taxa which it is coeval with. We must, in a desire for total data, also look at continental reconstruction: The last time Australia was in any way contiguous with any Laurasian continent was during the Jurassic, when through South America, Africa, and then to Eurasia, there was a marginal access northward. Despite this, collection in the intervening regions has not yielded ample material of a Laurasian nature invading South America, although the similarities between the North American Morrison Formation, the Portuguese Lourinhã Formation, and the Tanzanian Tendaguru Formation implies a transition during the Kimmeridgian and Tithonian of the Late Jurassic.

However, this occurs after the separation of Antarctica/Australia from the rest of Gondwana, and this occurs when only a few “peons” among Tyrannosauroidea were about (such as Dilong paradoxus [5]). None of these forms were advanced Tyrannosauridae, which contends with the argument made by Benson et al., averring a nearly or exceeding 80-my gap between the breakup of Gondwana and the appearance of Tyrannosauridae in the Campanian. And while Australovenator wintonensis [6] corresponds to an allosauroid form, the relationship of it to Fukuiraptor kitadaniensis [7] is in doubt as it was recoved as a basal, carcharodontosaurid in Hocknull et al. [6], but Fukuiraptor kitadaniensis was not, while Benson et al. [8] recovered as a carcharodontosaurid.

I would like to note that I do not disagree with either Herne et al. or Benson et al., but that one makes a compelling argument in favor of lack of reasonable anatomical detail, while the other makes the claim of essentially “What else could it be?” In the latter case, if the anatomy fits, the phylogeny must fit (pardon the reference), and everything else must follow. We can agree to both of these statements, while not discounting that either the pubis MUST be a tyurannosauroid, or it must not be. I am heavily inclined to doubt this identification, but based solely on biogeographical grounds, as it invokes either a deep, very deep radiation for the Tyrannosauridae, sensu Benson et al., or it shows massive convergences toward this condition, and that condition is potentially doubtful.

[1] Benson, R. B. J., Barrett, P. M., Rich, T. H. & Vickers-Rich, P. 2010. A southern tyrant reptile. Science 327:1613.
[2] Ali, J. R. & Huber, M. 2010. Mammalian biodiversity on Madagascar controlled by ocean currents. Nature 463:653-656.
[3] Herne, M. C., Nair, J. P. & Salisbury, S. W. 2010. Comment on “A southern tyrant reptile.” Science 329:1013c.
[4] Benson, R. B. J., Barrett, P. M., Rich, T. H., Vickers-Rich, P., Pickering, P. & Holland, T. 2010. Response to comment on “A southern tyrant reptile.” Science 329:1013d.
[5] Xu X., Norell, M. A., Kuang X., Wang X.-l, Zhao Q. & Jia C. 2004. Basal tyrannosauroids from China and evidence for protofeathers in tyrannosauroids. Nature 431:680-684.
[6] Hocknull, S. A., White, M. A., Tischler, T. R., Cook, A. G., Calleja, N. D., Sloan, T. & Elliott, D. A. 2009. New mid-Cretaceous (latest Albian) dinosaurs from Winton, Queensland, Australia. PLoS ONE 4(7):e6190.
[7] Azuma Y. & Currie, P. J. 2000. A new carnosaur (Dinosauria: Theropoda) from the Lower Cretaceous of Japan. Canadian Journal of Earth Sciences — Revue canadienne de sciences de la Tierre 37(3):1735-1753.
[8] Benson, R. B. J., Carrano, M. T & Brusatte, S. L. 2010. A new clade of archaic large-bodied predatory dinosaurs (Theropoda: Allosauroidea) that survived to the latest Mesozoic. Naturwissenschaften 97(1):71-78.

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