Terrestrisuchus

Terrestrisuchus is an extinct genus of very small early crocodylomorph that was about 76 centimetres (30 in) long. Fossils have been found in Wales and Southern England and date from near the very end of the Late Triassic during the Rhaetian, and it is known by type and only known species T. gracilis. Terrestrisuchus was a long-legged, active predator that lived entirely on land, unlike modern crocodilians. It inhabited a chain of tropical, low-lying islands that made up southern Britain, along with similarly small-sized dinosaurs and abundant rhynchocephalians. Numerous fossils of Terrestrisuchus are known from fissures in limestone karst which made up the islands it lived on, which formed caverns and sinkholes that preserved the remains of Terrestrisuchus and other island-living reptiles.

Terrestrisuchus was a small, slender crocodylomorph with very long legs, quite unlike modern crocodilians. It was initially estimated to have been between 49 and 76 centimetres (19 and 30 in) long, although this estimate may be based on juvenile specimens and fully grown Terrestrisuchus may have reached or exceeded 1 metre (3.3 ft) in length.

Its skull was long and narrow, with a tapering, pointed triangular snout lined with sharp curved teeth. The upper jaw margin was straight, and lacked a diastema (a gap in the tooth row) between the maxilla and the premaxilla. By contrast, the long and slender dentary bones of the lower jaw curved slightly upwards towards the front. Unlike modern crocodilians, the eye of Terrestrisuchus was supported by a scleral ring (a ring of ossicles).

The body was relatively short and shallow, and the spine was topped by paired rows of osteoderms running down from its neck down its back. These osteoderms are described as "leaf-shaped", being relatively longer than wide with a prominent spur at the front that slides under and interlocks with the scute in front of it. This provides a rigid support for the body and limited the flexibility of its spine, supporting its body on land. The hips of Terrestrisuchus had an elongated pubis, unlike living crocodilians. Terrestrisuchus is also known to have had tightly packed gastralia, or belly ribs. Its tail was particularly long, about twice the length of the head and body combined with an estimated 70 caudal vertebrae in total, and may have been used as a balance allowing the animal to rear up and run on its hind legs for brief periods.

Unlike modern crocodylians, the limbs of Terrestrisuchus were very long in proportion to the body and were held upright directly beneath it. The shape of the ankles and the bones in the hands and feet also suggest that Terrestrisuchus was digitigrade, with elongated metacarpus (wrist) and metatarsal bones that were pressed tightly together, similar to the feet of fast-running dinosaurs, suggesting that Terrestrisuchus was highly cursorial, adapted for running at high speeds. The pisiform bone in the wrist is notably smaller compared to early crocodyliforms such as Protosuchus, as well as modern crocodilians, indicating that Terrestrisuchus had less flexible wrists. Crush reconstructed Terrestrisuchus as a quadruped, with noticeably longer hind limbs than its forelimbs and its hips held high above the shoulder. However, based on these proportions it has also been suggested that Terrestrisuchus may have been bipedal instead. This question remained controversial in subsequent studies. A quantitative analysis of limb proportions suggested quadrupedal locomotion in early Crurotarsi in general, with quadrupedalism further suggested by a quantitative analysis of the animals' limb robusticity, whereas a study of femoral mid-diaphyseal cross sectional geometry supports bipedal locomotion.

Notably, the acetabulum (hip socket) of Terrestrisuchus is perforated and forms an opening between the hip bones. This feature is otherwise only known in dinosaurs (as well as a few other early crocodylomorphs) and is often regarded as a defining feature of that clade. Similarly, the femur of Terrestrisuchus has a distinct head that faces inwards towards the body, and fits into the hip socket at a right angle to the leg. This condition is described as "buttress-erect", and it is typical of dinosaurs and their close relatives but otherwise unheard of in pseudosuchians outside of basal crocodylomorphs. Other pseudosuchians with upright limbs were typically "pillar-erect", with their femurs attached into a hip-socket that faced directly downwards. The buttress-erect posture of Terrestrisuchus and other basal crocodylomorphs is unique amongst crocodile-line archosaurs, and restricted its posture to a permanently upright stance. Its posture was further restricted to an upright gait by the calcaneal tubercle on its heel bone pointing directly backwards from the foot, unlike the back-and-sideways facing tuber of modern, sprawling crocodilians.

The first fossils of Terrestrisuchus were discovered by Professor K. A. Kermack and Dr. Pamela Lamplugh Robinson in the spring of 1952, recovered from the Pant-y-ffynon Quarry located near Cowbridhe, Glamorgan in South Wales. Their finds were presented by Kermack to the Linnean Society of London on October 8, 1953, and was recognised belonging to a "primitive crocodile or crocodile ancestor". No osteoderms had been identified yet at the time, which Kermack regarded as representing a "missing link" between modern crocodilians and the Triassic "thecodonts". The fossils included several well-preserved articulated partial skeletons and various isolated bones. Kermack refrained from naming the animal or nominating a type specimen, as preparation of the fossils was still ongoing. The specimens were eventually named and thoroughly described by P. J. Crush in 1984, with the generic name Terrestrisuchus chosen to emphasise the terrestrial lifestyle of this crocodylomorph, and the specific name from the Latin gracilis for its light, graceful build.

The Pant-y-ffynnon Quarry is composed mostly of Carboniferous limestone, but the fossils of Terrestrisuchus were recovered from Triassic sedimentary rocks that were deposited within fissures in the limestone (such as sandstones and marls). The age of the deposits has been historically debated, with older literature suggesting a Carnian to Norian age. However, palynological data has been used to determine a younger Rhaetian age, close to the very end of the Triassic. This estimate has been corroborated by Rhaetian index fossils such as conchostracans and geomorphological data.