''Scleromochlus'' (from el, σκληρός , 'hard' and el, μοχλός , 'lever') is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of small pterosauromorph

archosaurs Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avi ...

from the Late

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

period. The genus contains the type and only species ''Scleromochlus taylori'', named by

Arthur Smith Woodward Sir Arthur Smith Woodward, FRS (23 May 1864 – 2 September 1944) was an English palaeontologist, known as a world expert in fossil fish. He also described the Piltdown Man fossils, which were later determined to be fraudulent. He is not relate ...

in 1907.

Discovery

Its fossils have been found in the

Carnian The Carnian (less commonly, Karnian) is the lowermost stage of the Upper Triassic Series (or earliest age of the Late Triassic Epoch). It lasted from 237 to 227 million years ago (Ma). The Carnian is preceded by the Ladinian and is followed by t ...

Lossiemouth Sandstone The Lossiemouth Sandstone is a Middle to Late Triassic (Ladinian to Norian) age geological formation. It is exposed on the south side of the Moray Firth near Lossiemouth and near Golspie in Sutherland. Dinosaur remains are among the fossils tha ...

Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...

. The

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

was discovered around 1900 and is listed as specimen BMNH R3556, a partial skeleton preserved as an impression in

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

, with portions of the skull and tail missing.

Description

''Scleromochlus taylori'' was about long, with long hind legs; it may have been capable of four-legged and two-legged locomotion. Studies about its gait suggest that it engaged in

kangaroo Kangaroos are four marsupials from the family Macropodidae (macropods, meaning "large foot"). In common use the term is used to describe the largest species from this family, the red kangaroo, as well as the antilopine kangaroo, eastern gre ...

- or

springhare ''Pedetes'' is a genus of rodent, the springhares, in the family Pedetidae. Members of the genus are distributed across southern and Eastern Africa. Species A number of species both extant and extinct are classified in the genus ''Pedetes''. ...

-like

plantigrade 151px, Portion of a human skeleton, showing plantigrade habit In terrestrial animals, plantigrade locomotion means walking with the toes and metatarsals flat on the ground. It is one of three forms of locomotion adopted by terrestrial mammals. T ...

hopping; however, a 2020 reassessment of ''Scleromochlus'' by Bennett suggested that it was a "sprawling quadrupedal hopper analogous to frogs." If ''Scleromochlus'' is indeed related to

pterosaurs Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 to ...

, this may offer insight as to how the latter evolved, since early pterosaurs also show adaptations for saltatorial locomotion.

Classification

A lightly built

cursorial A cursorial organism is one that is adapted specifically to run. An animal can be considered cursorial if it has the ability to run fast (e.g. cheetah) or if it can keep a constant speed for a long distance (high endurance). "Cursorial" is often u ...

animal, its

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

position has been debated; as different analyses have found it to be either the basal-most

ornithodira Avemetatarsalia (meaning "bird metatarsals") is a clade of diapsid reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs. Dinos ...

n, the sister-taxon to Pterosauria, or a basal member of

Avemetatarsalia Avemetatarsalia (meaning "bird metatarsals") is a clade of diapsid reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs. Dinos ...

that lies outside of

Ornithodira Avemetatarsalia (meaning "bird metatarsals") is a clade of diapsid reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs. Dinos ...

. In the phylogenetic analyses conducted by Nesbitt ''et al.'' (2017) ''Scleromochlus'' was recovered either as a basal member of

Dinosauromorpha Dinosauromorpha is a clade of avemetatarsalian archosaurs (reptiles closer to birds than to crocodilians) that includes the Dinosauria (dinosaurs) and some of their close relatives. It was originally defined to include dinosauriforms and lager ...

or as a non-

aphanosauria Aphanosauria ("hidden lizards") is an extinct group of reptiles distantly related to dinosaurs (including birds). They are at the base of a group known as Avemetatarsalia, one of two main branches of archosaurs. The other main branch, Pseudosuchi ...

n, non-pterosaur basal avemetatarsalian. However, the authors stressed that scoring ''Scleromochlus'' was challenging given the small size and poor preservation of the fossils, and stated that it could not be scored for many of the important characters that optimize near the base of Avemetatarsalia. In 2020, Bennett interpreted ''Scleromochlus'' as possessing certain characteristics, including

osteoderm Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, temnospondyls (extinct amp ...

s and a

crurotarsal A crurotarsal joint is one that’s situated between the bones of ''crus'', i.e. shin (tibia and fibula) and the proximal tarsal bones, i.e. astragalus and calcaneum. The ankle joint of therian mammals (marsupials and placentals) is a crurotarsal ...

morphology of the ankle, which suggested that ''Scleromochlus'' was not closely related to ornithodirans. Instead, he argued for a position of ''Scleromochlus'' among the

Doswelliidae Doswelliidae is an extinct family of carnivorous archosauriform reptiles that lived in North America and Europe during the Middle to Late Triassic period. Long represented solely by the heavily-armored reptile ''Doswellia'', the family's com ...

or elsewhere among basal members of the

Archosauriformes Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles that developed from archosauromorph ancestors some time in the Latest Permian (roughly 252 million years ago). It was defined by Jacques Gauthi ...

. However, in 2022, Foffa and colleagues reconstructed a complete skeleton using microcomputed tomographic scans of the 7 specimens found to date. This enabled a new phylogenetic analysis to be undertaken, which strongly supported the hypothesis that ''Scleromochlus'' was a member of the

Pterosauromorpha Pterosauromorpha (meaning "pterosaur-like forms") is one of the two basic divisions of Ornithodira that includes pterosaurs and all taxa that are closer to them than to dinosaurs and their close relatives (i.e. Dinosauromorpha). In addition to pt ...

– either as a genus of the

Lagerpetidae Lagerpetidae (; originally Lagerpetonidae) is a family of basal avemetatarsalians. Though traditionally considered the earliest-diverging dinosauromorphs (reptiles closer to dinosaurs than to pterosaurs), fossils described in 2020 suggest that ...

family (shown to be a part of Pterosauromorpha in 2020) or as the

sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and t ...

to pterosaurs and lagerpetids. Previous alternative classifications were demonstrated to have been based on misinterpretations of incomplete or ambiguous anatomical features found in the fossil record.

References

{{Taxonbar, from=Q133237 Pterosauromorpha Triassic archosaurs Carnian genera Late Triassic reptiles of Europe Triassic Scotland Fossils of Scotland Fossil taxa described in 1907 Taxa named by Arthur Smith Woodward Prehistoric reptile genera