Friday, November 18, 2011

The Giant Turtle Therizinosaurus

Therizinosaurus, you look... unwell. Reconstruction by K. K. Fierova, from Maleyev (1954).

I am quite fond of old, weird reconstructions, and the initial classification of Therizinosaurus cheloniformis as a "turtle-like reptile"1 resulted in the magnificent specimen above. So how could the veritable Jabberwocky we're all familiar with be misinterpreted to such a colossal degree?

This odd phrasing is mirrored in the scientific name ("saurus" = lizard, "cheloniformis" = turtle-like). Malayev (1954) linked Therizinosaurus with members of Protostegidae and thus (probably) didn't intend to suggest another clade of reptiles which converged on turtles. Bizarrely, Rozhdestvensky (1974) claimed Malayev/Maleev classified Therizinosaurus as a "turtle-like pangolin"! Rozhdestvensky (1977) does not reiterate that statement, and further notes that another worker (Sukhanov) classified Therizinosaurus as a turtle; I unfortunately cannot find that source ("The subclass Testudinata" in Osnovy Paleontologii).

Therizinosaurus in its non-turtle form. From Wikipedia Commons.

Malayev (1954) described Therizinosaurus from scrappy remains: a metacarpal fragment, 3 manual unguals, and rib fragments (Zanno 2010). One of the ribs was an estimated 1.5 meters long when complete and was used to calculate a maximum body width of 3.25 meters (10'8") and body length of 4.5 m (14'9") (Malayev 1954); this is of course quite a bit larger than even the largest known Stupendemys geographicus. The rib was noted to lack costal elements, which is curious since turtle skeletons generally look like this:

 
Common Snapping Turtle (Chelydra serpentina) skeleton. Note the plastron is missing. From Wikipedia Commons.

Surprisingly, this is not necessarily a critical flaw, as (all?) turtles have distinct ribs during development before the carapace is fully formed (Wyneken 2001, fig. 90; Sánchez-Villagra 2009, figs. 3, 4). Malayev (1954) did not mention this nor the obvious possibility of a multi-ton hatchling. Instead, the "form of the ribs" was compared to Archelon and Protostega:

Archelon skeleton. From Wikipedia Commons.

The similarity is very general and Malayev (1954) does not list any specific shared characteristics. Due to the lack of costal elements, Malayev (1954) speculated that Therizinosaurus was in a distinct clade and in life had "barely developed or almost completely absent bony armor". It is incredibly strange that the Leatherback Seaturtle (Dermochelys coriacea) was not mentioned, as it entirely lacks costal elements and instead has thousands of dermal ossicles (Cebra-Thomas et al. 2005). The skeleton (sans ossicles) looks like an attempt by turtles to become "normal" tetrapods again.. until you notice the pectoral girdle within the ribcage:

From Wikipedia Commons

The rib material used to describe Therizinosaurus cheloniformis is apparently not from a therizinosaur at all, but a sauropodomorph (Zanno 2010 citing Rozhdestvensky 1970). Isn't it a major problem that the holotype is a chimera? Whatever the case, Therizinosaurus cheloniformis has been re-described a few more times and other rib material has been referred to the species (Zanno 2010). However, all of the diagnostic traits (and most of the material) are from the forelimbs (Zanno 2010).

From Wikipedia Commons.

Malayev (1954) interpreted the metacarpal and phalanges to be "powerful swimming organs" and suggested the huge claws were used for "cutting aquatic vegetation or for another functions, constrained by movement and acquiring food". The longest phalanyx was 60-65 cm long, not including the keratin covering (Malayev 1954), which suggests that the claws were ridiculously huge in life, even for a turtle-like reptile with a 4.5 meter body. I have observed turtles using their claws to climb and tear apart food (maybe what Malayev had in mind...), but clearly claws this disproportionate were doing something special. Something like this:



I like to think that Therizinosaurus, despite not being turtle-shaped anymore, waved its giant claws seductively in the faces of prospective mates.

References:

Cebra-Thomas, J., Tan, F., Sistla, S., Estes, E., Bender, G., Kim, C., Riccio, P., and Gilbert S. F. (2005). How the Turtle Forms its Shell: A Paracrine Hypothesis of Carapace Formation. Journal of Experimental Zoology 304B, 558-569. Available.

Maleyev, E. A. (1954). A new turtle-like reptile from Mongolia. Priroda 3, 106-108. Available.

Rozhdestvensky, A. K. (1977). The study of Dinosaurs in Asia. Journal of the Palaeontological Society of India 20, 102-119. Available.

Rozhdestvensky, A. K. (1974). History of the dinosaur fauna of Asia and other continents and questions concerning paleogeography. Transactions of the Joint Soviet–Mongolia Paleontological Expedition 1, 107–131. Available.

Rozhdestvensky, A. K. (1970). On the gigantic claws of mysterious Mesozoic reptiles. Palaeontological Journal 1, 131-141.

Sánchez-Villagra, M. R., Müller, H., Sheil, C. A., Scheyer, T. M., Nagashima, H., and Kuratani, S. (2009).  Skeletal Development in the Chinese Soft-Shelled Turtle Pelodiscus sinensis (Testudines: Trionychidae). Journal of Morphology 270, 1381-1399. Available.

Wyneken, J. (2001). The Anatomy of Sea Turtles. U.S. Dept Commerce NOAA Tech Mem NMFS SEFSC-470. Available.

Zanno, L. E. (2010). A taxonomic and phylogenetic re-evaluation of Therizinosauria (Dinosauria: Maniraptora). Journal of Systematic Palaeontology 8(4), 503-543. Draft Available.

1 comment:

rockylanding said...

The photo of the chelydra serpentina skeleton shows incorrect anatomy. The pelvis is not only backward, but rotated 90 degrees.