General Systematics of the Theropoda

On the tree of the archosaurs, theropods are resolved as the sister-group to the Sauropodomorpha, the clade containing the giant herbivores. This post is a requested general run through their systematics.

The tree above shows the relationships as mentioned in the text. The branch lengths have no meaning, they were drawn to convenience (I made the tree by hand in Inkscape, it’s not the result of any analysis).

First, we need to put the theropods in their historical context. The rise of the theropods was not due to a competitive advantage – there is little evidence for a rapid “arms race” that occurred, with the carnivorous adaptations of the theropods coming slowly. Instead, the key to their rise was a mass extinction that killed off the dominant organisms of the time, the Crurotarsi, and the dinosaurs simply replaced them.

By 230 Ma, the theropods had begun their radiation, with the currently-known basalmost theropod being Herrerasaurus (Novas, 1994) found in Argentinian deposits of that age, although its position as a basal theropod isn’t universally accepted. If you need a safe name of a basal theropod, Eoraptor is a good choice, and we’re in luck, since it’s found in the very same formation, the Ischigualasto Fm. (Sereno et al., 1993).

Those two are sister to the “main” theropods, the Neotheropoda, a clade consisting of the Ceratosauria and the Tetanurae, both of which had originated by the Late Triassic. The former were initally dominant, radiating into the ceratosauroids (e.g. Majungasaurus) and coelophysoids (e.g. Dilophosaurus), but that was short-lived, and by the middle of the Jurassic period, the Tetanurae had eclipsed them.

The Tetranurae were the large theropods we all know of, the ones with the giant mouth and the tiny forearms (exemplified by the last panel above). The Megalosauridae were the earliest ones (e.g. Torvosaurus), and the Tetanurae soon radiated into the Allosauridae and Coelurosauria, the latter of which spread fairly quickly over the globe so that by the early Cretaceous, they were distributed globally; this was partly helped by the fact that they could fly (birds are coelurosaurs).

The Allosauridae include such dinosaurs as Allosaurus and Sinraptor, and they were highly-successful, although it is true that the coelurosaurs are the only ones still surviving today. Historical contingency’s a bitch. The coelurosaurs include Ornitholestes at the base, and four clades: the Therizinosauridae, the Ornithomimidae, the Tyrannosauridae, and the Maniraptora; the latter two clade together (Turner et al., 2007). Another family, its position not known for certain yet, is the Alvarezsauridae, a family characterised by highly-specialised forelimbs thought to be adapted to digging into soil, as modern anteaters do; however, given the shortness of the limbs and bipedalism, they probably could only dig through termite towers as diagrammed above (Senter, 2005).

The Maniraptora are of special interest, as they contain the birds, as well as the oviraptorosaurs and the deinonychosaurs, the latter of which is the sister group to the birds. It’s comprised of two families: the slender Troodontidae and the successful Dromaeosauridae, the latter of which includes the smallest-known non-avian dinosaur, Microraptor zhaoianus (Xu et al., 2000).

The oviraptors are well-known for their exhibition of parental care, as evidenced by findings of adult skeletons crouching over their own eggs in specially-built nests, as in brooding (e.g. Clark et al., 1999). I’m not sure of the extent of such behaviour among other non-avian dinosaurs.

As of the end of the 20th century, thanks to the discovery of all the “feathered dinosaurs” from China, we know that birds are theropods, although the idea of birds was first presented by Huxley (1868), on the basis of Compsognathus (found in Solnhofen, the same place as Archaeopteryx), going against the other idea of the time that birds descended from ornithopods. However, both of these ideas were eclipsed by the monumental Heilmann (1926), who concluded that the similarities between theroods and birds are convergent, or else Dollo’s Law would be broken (the assumption was that dinosaurs had lost their clavicles, and so its reappearance in birds is a violation).

It wasn’t until Ostrom (1976) that Heilmann’s ideas were challenged. Ostrom (1969) described Deinonychus, and his 1976 study identified many apomorphies shared by it and Archaeopteryx (such as the crescent-shaped wrist and the three-fingered hand), and so blowing the debate about the origin of birds wide open again, and by now, there is so much evidence both from the fossil record and from systematics, that it is almost universally accepted as fact that birds are theropods (Weischampel et al., 2004). I illustrated the major lines of evidence in this post.

The traditional place of Archaeopteryx as the basalmost bird has been called into question by a recent analysis that recovers it in a clade together with a new fossil called Xiaotingia as sister to the birds (Xu et al., 2011), however the same dataset examined using a different methodology recovers Archaeopteryx at the base of the birds again (Lee & Worthy, in press), so its position is still up in the air; this uncertainty could have implications for how we consider the early evolution of birds. What can be said for sure is that at least some early birds could be pretty large, as exemplified by the Late Cretaceous Kazakh Samrukia nessovi, reconstructed above in flight and flightless form (Naish et al., 2012) – it’s only known from its jaw, so its locomotion style can’t be confirmed.

The key predatory adaptations of theropods include the midmandibular joint, which is what gives them the very flexible bite. Their three-fingered hand was pretty powerful, allowing them to grasp their prey. They were bipedal, and could evidently run pretty fast with great balance enabled by their long, stiff tail; another adaptation for speed was the pneumatic diverticulae of their bones, making them more hollow and thus lighter.

References:

Clark JM, Norell M & Chiappe LM. 1999. An oviraptorid skeleton from the late Cretaceous of Ukhaa Tolgod, Mongolia, preserved in an avianlike brooding position over an oviraptorid nest. American Museum Novitates 3265, 1-36.

Heilmann G. 1926. The Origin of Birds.

Huxley TH. 1868. On the animals which are most nearly intermediate between birds and reptiles. Geological Magazine 5, 357-365.

Lee MSY & Worthy TH. in press. Likelihood reinstates Archaeopteryx as a primitive bird. Biology Letters.

Naish D, Dyke G, Cau A, Escuillié F & Godefroit P. 2012. A gigantic bird from the Upper Cretaceous of Central Asia. Biology Letters 8, 97-100.

Novas FE. 1994. New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis (Theropoda: Herrerasauridae) from the Ischigualasto Formation (Upper Triassic) of Argentina. Journal of Vertebrate Palaeontology 13, 400-423.

Ostrom JH. 1969. Osteology of Deinonychus antirrhopus, an unusual theropod from the Lower Cretaceous of Montana. Peabody Museum of Natural History Bulletin 30, 1-163.

Ostrom JH. 1976. Archaeopteryx and the origin of birds. Biological Journal of the Linnean Society 8, 91-182.

Senter P. 2005. Function in the stunted forelimbs of Mononykus olecranus (Theropoda), a dinosaurian anteater. Paleobiology 31, 373-381.

Sereno PC, Forster CA, Rogers RR & Monetta AM. 1993. Primitive dinosaur skeleton from Argentina and the early evolution of Dinosauria. Nature 361, 64-66.

Turner AH, Pol D, Clarke JA, Erickson GM & Norell MA. 2007. A Basal Dromaeosaurid and Size Evolution Preceding Avian Flight. Science 317, 1378-1381.

Weischampel DB, Dodson P & Osmolska H (eds.). 2004. The Dinosauria.

Xu X, Zhou Z & Wang X. 2000. The smallest known non-avian theropod dinosaur. Nature 408, 705-708.

Xu X, You H, Du K & Han F. 2011. An Archaeopteryx-like theropod from China and the origin of Avialae. Nature 475, 465-470.

Footnote: I hope this post makes up for the lack of Top Papers of the Week – I had a file screw-up and my master list of papers was lost, and I can’t be fucked to do it all again. Sorry.

 

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