''Galesaurus'' (from the Greek roots for 'weasel' and 'lizard') is an extinct genus of carnivorous
cynodont
The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide variety ...
therapsid
Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented more ...
that lived between the
Induan
The Induan is the first age of the Early Triassic epoch in the geologic timescale, or the lowest stage of the Lower Triassic series in chronostratigraphy. It spans the time between 251.902 Ma and Ma (million years ago). The Induan is sometimes ...
and the
Olenekian
In the geologic timescale, the Olenekian is an age in the Early Triassic epoch; in chronostratigraphy, it is a stage in the Lower Triassic series. It spans the time between Ma and Ma (million years ago). The Olenekian is sometimes divided into ...
stages of the
Early Triassic
The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which is a un ...
in what is now
South Africa
South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...
. It was incorrectly classified as a
dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
by
Sir Richard Owen
Sir Richard Owen (20 July 1804 – 18 December 1892) was an English biologist, comparative anatomist and paleontologist. Owen is generally considered to have been an outstanding naturalist with a remarkable gift for interpreting fossils.
Ow ...
in 1859.
Notably, ''Galesaurus'' was mentioned in the first issue of
Nature
Nature, in the broadest sense, is the physics, physical world or universe. "Nature" can refer to the phenomenon, phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. ...
in 1869, where
T. H. Huxley
Thomas Henry Huxley (4 May 1825 – 29 June 1895) was an English biologist and anthropologist specialising in comparative anatomy. He has become known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution.
The stor ...
erroneously expressed confidence that it would eventually be shown to be a dinosaur.
Description
![Galesaurus planiceps life restoration](https://upload.wikimedia.org/wikipedia/commons/0/0e/Galesaurus_planiceps_life_restoration.jpg)
The largest ''Galesaurus'' skull discovered is roughly long. Larger remains indicate that an adult ''Galesaurus'' is roughly long. Cynodonts, including ''Galesaurus,'' are believed to have had sprawling postures.
Examination of ''Galesaurus'' reveals two distinct morphs, a gracile and a robust morph. The main differences between the two morphs lie in the pectoral and pelvic girdles, as well as subtle differences in the fore and hind limbs. The morphological differences may be due to
sexual dimorphism
Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...
,
ontogeny
Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the stu ...
, or the presence of two subspecies.
Skull
The skull of ''Galesaurus'' is generally wide and low, the widest part being the region of the
zygomatic arch
In anatomy, the zygomatic arch, or cheek bone, is a part of the skull formed by the zygomatic process of the temporal bone (a bone extending forward from the side of the skull, over the opening of the ear) and the temporal process of the zygomati ...
es. The snout is blunt.
The
nasal bone
The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and by their junction, form the bridge of the upper one third of the nose.
Eac ...
s of ''Galesaurus'' are unusually large, they are constricted in the middle and extend over the anterior of the nostrils. A sheet of bone forms the septomaxilla that lines the floor of the
naris
A nostril (or naris , plural ''nares'' ) is either of the two orifices of the nose. They enable the entry and exit of air and other gasses through the nasal cavities. In birds and mammals, they contain branched bones or cartilages called turbi ...
and extends backwards between the nasal and
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. The t ...
. The skull has a septomaxillary foramen. The maxilla makes up a significant section of the lateral wall of the snout and contacts the lacrimal and
jugal
The jugal is a skull bone found in most reptiles, amphibians and birds. In mammals, the jugal is often called the malar or zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species.
Anatomy ...
posteriorly. Formina perforates the maxilla, especially in the area of the canine. Two large foramina, above the fifth and sixth postcanines, are present. The ascending process of the premaxilla dorsally overlaps the nasal. The anterior of the ascending process has a small opening call the anterior premaxillary foramen.
''Galesaurus'' has large, pentagonal shaped lacrimals with a flat outer surface. A fossa is present medial to the crista lacrimalis. This fossa is connected by two canals, one above the other, to the lacrimonasal canal that opens into the nasal cavity. The prefrontals extend halfway along the border of the lacrimals until they meet the
postorbitals at the middle of the upper border of the orbits, forming the upper orbital margin. As in other cynodonts, the frontal is excluded from the orbital margin by the prefrontal and postorbital.
