Gracilisuchus, meaning “the slender crocodile”, was a 2-foot-long reptile which lived in South America during the middle of the Triassic Period. Its remains were discovered in northwestern Argentina within the rocks of the Chañares Formation, which are dated to about 235 million years ago – that’s five million years prior to the oldest-known dinosaurs. It was distantly related to crocodiles, but it and its close relatives lived and hunted on land rather than in the water. In 1972, the animal was officially named Gracilisuchus stipanicicorum by Prof. Alfred S. Romer of Harvard University.

As of 2017, six specimens of Gracilisuchus have been found. Careful examination of the bones has led paleontologists to assert that Gracilisuchus is a primitive relative of crocodiles. Originally believed to be a member of the family Ornithosuchidae, a re-evaluation made in 2014 (and re-asserted in 2017) states that it belongs within its own family named Gracilisuchidae. Other related species within this family include Turfanosuchus and Yonghesuchus, both of which come from China.

Gracilisuchus possessed a triangular skull with massive eye sockets, indicating that this animal had huge eyes in proportion to the rest of its head. This could either mean that this animal was nocturnal, or perhaps this was a juvenile and the adult was much larger. The teeth are slightly laterally compressed, making them oval-shaped in cross-section, and are distinctly recurved backwards. The skull did not possess any “nutrient foramina” (small holes arranged in a line running on the surface of the skull close to where the teeth are) on either the upper or lower jaws. These indentations are often used by paleontologists and paleo-artists as evidence for the presence of lips, so it’s safe to assume that Gracilisuchus, like its modern-day crocodilian relatives, didn’t have any lips at all.

Like many primitive crocodilian relatives, Gracilisuchus possessed a double-row of osteoderms running down the middle of its back, extending from the back of its skull to the tail. However, since the end of the tail wasn’t found with the specimen, we don’t know if the osteoderms extended all the way to the tail’s tip. Each of these osteoderms possessed a low ridge running down the middle front-to-back, flared slightly outwards to the sides. The osteoderms are one-half the length of the vertebrae beneath them (meaning there are four osteoderms over each backbone), with the osteoderm in the front overlapping the osteoderm behind it like overlapping roof tiles.

Gracilisuchus had longer back legs compared to its front legs (the front legs were three-fifths the length of the back legs), indicating that it walked on just two legs rather than four. However, it also appears to have been poorly balanced, with a distinctly front-heavy build. Therefore, it’s likely that it spent much of its time walking on all fours, but was able to run on just two legs for short distances, like some species of lizards can today.

Below is a drawing of Gracilisuchus, made with No. 2 and No. 3 pencil.

Gracilisuchus. © Jason R. Abdale (January 18, 2022).

As always, dear reader and/or illustrator, keep your pencils sharp.

Bibliography (arranged in order of publication)

Romer, Alfred Sherwood (1972). “The Chañares (Argentina) Triassic Reptile Fauna. XIII. An Early Ornithosuchid Pseudosuchian, Gracilisuchus stipaniciorum, Gen. et Sp. Nov.” Breviora: Museum of Comparative Zoology, article number 389 (August 11, 1972). Pages 1-24.

Butler, Richard J.; Sullivan, Corwin; Ezcurra, Martín D.; Liu, Jun; Lecuona, Agustina; Sookias, Roland B. (2014). “New clade of enigmatic early archosaurs yields insights into early pseudosuchian phylogeny and the biogeography of the archosaur radiation”. BMC Evolutionary Biology, volume 14, article number 128 (June 10, 2014).

Lecuona, Augustina; Desojo, Julia B.; Pol, Diego (2017). “New information on the postcranial skeleton of Gracilisuchus stipanicicorum (Archosauria: Suchia) and reappraisal of its phylogenetic position”. Zoological Journal of the Linnean Society, volume 20 (February 1, 2017). Pages 1-40.

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