''Hylonomus'' (; ''hylo-'' "forest" + ''nomos'' "dweller") is an extinct

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 ...

reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...

that lived during the

Bashkirian The Bashkirian is in the International Commission on Stratigraphy geologic timescale the lowest stage (stratigraphy), stage or oldest age (geology), age of the Pennsylvanian (geology), Pennsylvanian. The Bashkirian age lasted from to Mega annu ...

stage of the

Late Carboniferous Late or LATE may refer to: Everyday usage * Tardy, or late, not being on time * Late (or the late) may refer to a person who is dead Music * Late (The 77s album), ''Late'' (The 77s album), 2000 * Late (Alvin Batiste album), 1993 * Late!, a pseudo ...

. It is the earliest known

crown group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor ...

amniote Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial animal, terrestrial and semiaquatic vertebrates. Amniotes evolution, evolved from amphibious Stem tet ...

and the oldest known unquestionable reptile, with the only known species being ''Hylonomus lyelli''. Despite being amongst the oldest known reptiles, it is not the most primitive member of the group, being a eureptile more derived than either parareptiles or

captorhinids Captorhinidae is an extinct family of tetrapods, traditionally considered primitive reptiles, known from the late Carboniferous to the Late Permian. They had a cosmopolitan distribution across Pangea. Description Captorhinids are a clade of sm ...

Discovery and naming

''Hylonomus lyelli'' was first described by

John William Dawson Sir John William Dawson (1820–1899) was a Canadian geologist and university administrator. Life and work John William Dawson was born on 13 October 1820 in Pictou, Nova Scotia, where he attended and graduated from Pictou Academy. Of Sco ...

in 1860. The species' name was given in honor of Dawson's teacher, the geologist Sir Charles Lyell. While it has traditionally been included in the group

Protorothyrididae Protorothyrididae is an extinct family (biology), family of small, lizard-like reptiles belonging to Eureptilia. Their skulls did not have Fenestra (anatomy), fenestrae, like the more derived diapsids. Protorothyridids lived from the Late Carbon ...

, it has since been recovered outside this group.

Formerly assigned species

Dawson also attributed two other species ''H. aciedentatus'' and ''H. wymani'' when he described ''H. lyelli'' in 1860, and later described two more species ''H. multidens'' and ''H. latidens'' in 1882. In 1966,

Robert L. Carroll Robert "Bob" Lynn Carroll (May 5, 1938 – April 7, 2020) was an American–Canadian vertebrate paleontologist who specialised in Paleozoic and Mesozoic amphibians and reptiles. Biography Carroll was an only child and grew up on a farm nea ...

suggested that ''H. latidens'' is synonymous with the type species ''H. lyelli'' and that ''H. multidens'' belongs to a different genus of 'microsaur' which he named as ''Novascoticus''. Both ''H. aciedentatus'' (also known as ''Smilerpeton aciedentatum'') and ''H. wymani'' (RM 3061-9) are later reclassified as specimens of '' Dendrerpeton acadianum''.

Description

''Hylonomus'' was long (including the tail). Most of them are 20 cm long and probably would have looked rather similar to modern

lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...

s. It had small sharp teeth and it likely ate small invertebrates such as

millipede Millipedes (originating from the Latin , "thousand", and , "foot") are a group of arthropods that are characterised by having two pairs of jointed legs on most body segments; they are known scientifically as the class Diplopoda, the name derive ...

s or early

insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...

s. Specimens of ''Hylonomus'' indicate that their bodies are covered with horny scales. They are also described as having slender and lightweight leg and arm bones, long and slim hands and feet, a narrow and tongue-shaped part in the roof of the mouth, a deep groove on a certain bone in the skull, a bumpy structure on the back bones, changes in the height of certain back bone parts, a hole in a specific place on the skull, arm and leg bones that are the same length, a short fourth toe bone compared to the shin bone, a short fifth toe bone compared to the fourth toe bone, long neck bones, and a well-developed opening below the eye. Hylonomus Scale

Fossils of the basal

pelycosaur Pelycosaur ( ) is an older term for basal or primitive Late Paleozoic synapsids, excluding the therapsids and their descendants. Previously, the term mammal-like reptile was used, and Pelycosauria was considered an order, but this is now thoug ...

Archaeothyris ''Archaeothyris'' is an extinct genus of ophiacodontid synapsid that lived during the Late Carboniferous and is known from Nova Scotia. Dated to 306 million years ago, ''Archaeothyris'', along with a more poorly known synapsid called ''Echinerpet ...

'' and the basal

diapsid Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The earliest traditionally identified diapsids, the araeosc ...

Petrolacosaurus ''Petrolacosaurus'' ("rock lake lizard") is an extinct genus of diapsid reptile from the late Carboniferous period. It was a small, long reptile, and one of the earliest known reptiles with two temporal fenestrae (holes at the rear part of the ...

'' are also found in the same region of Nova Scotia, although from a higher stratum, dated approximately 6 million years later. Fossilized footprints found in

New Brunswick New Brunswick is a Provinces and Territories of Canada, province of Canada, bordering Quebec to the north, Nova Scotia to the east, the Gulf of Saint Lawrence to the northeast, the Bay of Fundy to the southeast, and the U.S. state of Maine to ...

have been attributed to ''Hylonomus'', at an estimated age of 315 million years.

Paleoecology

Fossils of ''Hylonomus'' have been found in the remains of fossilized club moss stumps in the Joggins Formation, Joggins, Nova Scotia, Canada. It is supposed that, after harsh weather, the club mosses would crash down, with the stumps eventually rotting and hollowing out. Small animals such as ''Hylonomus'', seeking shelter, would enter and become trapped, starving to death. An alternative hypothesis is that the animals made their nests in the hollow tree stumps.

In popular culture

''Hylonomus lyelli'' was named the Provincial Fossil of Nova Scotia in 2002.

References

External links

Fossils of Nova Scotia - The Tree Stump Animals

Early Researchers & Finds of the Joggins Fossil CliffsHylonomus: Provincial Fossil of Nova Scotia

A photograph of the disarticulated skeleton, credited to J. CalderAnother photo of the specimen, from Dr. Melissa Grey's twitter account
{{Taxonbar, from=Q132458 Protorothyrididae Carboniferous reptiles of North America Transitional fossils Monotypic prehistoric reptile genera Paleozoic life of Nova Scotia Pennsylvanian genus first appearances Pennsylvanian genus extinctions Fossil taxa described in 1860