The tiny croc that can’t keep its trap shut: Orthosuchus

It’s tiny and well known from 3D skeletal material.
And yet, for a long time Orthosuchus (Fig. 1) was a bit of an enigma in traditional paleontology.

Figure 1. The well-known skull of tiny Orthosuchus. Note the concave maxilla and dentary, resulting in a large gap.

Figure 1. The well-known skull of tiny Orthosuchus SAM-PK-K409. Note the concave maxilla and dentary, resulting in a large gap. Was the maxilla deformed? Maybe. Note the mid-skull position of the internal nares (choanae). Skull shown more than twice natural size. Dentary based on BP/1/7979,

Orthosuchus stormbergi (Nash 1968; Early Jurassic 196mya; 60cm in length; SAM-PK-K409, Fig. 1) is a tiny crocodiliform close to Protosuchus, but closer to Sichuanosuchus (Fig. 2) in the large reptile tree (LRT, 1562 taxa). The low flat skull is covered in shallow pits. The upper and lower jaws attempt to close but with a large gap between them. One wonders if long, filter-feeding teeth bridged that gap. The choanae are midway betwen the nose and throat.

Figure 2. Sichuanosuchus is another small protosuchid croc.

Figure 2. Sichuanosuchus is another small protosuchid croc.

Dollman et al. 2017 recently reported,
“The phylogenetic position of Orthosuchus is not solidified, it has previously been recovered as either a basal ‘protosuchian’ (Pol et al. 2004) or derived ‘protosuchian’ (Pol et al. 2014) just outside of Protosuchidae.” No mention was made of Sichuanosuchus in the text.

Later, Dollman et al. 2018
did mention and nest Sichuanosuchus more or less close to Protosuchus and Orthosuchus, and also close to Fruitachampsa.

Figure 3. Subset of the LRT with the addition of Lagosuchus next to Saltopus among the basal bipedal Crocodylomorpha. The nesting of skull-only Yonghesuchus near the skull-less taxa provides clues to the morphology of the skulls in the headless taxa.

Figure 3. Subset of the LRT with the addition of Lagosuchus next to Saltopus among the basal bipedal Crocodylomorpha. The nesting of skull-only Yonghesuchus near the skull-less taxa provides clues to the morphology of the skulls in the headless taxa.

References
Dollman KN, Vigliett PA and Choiniere JN 2017. A new specimen of Orthosuchus stormbergi (Nash 1968) and a review of the distribution of Southern African Lower Jurassic crocodylomorphs. Historical Biology 31(5):653–654.
Dollman KN, ClarkJM, Norell MA, Xu X and Choiniere JN 2018. Convergent evolution of a eusuchian-type secondary palate within Shartegosuchidae. American Museum Novitates. 3901: 1–23.
Nash D 1968. A crocodile from the Upper Triassic of Lesotho. Journal of Zoology. London 156:163–179.
Nash DS 1975. The Morphology and Relationships of a Crocodilian, Orthosuchus stormbergi, from the Upper Triassic of Lesotho. Annals of the South African Museum 67: 227-329.

wiki/Protosuchus
wiki/Orthosuchus
wiki/Sichuanosuchus

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