''Echinerpeton'' is an extinct
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
of
synapsid
Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptil ...
, including the single species ''Echinerpeton intermedium'' from the
Late Carboniferous
Late may refer to:
* LATE, an acronym which could stand for:
** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia
** Local-authority trading enterprise, a New Zealand business law
** Local average treatment effect, ...
of
Nova Scotia
Nova Scotia ( ; ; ) is one of the thirteen provinces and territories of Canada. It is one of the three Maritime provinces and one of the four Atlantic provinces. Nova Scotia is Latin for "New Scotland".
Most of the population are native Eng ...
, Canada. The name means 'spiny lizard' (Greek). Along with its contemporary ''
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 '' Echinerpe ...
'', ''Echinerpeton'' is the oldest known synapsid, having lived around 308 million years ago. It is known from six small, fragmentary fossils, which were found in an outcrop of the
Morien Group near the town of
Florence
Florence ( ; it, Firenze ) is a city in Central Italy and the capital city of the Tuscany Regions of Italy, region. It is the most populated city in Tuscany, with 383,083 inhabitants in 2016, and over 1,520,000 in its metropolitan area.Bilan ...
.
The most complete specimen preserves articulated vertebrae with high
neural spine
The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...
s, indicating that ''Echinerpeton'' was a sail-backed synapsid like the better known ''
Dimetrodon'', ''
Sphenacodon'', and ''
Edaphosaurus
''Edaphosaurus'' (, meaning "pavement lizard" for dense clusters of teeth) is a genus of extinct edaphosaurid synapsids that lived in what is now North America and Europe around 303.4 to 272.5 million years ago, during the Late Carboniferous to ...
''. However, the relationship of ''Echinerpeton'' to these other forms is unclear, and 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 ...
placement among basal synapsids remains uncertain.
Description
''Echinerpeton'' is known from six specimens, five housed in the
Museum of Comparative Zoology
A museum ( ; plural museums or, rarely, musea) is a building or institution that cares for and displays a collection of artifacts and other objects of artistic, cultural, historical, or scientific importance. Many public museums make thes ...
and a sixth in the
Redpath Museum
The Redpath Museum (french: Musée Redpath) is a museum of natural history belonging to McGill University and located on the university's campus at 859, rue Sherbrooke Ouest (859 Sherbrooke Street West) in Montreal, Quebec. It was built in 1882 ...
: the holotype MCZ 4090, which consists of a partial postcranial skeleton and some jaw fragments; MCZ 4091, which includes vertebrae and an
interclavicle
An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals (marsupials and placentals) are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. In t ...
; MCZ 4092, a left
maxilla
The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. T ...
or upper jaw bone; MCZ 4093, a partial right maxilla; MCZ 4094, including three
neural arch
The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...
es or vertebral spines; and RM 10057, consisting of a right maxilla, neural arch, rib, and a
phalanx
The phalanx ( grc, φάλαγξ; plural phalanxes or phalanges, , ) was a rectangular mass military formation, usually composed entirely of heavy infantry armed with spears, pikes, sarissas, or similar pole weapons. The term is particularly ...
or finger bone. Since all other specimens besides the holotype are isolated bone fragments, their assignment to the same species is not certain. The maxillae are distinct in having straight lower margins, distinct from the often curved jaws of ophiacodontids and sphenacodontids but similar to the straight jaws of some other synapsids like ''Archaeothyris'', ''
Haptodus
''Haptodus'' is an extinct genus of basal sphenacodont, member of the clade that includes therapsids and hence, mammals. It was at least in length. It lived in present-day France during the Early Permian. It was a medium-sized predator, feeding ...
'', and ''
Varanops''. The dentary or lower jaw bone has a slight upward curve. The teeth of both the upper and lower jaws are small and cone-shaped, some having slightly serrated edges, and are only differentiated by slight differences in length (some other synapsids have teeth that vary greatly and shape across their jaws). The three forward-most dentary teeth are angled slightly outward as in more derived synapsids such as ''
Dimetrodon'' and ''
Sphenacodon''. Several features, including straight-margined maxillae and simple conical teeth, are also seen in the earliest reptiles.
[ Twenty-three presacral (neck and back) vertebrae are preserved in the holotype, although several may be missing because the typical number of presacral vertebrae in early synapsids is 27. The ]centra
Centra is a convenience shop chain that operates throughout Ireland. The chain operates as a symbol group owned by Musgrave Group, the food wholesaler, meaning the stores are all owned by individual franchisees.
