Updated April 8, 2020
with more taxa in the clade Crocodylomorpha (Fig. 1).
The lumbering quadrupedal alligators and crocodiles we know today evolved from bipedal sprinters in the Triassic. Some were large. Others were tiny (Fig. 1). All lived during the Middle and Late Triassic alongside the first bipedal dinosaurs and their other cousins, the rauisuchids. A few of these, such as Smok and Postosuchus, also experimented with a bipedal stance.
True “crocs” are not to be confused with poposaurs (popsauroids), such as Poposaurus and Effigia, which are widely considered to be crocodylians or rauisuchids. In the heretical view, supported by a large analysis (Fig. 2), poposaurs were dinosaurs with ankle issues, as we learned earlier.
An Origin from Basal Rauisuchids
The first clade (Fig. 2) leading to basal crocodylomorphs includes Decuriasuchus, Lewisuchus and Pseudhesperosuchus. At present these three constitute the base of the Archosauria (defined as the most recent common ancestor of living crocs and birds and all of its descendants).
Decuriasuchus (França, Ferigolo and Langer 2011) was originally nested with Prestosuchus, but the tree was poorly resolved and neither Vjushkovia, Pseudhesperosuchus nor Lewisuchus was included. Here Decuriasuchus was derived from a sister of Vjushkovia, a quadrupedal basal rauisuchid. Decuriasuchus was, at best, an occasional biped based on its elongated torso, short legs and small feet.
Tiny Calcaneal Tuber
On Decuriasuchus, Lewisuchus and Pseudhesperosuchus there was no large calcaneal “heel.” In this way they were similar to theropods, sauropomorphs and ornithischians by convergence. Large calcaneal tubers are found in phytosaurs, rauisuchians, derived crocodylomorphs, Turfanosuchus and poposaurids, most by convergence and all with distinctive and unique shapes. (That should be a good topic for a future blog!)
Lewisuchus and Pseudhesperosuchus
Lewisuchus (Romer 1972) and Pseudhesperosuchus (Bonaparte 1969) nested as sisters, but it was likely that Lewisuchus was the first of the two to develop a bipedal stance based only on its size. If so, this pattern would follow the size reduction preceding major morphological changes in basal mammals, derived pterosaurs, etc. In either case, for its size, Lewisuchus would also have led more gradually into all the other little basal crocodylomorphs to follow (Fig. 1) as well as basal dinosaurs of which we know very little at present (Lagerpeton not withstanding).
Lewisuchus and Pseudhesperosuchus are important to our understanding of basal archosaurs because they do not have an anterior-leaning quadrate, a key trait of all crocodylomorphs closer to Gracilisuchus. Pseudhesperosuchus had an elongated radiale and ulnare, a typical crocodylomorph trait retained by Trialestes, a dinosaur predecessor.
Gracilisuchus (Romer 1972) was originally considered an “ornithosuchid pseudosuchian,” which means it was hard to nest. At one time it was considered a basal dinosaur. Brusatte et al. (2010) nested it between aetosaurs and Revueltosaurus. Here it nests as THE basal crocodylomorph. Nesbitt (2011) reported, “The forelimb assigned to Gracilisuchus by Romer (1972c) is too small for the size of the holotype.” Here it appears to be the correct size because all sister taxa have even smaller forelimbs. Nesbitt (2011) nested Gracilisuchus with Turfanosuchus. They remain sisters in the present study, but Turfanosuchus does not have a quadrate leaning toward the postorbital medial to the quadratojgual and the squamosal does not produce such a distinct shelf.
Scleromochlus and Saltopus
Scleromochlus (Woodward 1907) has been studied and analyzed for over a century, but rarely with other bipedal crocs. Unfortunately this has led to a long standing problem that has been feeding on itself for several decades. Padian (1984), Sereno (1991), Bennett (1996), Benton (1999) Senter (2003) Hone and Benton (2008) all nested Scleromochlus with pterosaurs! This blog dismissed that earlier based on Peters (2002) and the large reptile tree. Basically Scleromochlus was a smaller sister to Gracilisuchus, nothing more. Saltopus was another Gracilisuchus sister with longer legs and a more gracile vertebral column.
Terrestrisuchus, Saltoposuchus, Dromicosuchus, Hesperosuchus and Pedeticosaurus
The last five bipedal crocs were broadly similar to the others and phylogentically preceded quadrupedal crocs, such as Protosuchus. While some workers and artists try to force Saltoposuchus, Dromicosuchus, Hesperosuchus and Pedeticosaurus into quadrupedal poses, to do so produces awkward reconstructions with necks oriented downward and skulls tipped too far up. None of these had the elongated coracoid of living crocs.
The Advantages of Bipedalism
Wikipedia provides a good introduction to the advantages of a bipedal configuration here. Principally a bipedal configuration raises the head to see beyond short obstructions. It permits both sides of the lungs to expand while running and it permits the hands to do something else. Unfortunately, crocs did not take advantage of this last opportunity because their hands remained small and unspecialized.
Certain crocodylomorphs of the Jurassic developed a mammal-like dentition, flippers and other odd morphologies, but they never again developed a bipedal stance, evidently losing out in the competition with dinosaurs.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Allen D 2003. When Terrestrisuchus gracilis reaches puberty it becomes Saltoposuchus connectens!”. Journal of Vertebrate Paleontology 23 (3): 29A.
Bennett SC 1996. The phylogenetic position of the Pterosauria within the Archosauromorpha. Zoolological Journal of the Linnean Society 118: 261–308.
Benton MJ 1999. Scleromochlus taylori and the origin of the pterosaurs. Philosophical Transactions of the Royal Society London, Series B 354 1423-1446. Online pdf
Benton MJ and Walker AD 2011. Saltopus, a dinosauriform from the Upper Triassic of Scotland. Earth and Environmental Science Transactions of the Royal Society of Edinburgh: 101 (Special Issue 3-4):285-299. DOI:10.1017/S1755691011020081
Crush PJ 1984. A late upper Triassic sphenosuchid crocodilian from Wales. Palaeontology 27: 131-157.
França MAG, Ferigolo J and Langer MC 2011. Associated skeletons of a new middle Triassic “Rauisuchia” from Brazil. Naturwissenschaften.
Hone DWE and Benton MJ 2008. Contrasting supertree and total evidence methods: the origin of the pterosaurs. Zitteliana B28:35–60.
Huene FR 1910. Ein primitiver Dinosaurier aus der mittleren Trias von Elgin. Geol. Pal. Abh. n. s., 8:315-322.
Padian K. 1984. The Origin of Pterosaurs. Proceedings, Third Symposium on Mesozoic Terrestrial Ecosystems, Tubingen 1984. Online pdf
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. Hist Bio 15: 277–301.
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna; XIV, Lewisuchus admixtus, gen. et sp. nov., a further thecodont from the Chañares beds. Breviora 390:1-13
Senter P 2003. Taxon Sampling Artifacts and the Phylogenetic Position of Aves. PhD dissertation. Northern Illinois University, 1-279.
Sereno PC 1991. Basal archosaurs: phylogenetic relationships and functional implications. Journal of Vertebrate Paleontology 11 (Supplement) Memoire 2: 1–53.
Woodward AS 1907. On a new dinosaurian reptile (Scleromochlus taylori, gen. et sp. nov.) from the Trias of Lossiemouth, Elgin. Quarterly Journal of the Geological Society 1907 63:140-144.