Dentition
The upper teeth of ''Galesaurus'' are located on the alveolar ridges of the premaxilla and maxilla. Palatal teeth are absent in ''Galesaurus''.
The first postcanine of ''Galesaurus'' only has one cusp, while the other postcanine teeth are flattened and have two curved cusps. The second tooth contains a long anterior cusp and a short posterior cusp. The base of the incisors is wide, though the crown tapers to a point. The canines and postcanines have been pushed to the outer rim of the maxilla, allowing the lower teeth enough room to lie medially to the upper teeth when the jaw is closed. Postcanine tooth replacement is believed to have occurred throughout life.
The maxilla forms a large portion of the lateral wall of the snout. It is composed of a corpus, frontal process, zygomatic process, palatal process, and alveolar process. The corpus is long, inconspicuous, and encloses the sinus maxillaris cavity. The sinus maxillaris opens into the nasal cavity dorsal to the secondary palate and anterior to the palatine. It extends anteriorly, and continues to narrow until the cavity ends below the posterior elongation of the roots of the canine. This cavity is hypothesized to have served as the cynodont equivalent to the mammalian canalis alveolaris. There are two large foramina in the maxilla, above the fifth and sixth postcanines.
Discovery and naming
![Galesaurus block](https://upload.wikimedia.org/wikipedia/commons/3/3f/Galesaurus_block.png)
The first ''Galesaurus'' specimen was originally discovered in the
Karoo Basin
The Karoo Supergroup is the most widespread stratigraphy, stratigraphic unit in Africa south of the Kalahari Desert. The group (stratigraphy), supergroup consists of a sequence of units, mostly of nonmarine origin, deposited between the Pennsylvan ...
of South Africa and described by the naturalist Sir Richard Owen in 1859. Owen named the specimen ''Galesaurus planiceps'', but incorrectly classified ''Galesaurus'' as a new species of dinosaur. Owen assumed that ''Galesaurus'' was reptilian because its skull resembled ''
Rhopalodon
''Rhopalodon'' is an extinct genus of therapsids from the Permian of Russia. It has been variously classified as a dinosaur, a dinocephalian, or another branch of Reptilia. ''Rhopalodon'' is notable for being among the first reptiles mentioned in ...
'', a synapsid that had also been misclassified as a dinosaur. Despite classifying ''Galesaurus'' as a dinosaur, Owen noted that ''Galesaurus'' was remarkably mammal-like. Owen's ''Galesaurus'' type specimen was considerably crushed and the teeth were poorly preserved. Only recently have articulated skeletons of ''Galesaurus'' been found, whose well-preserved postcranial bones yield a better understanding of Galesaurus morphology; it is now considered to be a
cynodont
The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide variety ...
.
Other ''Galesaurus planiceps'' fossils that were initially identified as ''Glochinodon detinens'' in 1916 and ''Glochinodontoides gracilis'' in 1924 were synonymized subjectively with ''Galesaurus planiceps'' in 1972.
Classification
''Galesaurus'' is a member of the clade
Epicynodontia
Epicynodontia is a clade of cynodont therapsids that includes most cynodonts, such as galesaurids, thrinaxodontids, and Eucynodontia (including mammals). It was erected as a stem-based taxon by Hopson and Kitching (2001) and defined as the mo ...
, which is within the infraorder Cynodontia, the ancestor group of all mammals. ''Galesaurus'' is also a member of the family ''Galesauridae,'' which includes the closest relatives of ''Galesaurus, Cynosaurus'' and ''Progalesaurus.''
Paleobiology
Locomotion
During cynodont locomotion, the axial skeleton is unlikely to have flexed and extended in the sagittal plane as it does in mammals. Instead, cynodonts are believed to moved by
lateral undulation
Undulatory locomotion is the type of motion characterized by wave-like movement patterns that act to propel an animal forward. Examples of this type of gait include crawling in snakes, or swimming in the lamprey. Although this is typically the ...
, the typical axial movement of reptiles. The imbricating coastal plates in cynodonts may be analogous to the expanded ribs in certain
edentates
Xenarthra (; from Ancient Greek ξένος, xénos, "foreign, alien" + ἄρθρον, árthron, "joint") is a major clade of placental mammals native to the Americas. There are 31 living species: the anteaters, tree sloths, and armadillos. E ...