The chain has three different ...
or centers of the vertebrae are slightly compressed, similar to the compression seen in the vertebrae of ophiacodontids. There are large spaces between the centra where bones called intercentra could fit, although no intercentra are preserved in the six ''Echinerpeton'' specimens. The loose connection between the centra and intercentra is one of the primitive features of ''Echinerpeton'', since other early synapsids have intercentra that fit tightly with the centra as part of an evolutionary progression toward completely fused vertebral elements. The most prominent feature of the vertebrae of ''Echinerpeton'' are their tall neural spines, which can be up to seven times higher than they are wide. They are similar in proportion to the spines of ''Sphenacodon'', although ''Echinerpeton'' is considerably smaller in overall size. The neural spines of the holotype are thinnest at their tips, suggesting that MCZ 4090 may have been an immature individual with poorly ossified
Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...
bones. By contrast, MCZ 4094 has neural spines that are thicker at their tips and are slightly larger in size, both of which are possible indications that the specimen represents an adult individual. The first vertebra preserved in the series, the axis bone, has a neural spine that is low and broad like those of many other synapsids. The axis is most similar to those of ophiacodontids because it widens toward the top, and unlike those of sphenacodontids which widen about midway up the spine and then narrow at the top. To either side of the neural spines are smaller transverse processes, which have struts of bones extending from them that were described as a "webbing." This "webbing" helps connect the vertebrae to the ribs, and is otherwise only seen in ophiacodontids.[
Parts of the ]appendicular skeleton
The appendicular skeleton is the portion of the skeleton of vertebrates consisting of the bones that support the appendages. There are 126 bones. The appendicular skeleton includes the skeletal elements within the limbs, as well as supporting shou ...
(limbs, hips, and shoulder bones) are preserved in ''Echinerpeton'' specimens, including the interclavicle
An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals (marsupials and placentals) are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. In t ...
, scapula
The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eith ...
(shoulder blade), the lower portion of the humerus (upper arm bone), the ilium (a hip bone), the upper portion of the femur
The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates wit ...
(upper leg bone), the tibia
The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...
and fibula
The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity i ...
(lower leg bones), astragalus
''Astragalus'' is a large genus of over 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae and the subfamily Faboideae. It is the largest genus of plants in terms of described species. The genus is native to tempe ...
and calcaneum
In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is the point of the hock.
...
(ankle bones), and metatarsal
The metatarsal bones, or metatarsus, are a group of five long bones in the foot, located between the tarsal bones of the hind- and mid-foot and the phalanges of the toes. Lacking individual names, the metatarsal bones are numbered from the me ...
s (foot bones). The ilium of ''Echinerpeton'' is similar to those of early reptiles in that it is narrow and backward-pointing, while those of sphenacodontids are widened at their front to support the hip's connection with the sacral
Sacral may refer to:
*Sacred
Sacred describes something that is dedicated or set apart for the service or worship of a deity; is considered worthy of spiritual respect or devotion; or inspires awe or reverence among believers. The property ...
vertebrae. The astragalus has the same simple L-shape as those of ophiacodontids.[
]
Phylogeny
Reisz (1972) tentatively classified ''Echinerpeton'' as an ophiacodontid in its initial description, but later (1986) considered it an indeterminate "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'' had been used, and pelycosaur was considered an order, but this is ...
". Lee (1999) argued that the placement of ''Echinerpeton'' within Synapsida was not certain because it lacked any of the defining features or synapomorphies present in the group, all of which come from the skull. He claimed that all the features linking ''Echinerpeton'' with synapsids were also present in other basal amniotes, so it could not be placed definitively on the synapsid branch of Amniota (the other amniote branch is Sauropsida, or reptiles). Benson (2012) was the first to include ''Echinerpeton'' in a phylogenetic analysis, and considered it a "wildcard taxon" because it had three equally likely positions on the synapsid tree: one as the most basal synapsid, another as the sister taxon
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 ...
of a clade containing Caseasauria, Edaphosauridae
Edaphosauridae is a family of mostly large (up to 3 meters or more) Late Carboniferous to Early Permian synapsids. Edaphosaur fossils are so far known only from North America and Europe.
Characteristics
They were the earliest known herbivorous a ...
, and Sphenacodontia
Sphenacodontia is a stem-based clade of derived synapsids. It was defined by Amson and Laurin (2011) as "the largest clade that includes '' Haptodus baylei'', '' Haptodus garnettensis'' and '' Sphenacodon ferox'', but not '' Edaphosaurus pogoni ...
, and a third as an ophiacodontid more derived than ''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 '' Echinerpe ...
''. Benson also found that the inclusion of ''Echinerpeton'' in his analysis was causing large polytomies, or unresolved relationships, in the strict consensus tree.[ Mann and Patterson (2019) described new material of ''Echinerpeton'', including substantial cranial remains, and recovered it as a member of Ophiacodontidae.]
References
{{Taxonbar, from=Q5332443
Carboniferous synapsids of North America
Prehistoric synapsid genera
Taxa named by Robert R. Reisz
Fossil taxa described in 1972
Paleozoic life of Nova Scotia