The origin of dinosaur hands

Dinosaurs have a variety of hands.
Everyone knows T-rex had but two fingers. Birds have fused fingers derived from fingers 1-3 that are not fused in the basal bird, Archaeopteryx. Basal sauropodomorphs had five fingers, but later giants reduced this to a U-shaped column of metacarpals.

Figure 1. the manus and carpus of several basal bipedal crocs and basal dinosaurs showing the similarities and differences. Click to enlarge.

Figure 1. the manus and carpus of several basal bipedal crocs and basal dinosaurs showing the similarities and differences. Click to enlarge.

The currently known basalmost dinosaur
Herrrerasaurus, had five metacarpals, but digit 4 was a vestige and digit 5 was absent on a vestigial metacarpal (Fig. 1).

Proximal outgroups to the Dinosauria
among basal bipedal crocs appear to have had four digits and a vestigial metacarpal 5 (Fig. 1), but this may be due to poor preservation and excavation more than in vivo reality.

Among other basal crocs,
digits 1-4 are sometimes known, sometimes partly known (distal phalanges tend to disappear first) and sometimes not known. In all basal crocs, metacarpals 1-3 appear to increase in length laterally and are aligned distally.

The carpus in bipedal basal crocs
includes an elongate ulnare and an even longer radiale (both are proximal wrist bones). No dinosaurs have elongate proximal wrist bones, a key difference. But proto-dinosaurs, like Junggarsuchus and Trialestes, did.

A transitional croc/dino taxon with slightly longer proximal carpals has not been identified.

Like dinosaurs,
most crocs reduce metacarpals 4 and 5, both in length and diameter. But there are certain exceptions (Fig. 1), including Terrestrisuchus and the SMNS 12352 specimen. Junggarsuchus may also have a reduced digit 5, as it appears from the photo, distinct from the Clark et al. 2004 interpretation (Fig. 1) which includes a large metacarpal 5 and lacks a metacarpal 1, distinct from all known sister taxa.

Compared to dinos
the basal bipedal crocs appear to have shorter distal phalanges and smaller hands, as if they were more often in contact with the ground and not used to snare prey.

By contrast, in basal dinos
like Herrerasaurus and Tawa the distal phalanges were trenchant claws and the penultimate phalanges were likewise elongated. Among dino and croc ancestors only tiny Lewisuchus has such long dangerous claws and fingers, perhaps developed by convergence.

The hands of more ancient rauisuchians
are not often found and what is known suggests the hands were small with short distal phalanges. The hands of Eoraptor have fingers longer than metacarpals and relatively longer lateral metacarpals, but the fingers are not so long as in therpods. In Massospondylus there’s a combination of short fingers and large trenchant claws, probably for grabbing tree trunks for feeding high in the boughs.

The difference in manual claw size between basal dinos and basal bipedal crocs may be one of the key differences that enabled dinosaurs to rise to greatness in the Mesozoic.

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