, which may represent musculoskeletal adaptation to adopt a more characteristically mammalian posture by lifting the trunk off the ground. Cynodonts are also believed to have had propulsive movements in the humerus, which are typical in mammal locomotion. The presence of both reptile and mammal features in cynodont locomotion is indicative of a transition between the two classes.
Comparison of ''Galesaurus'' to ''Thrinaxodon''
''Galesaurus'' is often compared with ''Thrinaxodon'', a more derived basal cynodont, because ''Thrinaxodon'' is the best known of all the ''Epicynodonts''.
''Galesaurus'' and ''Thrinaxodon'' are also very similar in morphology, are both from the early Triassic, and are both found in the Karoo Basin in South Africa. Though ''Galesaurus'' and ''Thrinaxodon'' are similar in appearance, they have a number of differences in their skulls. In ''Galesaurus,'' the zygomatic arch height has positive allometry, which indicates that older individuals of ''Galesaurus'' had larger and more developed masseter muscles than in Thrinaxodon. The development of the angulation of the zygomatic arch in the adult ''Galesaurus'' indicates that the superficial masseter muscle also became more developed in comparison to ''Thrinaxodon''. Timing of the development of the posterior sagittal crest occurs later in ''Galesaurus'' than it does in ''Thrinaxodon''. The posterior sagittal crest develops in ''Thrinaxodon'' in the late juvenile stage, while it only appears in the adult stage of ''Galesaurus.'' While all adult ''Thrinaxodon'' develop an anterior sagittal crest, the structure is absent in most ''Galesaurus'' specimens. The absence of the anterior sagittal crest indicates that the anterior fibers of the temporalis muscle not as developed in ''Galesaurus'' as they were in ''Thrinaxodon.'' Skull width, which indicates lateral expansion of the zygomatic arches, varies between ''Galesaurus'' and ''Thrinaxodon. Galesaurus'' has a positively allometric skull width, while skull width in isometric in ''Thrinaxodon.'' This indicates that ''Galesaurus'' had a more developed adductor musculature. When taking the differences in adductor musculature and the large medial shift of the mandible within the temporal fenestra, it is hypothesized that ''Galesaurus'' had highly developed masseters. The external occipital crest of ''Galesaurus'' increased during growth, though it was absent in juvenile ''Thrinaxodon,'' and poorly developed in the adults. The size of the external occipital crest indicates enlarged, stronger nuchal muscles in ''Galesaurus,'' but relatively weaker nuchal muscles in ''Thrinaxodon. Galesaurus'' also had a larger maximum skull size than ''Thrinaxodon.'' The presence of comparatively thicker peripheral lamellar tissue in ''Thrinaxodon'' and excelerated ontogenetic development of the posterior sagittal crest suggests that ''Galesaurus'' reached sexual maturity later than ''Thrinaxodon.''
During ontogeny, both ''Galesaurus'' and ''Thrinaxodon'' undergo changes in posterior projection of the postorbital, posterior sagittal crest, and external occipital crest. Ontogenetic changes that were unique to ''Galesaurus'' include a large shift in the relative position of the mandible within the temporal fenestra, a change in the ectocranial morphology of the nasal-nasal suture, fusion of the exocciptal with three surrounding occipital bones, and development of sexual dimorphism in adults. In contrast, the ontogenetic changes that were unique to ''Galesaurus'' include the presence of an interpterygoid vacuity in small juveniles, change in the ectocranial trace of the frontal-parietal suture, the changing shape of the parietal foramen, and obliteration of the posterior parietal-parietal suture.
See also
*
List of therapsids
This list of therapsids is an attempt to create a comprehensive listing of all genera that have ever been included in the Therapsida excluding mammals and purely vernacular terms. The list includes all commonly accepted genera, but also genera tha ...
References
{{Taxonbar, from=Q2757963
Prehistoric cynodont genera
Induan life
Olenekian life
Early Triassic synapsids of Africa
Triassic South Africa
Fossils of South Africa
Fossil taxa described in 1859
Taxa named by Richard Owen