Tyrannosaurid Origins

 

The Bering Land Bridge is often the focus of attention by researchers looking at the origins of specific groups of dinosaurs that inhabited both Asia and North America. These include hadrosaurs, ceratopsids and, of course, tyrannosaurids but the actual processes that led to the varied faunal dispersals during the Cretaceous are poorly understood, complex and sometimes controversial.

It seems likely that during the Early Albian the two continents were still separated and this is supported by the dispersal patterns of both ammonites and belemnites which had cool water preferences suggesting that water from the Arctic was able to penetrate into the Boreal-Pacific basin (Iba & Tanabe 2007).

However, by the early-middle Late Albian, the land bridge has appeared since evidence for cooling water is absent and supplemented at this time by an increase and diversification in plants, such as ferns, cycads and some early conifers (Zakharov et al 2011). These are plants that thrive and require warmer conditions.

At about the same time, circa 100 million years ago, it is worth noting that hadrosaurids appear in the fossil record in both Asia and North America and it seems reasonable to assume that the first faunal exchanges took place at this time. The land bridge remained intact throughout the Cenomanian and the majority of the Turonian before disappearing again in the Coniacian and most of the Santonian before emerging yet again with the onset of the Campanian. There is conflicting evidence whether the land bridge remained in place throughout the Maastrichtian (eg Herman 2007 a, b; Krassilov 1981; Krassilov et al, 1990) although the recent increase and diversity in dinosaur fossils from Alaska suggests that it may have.    

I’ve previously referred to the origins of Tyrannosauridae and briefly discussed the different theories but it is one of those issues that have seemingly limitless permutations. I believe it is safe to say that the general consensus for many years is that Asia provided the likely launch pad for the clade but is it really as simple as that when basal tyrannosauroids are known, not only from Asia, but also from Europe, North America and, perhaps, even Australia (Benson et al 2010).

Asia does seem, on the face of it, to have a number of factors supporting its position as the launch pad for Tyrannosauridae. Certainly, tyrannosaurids in Asia often display primitive characteristics in relation to their American cousins and retain these basal traits throughout. Indeed, as I pointed out during my review of the Alioramus monograph, Tarbosaurus, in particular, displays several primitive characteristics that you would not expect to see in more derived American tyrannosaurines.

And yet, where the original tyrannosaurid ancestor originated from is still a matter of conjecture but there are more clues gradually being uncovered. A poorly preserved tyrannosauroid premaxillary tooth from the Aptian/Albian Cloverly Formation of North America (Zanno &Makovicky 2011) may be indicative of an endemic tyrannosauroid clade because the tooth lacks serrations. This is interesting because premaxillary teeth in all Asian tyrannosauroids predating Xiongguanlong (also Aptian/Albian)  lack serrations.

Since Late Jurassic tyrannosauroids are poorly represented in the fossil record by such animals as Stokesosaurus, and there are no premaxillary teeth known for these Morrison theropods, it is extremely difficult to make comparisons but it does preclude the possibility that characteristic unserrated premaxillary teeth may have originated in North America and spread to Asia via the land bridge.

 But I suggest that premaxillary teeth are not the best providers of morphological evidence when it comes to recognising genera and taxa. Aublysodon is the classic example of this and it is generally recognised that the unserrated premaxillary teeth that were attributed to this “taxon” are actually the juvenile teeth of other tyrannosaurids such as Daspletosaurus.

Of course, this kind of hypothesis poses massive problems – not least the lack of proper specimens.  In fact the whole interfaunal land exchange process suffers due to a combination of taphonomic distortion, poor preservation and simple straight forward sampling bias. Indeed, the fact that faunal exchange took place in the first place was almost certainly responsible for localised extinction and faunal turnover as disease was very likely spread between populations and animals from either continent likely to out compete those less evolved to survive. 

Appalachiosaurus

In addition to the origin of tyrannosaurids in western North America and Asia we also have the interesting, yet similar, situation regarding those on the other side on the Western Interior Seaway – the tyrannosaurs of Appalachia. Effectively cut off from the western radiation of Tyrannosauridae proper, these are represented by such animals as Dryptosaurus aquilunguis and Appalachiosaurus montgomeriensis and represent yet another grey area. The phylogenetic affinities of Appalachiosaurus and Dryptosaurus are still uncertain although it does seem likely that they are phylogentically closer to Tyrannosauridae as opposed to primitive Asian forms such as Xiongguanlong (but see about skull shape later). Appalachiosaurus is the older tyrannosauroid (Mid Campanian) whilst Dryptosaurus, despite its basal affinities, is Mid Maastrichtian.

 

The enigmatic Alectrosaurus

More basal tyrannosauroids are slowly coming to life. Averianov and Sues (2011) report on non-tyrannosaurid tyrannosauroid remains from the Turonian of Uzbekistan – some 10 million years older than Appalachiosaurus. Interestingly, and actually coeval with this new skeletal material, is the somewhat mysterious Alectrosaurus olseni from Mongolia. Despite its fragmentary nature, Alectrosaurus clearly demonstrates tyrannosauroid affinities that are similar to the Appalachian forms.

