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.

References
Bonaparte JF 1969. Dos nuevos “faunas” de reptiles triásicos de Argentina. Gondwana Stratigraphy. Paris: UNESCO. pp. 283–306.
Clark JM et al. 2000. A new specimen of Hesperosuchus agilis from the Upper Triassic of New Mexico and the interrelationships of basal crocodylomorph archosaurs. Journal of Vertebrate Paleontology 20 (4): 683–704.
doi:10.1671/0272-4634(2000)020[0683:ANSOHA]2.0.CO;2.
Clark JM, Xu X, Forster CA and Wang Y 2004. A Middle Jurassic ‘sphenosuchian’ from China and the origin of the crocodilian skull. Nature 430:1021-1024.
Novas FE 1994. New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis (Theropoda: Herrerasauridae) from the Ischigualasto
Reig OA 1963. La presencia de dinosaurios saurisquios en los “Estratos de Ischigualasto” (Mesotriásico Superior) de las provincias de San Juan y La Rioja (República Argentina). Ameghiniana 3: 3-20.
Sereno PC and Novas FE 1993. The skull and neck of the basal theropod Herrerasaurusischigualastensis. Journal of Vertebrate Paleontology 13: 451-476. doi:10.1080/02724634.1994.10011525.
Sereno PC, Forster CA, Rogers RR and Moneta AM 1993. Primitive dinosaur skeleton form Argentina and the early evolution of the Dinosauria. Nature 361, 64-66.
Sereno PC, Martínez RN and Alcober OA 2013. Osteology of Eoraptor lunensis (Dinosauria, Sauropodomorpha). Basal sauropodomorphs and the vertebrate fossil record of the Ischigualasto Formation (Late Triassic: Carnian-Norian) of Argentina. Journal of Vertebrate Paleontology Memoir 12: 83-179 DOI:10.1080/02724634.2013.820113

wiki/Eoraptor
wiki/Herrerasaurus
wiki/Sanjuansaurus
wiki/Pseudhesperosuchus

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