''Scleromochlus'' (from , 'hard' and , 'lever') is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of small pterosauromorph

archosaur Archosauria () or archosaurs () is a clade of diapsid sauropsid tetrapods, with birds and crocodilians being the only extant taxon, extant representatives. Although broadly classified as reptiles, which traditionally exclude birds, the cladistics ...

s from the

Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch a ...

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 th ...

Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...

. 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 rel ...

in 1907.

Discovery

Its fossils have been found in the

Carnian The Carnian (less commonly, Karnian) is the lowermost stage (stratigraphy), stage of the Upper Triassic series (stratigraphy), Series (or earliest age (geology), age of the Late Triassic Epoch (reference date), Epoch). It lasted from 237 to 227.3 ...

. The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

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

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

, with portions of the skull and tail missing.

named and described ''Scleromochlus taylori'' in 1907.

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 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. ...

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 are an extinct clade of flying reptiles in the Order (biology), order Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 million to 66 million years ago). Pterosau ...

, 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 ...

animal, its

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

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. Di ...

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

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

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. Di ...

. 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 avemetatarsalians ( archosaurs 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 lagerpetids ...

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 amph ...

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 crurotarsa ...

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 compos ...

or elsewhere among basal members of the

Archosauriformes Archosauriformes (Ancient Greek, Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles encompassing Archosaur, archosaurs and some of their close relatives. It was defined by Jacques Gauthier (1994) as the clade stemmin ...

. However, in 2022, Foffa and colleagues reconstructed a complete skeleton using microcomputed tomographic scans of the seven 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 Taxon, taxa that are closer to them than to dinosaurs and their close relatives (i.e. Dinosauromorpha). In addition ...

– either as a genus of the

Lagerpetidae Lagerpetidae (; originally Lagerpetonidae) is a family of basal avemetatarsalians (early-diverging members of the reptile lineage leading to birds and other dinosaurs). Though traditionally considered the earliest-diverging dinosauromorphs (ar ...

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 ...

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. Foffa et al. followed up with a more extensive redescription of ''Scleromochlus'' fossils in 2023.

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