 

The fly in the ointment (or perhaps the missing cog in the wheel) may very well be the Cedar Mountain tyrannosauroid which is even older (Cenomanian). This animal is represented by a number of isolated incrassate maxillary and dentary teeth (Kirkland et al 1997) and has been used to support faunal interchange between Asia and North America. Without more skeletal remains, however, the Cedar Mountain form remains enigmatic.

The other significant tyrannosauroid of note is Bistahieversor sealeyi from the Upper Campanian of New Mexico but this deep snouted form is very tyrannosaurid-like, albeit more basal, and was obviously part of the diversification and proliferation of Tyrannosauridae throughout Laramidia. It is interesting to note, however, that the Appalachian forms, such as Appalachiosaurus, retained the more primitive shallower snout of Asian tyrannosauroids, such as Dilong, after the Western Interior Seaway inundated the continent (Carr & Williamson 2010).

Thus it appears we may have two distinct clades of tyrannosaurs, Tyrannosauridae in the west and a more basal clade in the east although this hypothesis is unsupportable at the moment – but it is rather intriguing (Brusatte et al 2011).

So for now, at least, it does seem likely that the ancestry for all North American tyrannosaurs is likely to be Asian but that there is still the distinct possibility that an immigrant from North America may have planted the seed. I suspect that since the two continents were connected for a much longer period of time, than was first suspected, that tyrannosaur faunal  interchange took place on a much more frequent basis. Certainly, the biggest radiation within Tyrannosauridae itself was within North America and there is a distinct possibility that different taxa crossed back over the bridge to establish the tyrannosaurid dynasty of Asia including animals such as Alioramus and Tarbosaurus.

References

Averianov, A., Sues, H.-D., Skeletal remains of Tyrannosauroidea (Dinosauria: Theropoda) from the Bissekty Formation (Upper Cretaceous: Turonian) of Uzbekistan, Cretaceous Research (2011), doi: 10.1016/j.cretres.2011.11.009

Benson, R.B.J., P.M. Barrett, T.H. Rich, and P. Vickers-Rich. 2010a. A southern tyrant reptile. Science 327: 1613.

Brusatte, S. L. and Benson, R. B. J. and Norell, M. A. (2011) The Anatomy of Dryptosaurus aquilunguis (Dinosauria: Theropoda) and a Review of its Tyrannosauroid Affinities. American Museum Novitates, 3717. pp. 1-53. ISSN 0003-0082

Carr, Thomas D. and Williamson, Thomas E. (2010) 'Bistahieversor sealeyi, gen. et sp. nov., a new tyrannosauroid from New Mexico and the origin of deep snouts in Tyrannosauroidea', Journal of Vertebrate Paleontology, 30: 1, 1 — 16

Herman, A.B., 2007a. Paleoclimatic effects of Late Cretaceous–Paleocene straits from data on terrestrial biota. In: Baraboshkin, E.Y. (Ed.), Prolivy Severnogo Polushariyav Melu i Paleogene (The Straits of the Northern Hemisphere during Cretaceous and Paleogene). Geologicheskij Fakultet Moskovskogo Gosudarstvennogo Universiteta, Moscow, pp. 119–136 (in Russian).

Herman, A.B., 2007b. Comparative palaeofloristics of Albian through the Early Paleocene of Anadyr-Koryak and Northern Alaska subregions. Paper 3. Comparison of floras and floristic changes at the Cretaceous–Paleogene boundary. Stratigrafiya. Geologicheskaya Korrelyatsiya 15 (5), 74–82 (in Russian).

Iba, Y., Tanabe, K., 2007. Albian ammonite paleobiogeography in the North Pacific. 7^th International Symposium: Cephalopods — Present & Past (Sept. 2007), pp. 98–99. Abstract Volume. Sapporo.

Kirkland, J.I., Britt, B., Burge, D.L., Carpenter, K., Cifelli, R., Decourten, F., Eaton, J., Hasiotis, S. and Lawton, T., 1997, Lower to Middle Cretaceous dinosaur faunas of the central Colorado plateau: a key to understanding 35 million years of tectonics, sedimentology, evolution and biogeography. BYU Geology Studies, 42, 69−103.

Krassilov, V.A., 1981. Changes of Mesozoic vegetation and the extinction of dinosaurs. Palaeogeography, Palaeoclimatology, Palaeoecology 34, 201–224.

Krassilov, V.A., Golovneva, L.B., Nesov, L.N., 1990. Cycadophyte from the Late Cretaceous dinosaurs locality in North Koryakia. In: Krassilov, V.A. (Ed.), Non-marine Cretaceous of the USSR. Dalnevostochnoye Otdeleniye Rossijskoy Akademii Nauk, Vladivostok, pp. 213–215 (in Russian).

Zakharov, Y. D., Shigeta, Y., Popov, A., et al., 2011. Cretaceous Climatic Oscillations in the Bering Area (Alaska and Koryak Upland): Isotopic and Palaeontological Evidence. Sedimentary Geology, 235(1–2): 122–131, doi:10.1016/j.sedgeo.2010.03.012

Zanno, Lindsay E. and Makovicky, Peter J. (2011) 'On the earliest record of Cretaceous Tyrannosauroids in western North America: implications for an Early Cretaceous Laurasian interchange event', Historical Biology, First published on: 24 February 2011 (iFirst) To link to this Article: DOI: 10.1080/08912963.2010.543952