Restoring the manus of Lewisuchus

Earlier we looked at several hands (the manus) of basal bipedal crocs, some of which nest basal to dinosaurs. Today we’ll restore the manus of little Lewisuchus (Romer 1972, Fig. 1; note: Bittencourt et al. 2014 considered the digits proterochampsid pedal elements without restoring the pes according to their hypothesis). The picture tells the story.

Figure 1. Restoration of the manus of Lewisuchus based on patterns found in sister taxa.

Figure 1. Restoration of the manus of Lewisuchus based on patterns found in sister taxa. Click to enlarge.

Now, just to be clear,
if it’s difficult to determine whether these are pedal or manual elements, it’s also difficult to determine which metatarsal is which. Phylogenetic bracketing helps, but there is as much art here as science. Note that much of this data is based on line drawings. So, what I’m presenting here is a hypothesis. Not fact. Still, it’s good to see all this data together in one place.

It’s notable that
in Lewisuchus metacarpal 4 is half as long as metacarpals 2 and 3, and metacarpal 5 is a vestige. That leaves only metacarpal 1 absent. In sister taxa the fingers are often missing. And that pattern is what makes me consider this a manus.

note the apparent reduction of the metacarpals from Lewisuchus to Pseudheseperosuchus, Trialestes and Herrerasaurus. It’s also interesting to note that Lewisuchus had relatively longer fingers than did Junggarsuchus, but, let’s face it, the data is pretty crappy. Let’s all hope for some better future discoveries.

Still haven’t seen
the transitionally reduced proximal carpals between pseudhesperosuchids and basal dinos, but maybe we’ll find them close to PVL 4597, a taxon closer to dinosaurs, but only known from a pelvis and hind limb.

Bittencourt JS, Arcucci AB, Maricano CA and Langer MC 2014. Osteology of the Middle Triassic archosaur Lewisuchus admixtus Romer (Chañares Formation, Argentina) its inclusivity, and relationships amongst early dinosauromorphs. Journal of Systematic Palaeontology. Published online: 31 Mar 201. DOI:10.1080/14772019.2013.878758
Nesbitt SJ. et al. 2010. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature 464(7285):95-8
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna; XIV, Lewisuchusadmixtus, gen. et sp. nov., a further thecodont from the Chañares beds. Breviora 390:1-13



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