Caerorhachis
Encyclopedia
Caerorhachis is an extinct genus
of early tetrapod
from the Early Carboniferous of Scotland
. Its placement within Tetrapoda is uncertain, but it is generally regarded as a primitive member of the group. The type species
C. bairdi was named in 1977.
anthracosaur or a close relative of anthracosaurs. In this classification, Caerorhachis is a close ancestor of amniote
s, or tetrapods that lay eggs on land. Caerorhachis has also been classified as the sister taxon of temnospondyls, a large group of extinct amphibian
s, based on the presence of several primitive traits. In fact, when it was named in 1977, Caerorhachis was thought to be a dendrerpetontid
temnospondyl.
The vertebrae of Caerorhachis are more similar to anthracosaurs, however. As in all early tetrapods, the centrum
, or central part of the vertebra, is composed of two parts: the intercentrum and the pleurocentrum. While temnospondyls have large intercenta and small pleurocentra, Caerorhachis and anthracosaurs have larger pleurocentra than intercentra. A 2003 phylogenetic analysis of early tetrapods placed Caerorhachis outside the clade
that included temnospondyls and anthracosaurs in an ancestral position to both groups.
s across the skull that served as an adaptation for earlier aquatic tetrapods and their ancestors. The large, well developed limbs suggest it was able to move on land better than other early tetrapods like colosteids
and baphetids. Robert Holmes and Robert L. Carroll
, the first to describe Caerorhachis, interpreted it as "[an] animal spending much of its life in the damp mud on the margins of ponds or streams, feeding on stranded fish, or occasionally venturing into the water to catch aquatic larvae of other amphibians."
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of early tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
from the Early Carboniferous of Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
. Its placement within Tetrapoda is uncertain, but it is generally regarded as a primitive member of the group. The type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
C. bairdi was named in 1977.
Classification
Caerorhachis has usually been placed as a basalBasal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
anthracosaur or a close relative of anthracosaurs. In this classification, Caerorhachis is a close ancestor of amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...
s, or tetrapods that lay eggs on land. Caerorhachis has also been classified as the sister taxon of temnospondyls, a large group of extinct amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
s, based on the presence of several primitive traits. In fact, when it was named in 1977, Caerorhachis was thought to be a dendrerpetontid
Dendrerpetontidae
Dendrerpetontidae is a family of Temnospondyli.-References:*Benton, M.J. 2005. Vertebrate Palaeontology, Third Edition. University of Bristol...
temnospondyl.
The vertebrae of Caerorhachis are more similar to anthracosaurs, however. As in all early tetrapods, the centrum
Body of vertebra
The body is the largest part of a vertebra, and is more or less cylindrical in shape. For vertebrates other than humans, this structure is usually called a centrum....
, or central part of the vertebra, is composed of two parts: the intercentrum and the pleurocentrum. While temnospondyls have large intercenta and small pleurocentra, Caerorhachis and anthracosaurs have larger pleurocentra than intercentra. A 2003 phylogenetic analysis of early tetrapods placed Caerorhachis outside the clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
that included temnospondyls and anthracosaurs in an ancestral position to both groups.
Paleobiology
Caerorhachis is thought to have had a primarily terrestrial lifestyle. It lacks the lateral lineLateral line
The lateral line is a sense organ in aquatic organisms , used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...
s across the skull that served as an adaptation for earlier aquatic tetrapods and their ancestors. The large, well developed limbs suggest it was able to move on land better than other early tetrapods like colosteids
Colosteidae
The Colosteidae are a family of amphibian tetrapods that lived in the Carboniferous period. They appear to be a fairly primitive group, a sister group to the reptile-like amphibians and the other temnospondylans....
and baphetids. Robert Holmes and Robert L. Carroll
Robert L. Carroll
Robert Lynn Carroll is a vertebrate paleontologist who specialises in Paleozoic and Mesozoic amphibians and reptiles.Carroll was an only child and grew up on a farm near Lansing, Michigan...
, the first to describe Caerorhachis, interpreted it as "[an] animal spending much of its life in the damp mud on the margins of ponds or streams, feeding on stranded fish, or occasionally venturing into the water to catch aquatic larvae of other amphibians."