Terrestrisuchus Scale

''Terrestrisuchus'' is an extinct genus of very small early crocodylomorph that was about 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.

Description

''Terrestrisuchus'' was a small, slender crocodylomorph with very long legs, quite unlike modern crocodilians. It was initially estimated to have been between long, although this estimate may be based on juvenile specimens and fully grown ''Terrestrisuchus'' may have reached or exceeded 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 ring of bony ossicles, the sclerotic ring.

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.

Limbs and posture

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.

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.

History of discovery

The first fossils of ''Terrestrisuchus'' were discovered by Professor K. A. Kermack and Dr. P. L. 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.

Additional material attributed to ''Terrestrisuchus'' has been discovered in other Late Triassic fissure deposits in South Wales and Bristol, including the Ruthin Quarry in Wales and the Tytherington and Cromhall quarries near Bristol, as well as a possible specimen from Durdham Down. The fossils of ''Terrestrisuchus'' were originally housed at University College, London before being transferred to the Natural History Museum in London where they are currently stored.

Classification

''Terrestrisuchus'' was originally classified as a member of the crocodylomorph suborder " Sphenosuchia", a group that included various other similar long-legged early crocodylomorphs and was considered to be a separate radiation from the group that all later crocodylomorphs would evolve from. However, this classification was made prior to the invention of cladistic phylogenetic analyses, which has since demonstrated that "Sphenosuchia" is an unnatural grouping (paraphyletic), meaning that "sphenosuchians" are not all descended from a single common ancestor to the exclusion of all other crocodylomorphs. Instead, the "sphenosuchians" are a grade of basal crocodylomorphs that lead up to the more derived crocodyliforms. Nonetheless, ''Terrestrisuchus'' has consistently been recovered within this grade, as shown in the cladogram below from Leardi ''et al.'' (2017):

In 1988, palaeontologists Michael Benton and James Clark first proposed that specimens of ''Terrestrisuchus'' in fact represented the juveniles of the related "sphenosuchian" from Germany, '' Saltoposuchus''. The similarity between ''Terrestrisuchus'' and ''Saltoposuchus'' had been recognised since its description, so much so that Crush even named a new family, the Saltoposuchidae, to accommodate the two genera. However, Benton and Clark considered that the characters Crush identified to separate the two taxa as invalid, and so that the two were likely to belong to at least the same genus. This hypothesis was rejected by Sereno and Wild in 1992, who claimed to have identified additional differences between the two genera, although Clark ''et al.'' (2001) considered these differences to be dubious or due to the size difference between their remains. In 2003, palaeontologist David Allen identified juvenile features in ''Terrestrisuchus'', and believed all the differing traits between it and ''Saltoposuchus'' to be ontogenetically variable, and so were otherwise indistinguishable.

However, in 2013 a re-evaluation of the hypothesis concluded that the available evidence was not consistent with the two species being synonymous, and it is likely that ''Terrestrisuchus'' is indeed its own genus. This included the non-overlapping geographic and stratigraphic ranges of the two taxa, with ''Terrestrisuchus'' being at least "several million years" younger than ''Saltoposuchus'', as well as inconsistencies in the patterns of the fusion of their vertebrae and the proportions of the hind limb during growth compared to other crocodylomorph growth series. Nonetheless, the two genera do appear to be closely related and are consistently recovered together in phylogenetic analyses. The analysis of Leardi ''et al.'' (2017) in particular finds them to clade together with the South African '' Litargosuchus'', and they recognised that this clade could approximate the Saltoposuchidae as conceived by Crush in 1984. The shape and construction of its hip bones, particularly the elongated pubis, indicate that they were rigidly sutured together and that its pubis was not mobile as it is in modern crocodilians. This indicates that ''Terrestrisuchus'' would not have utilised the hepatic piston method of breathing found in modern crocodilians.

Metabolism and growth

The microstructure of the limb bones of ''Terrestrisuchus'' show that it was well vascularised and contained large amounts of energy-consuming fibrolamellar bone tissue, indicating a relatively fast growth-rate for ''Terrestrisuchus'' compared to other archosauriforms and even other pseudosuchians. Such a high growth rate is in agreement with an elevated, "warm-blooded" metabolism. However, the related "sphenosuchian" ''Hesperosuchus'' was found to have a slower, more typical crocodilian-like growth rate, and so it is possible that the high growth rate of ''Terrestrisuchus'' was due to the sampled specimens being immature and still rapidly growing, and that adults had a slower metabolism.

Palaeoecology

''Terrestrisuchus'' was a coastal species, unusual for basal crocodylomorphs, which are typically known from inland floodplain environments. In the Late Triassic, the Pant-y-ffynnon Quarry and other quarries were part of ancient islands in a palaeo- archipelago that stretched across southern Wales and England to Bristol. The islands were forested karstic environments, riddled with fissures, sinkholes and caverns eroded into the limestone, environments which long limbed, agile reptiles like ''Terrestrisuchus'' may have been well suited to inhabit.

On the Pant-y-ffynnon palaeo-island, ''Terrestrisuchus'' coexisted with other archosaurs such as the similarly long-legged, enigmatic pseudosuchian '' Aenigmaspina'', the herbivorous sauropodomorph dinosaur '' Pantydraco'', and the coelophysoid theropod '' Pendraig''. Rhynchocephalians (relatives of modern tuataras) were very abundant and are known from at least three species including '' Clevosaurus cambrica'', '' Diphydontosaurus'' and at least one or two unnamed species, which likely formed a large part of the diet of ''Terrestrisuchus'' (possibly evidenced by bite marks on the bones of ''Clevosaurus'' that likely belong to ''Terrestrisuchus''). ''Terrestrisuchus'' was a relatively rare component of the island's fauna, an expected relationship for a predatory animal.

References

{{Taxonbar, from=Q981048

Terrestrial crocodylomorphs
Triassic crocodylomorpha
Rhaetian life
Late Triassic reptiles of Europe
Triassic England
Triassic Wales
Fossils of England
Fossils of Wales
Fossil taxa described in 1984
Prehistoric pseudosuchian genera