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The 'prosauropods' are one group of dinosaurs that seemingly don't get no respect. While most other groups have their swarms of enthusiasts, there are relatively few inclined to shout their enthusiasm for non-sauropod sauropodomorphs from the roof-tops. Pop culture has a tendency to gloss them over: in the 1990s TV series Walking with Dinosaurs, for instance, their appearance was limited to a brief cameo at the end of the first episode. Despite this, they are perhaps the most 'dinosaur-y' of all dinosaurs, if comparisons with generic 'dinosaur' depictions are to be made.
The name 'Anchisauria' was introduced by Galton & Upchurch (2004) for the most exclusive clade uniting the genera Anchisaurus and Melanorosaurus. Galton & Upchurch were working under the framework that prosauropods formed a monophyletic sister group to the sauropods, but subsequent phylogenetic analyses have placed sauropods close to Melanorosaurus and hence within Anchisauria (Yates 2010; Yates et al. 2010; Pol et al. 2011). The name 'Anchisauria' can be translated as 'near lizards', but they are more properly near sauropods. Still, because this is to be a prosauropod-centred post, I will ignore the sauropods from this point on unless they insist on pushing their way in (presumably not a difficult task for a sauropod).
The two anchoring genera remain the most consistent non-sauropod members of the clade. The South American Riojasaurus, placed within Melanorosauridae by Galton & Upchurch (2004), has subsequently been placed outside Anchisauria. The Argentinian Lessemsaurus was also treated by those authors as a melanorosaurid, but may be a basal sauropod proper, while the status of the English Camelotia needs more work (Pol et al. 2011 were unable to resolve its position between Anchisauria and its close relatives). The Chinese Yunnanosaurus was placed within Anchisauria by Yates (2010), but other analyses have disagreed. Two recent genera, Aardonyx Yates et al. 2010 and Leonerasaurus Pol et al. 2011 are currently regarded as anchisaurians.
Anchisaurus polyzelus, from the early Jurassic of Connecticut, reached about four metres in length and is represented by the remains of a number of individuals. Some of these have been described as separate species such as Ammosaurus major and Yaleosaurus colurus, but Yates (2010) regarded them as representing a single species. This makes the '2.5 m' estimate of length given for this species by Galton & Upchurch (2004) too small, as based on a potential juvenile. Nevertheless, it was evidently such a good number that the fossil record apparently decided not to let it pass: the Argentinian anchisaur Leonerasaurus taquetrensis is about that size. The South African Aardonyx celestae was probably comparable to size to Anchisaurus* [Update: Spectacular reading fail on my part. A. celestae was about twice the size of Anchisaurus. See comments below].
*Actually, the scale bar given for the skeletal reconstruction of A. celestae by Yates et al. (2010) would seem to indicate that is must have been the smallest sauropodomorph ever. One can only assume that its size was meant to indicate 500 mm, not '500 µm' [Update: Ignore this. I am a twit. See comments below].
Melanorosaurus readi was quite a bit larger, close to eight metres, and phylogenetic analyses have accordingly placed it as the closest relative to sauropods. Interestingly, M. readi was nevertheless quite a bit earlier than the other non-sauropod anchisaurs, being late Triassic rather than early Jurassic, and the smaller anchisaurs evidently survived the evolution of their larger cousins by some time. As well as its larger size, M. readi resembled sauropods in being an obligate quadruped. The other anchisaurs retained their plesiomorphic bipedality; the forelimbs of Aardonyx indicate that it was probably unable to adopt a comfortably quadrupedal stance (being unable to pronate its hands to a great degree, it would have had to rest them on their sides if it tried to do so). Pol et al. (2011) placed Leonerasaurus closer to the sauropods and Melanorosaurus than either Anchisaurus or Aardonyx, but the distal part of its forelimbs are unfortunately unknown.
REFERENCES
Galton, P. M., & P. Upchurch. 2004. Prosauropoda. In: Weishampel, D. B., P. Dodson & H. Osmólska (eds) The Dinosauria, 2nd ed., pp. 232-258. University of California Press.
Pol, D., A. Garrido & I. A. Cerda. 2011. A new sauropodomorph dinosaur from the Early Jurassic of Patagonia and the origin and evolution of the sauropod-type sacrum. PLoS One 6 (1): e14572.
Yates, A. M. 2010. A revision of the problematic sauropodomorph dinosaurs from Manchester, Connecticut and the status of Anchisaurus Marsh. Palaeontology 53 (4): 739-752.
Yates, A. M., M. F. Bonnan, J. Neveling, A. Chinsamy & M. G. Blackbeard. 2010. A new transitional sauropodomorph dinosaur from the Early Jurassic of South Africa and the evolution of sauropod feeding and quadrupedalism. Proceedings of the Royal Society of London Series B—Biological Sciences 277: 787-794.
*Actually, the scale bar given for the skeletal reconstruction of A. celestae by Yates et al. (2010) would seem to indicate that is must have been the smallest sauropodomorph ever. One can only assume that its size was meant to indicate 500 mm, not '500 µm'.
ReplyDeleteSir, "500 µm" refers to figure 2e (slice of bone), not 2d (life restoration), where the scale is "1 m". That means Aardonyx was about 9 m in length. Moreover, the studied specimens were still growing at the time of death (Yates et al. 2010).
I fail reading comprehension forever!
ReplyDeleteThank you for pointing that out, too. The post has been edited accordingly.
ReplyDelete