| Galesaurus | |
|---|---|
 | |
| Specimen in a block | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Therapsida |
| Clade: | Cynodontia |
| Family: | †Galesauridae |
| Genus: | †Galesaurus Owen, 1859 |
| Type species | |
| Galesaurus planiceps Owen, 1859 | |
Galesaurus (from the Greek roots for 'weasel' and 'lizard') is anextinctgenus of carnivorouscynodonttherapsid that lived between theInduan and theOlenekian stages of theEarly Triassic in what is nowSouth Africa.[1] It was incorrectly classified as adinosaur bySir Richard Owen in 1859.
Notably,Galesaurus was mentioned in the first issue ofNature in 1869, whereT. H. Huxley erroneously expressed confidence that it would eventually be shown to be a dinosaur.[2]
The largestGalesaurus skull discovered is roughly 12 centimetres (4.7 in) long. Larger remains indicate that an adultGalesaurus is roughly 75 to 80 centimetres (30 to 31 in) long.[citation needed] Cynodonts, includingGalesaurus, are believed to have had sprawling postures.[3]
Examination ofGalesaurus 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 tosexual dimorphism,ontogeny, or the presence of two subspecies.[4]
The skull ofGalesaurus is generally wide and low, the widest part being the region of thezygomatic arches. The snout is blunt.[5]
Thenasal bones ofGalesaurus 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 thenaris and extends backwards between the nasal andmaxilla. The skull has a septomaxillary foramen. The maxilla makes up a significant section of the lateral wall of the snout and contacts thelacrimal andjugal posteriorly.[6] Formina perforates the maxilla, especially in the area of thecanine. Two large foramina, above the fifth and sixth postcanines, are present. The ascending process of thepremaxilla dorsally overlaps the nasal. The anterior of the ascending process has a small opening call the anterior premaxillary foramen.[4]
Galesaurus has large, pentagonal shaped lacrimals with a flat outer surface. A fossa is present medial to thecrista 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 thepostorbitals 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.[4]
The teeth ofGalesaurus were attached to the jaws viagomphoses.[7] The upper teeth ofGalesaurus are located on the alveolar ridges of the premaxilla and maxilla.Palatal teeth are absent inGalesaurus.[5] The first postcanine ofGalesaurus 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 theincisors 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.[4] In the anterior dentition, meanwhile, incisors were also continuously replaced throughout life; canine replacement, however, ceased upon the onset of skeletal maturity.[8]
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.[5]
The firstGalesaurus specimen was originally discovered in theKaroo Basin of South Africa and described by the naturalist Sir Richard Owen in 1859. Owen named the specimenGalesaurus planiceps, but incorrectly classifiedGalesaurus as a new species of dinosaur. Owen assumed thatGalesaurus was reptilian because its skull resembledRhopalodon, a synapsid that had also been misclassified as a dinosaur. Despite classifyingGalesaurus as a dinosaur, Owen noted thatGalesaurus was remarkably mammal-like. Owen'sGalesaurus type specimen was considerably crushed and the teeth were poorly preserved. Only recently have articulated skeletons ofGalesaurus been found, whose well-preserved postcranial bones yield a better understanding of Galesaurus morphology; it is now considered to be acynodont.[4][9]
OtherGalesaurus planiceps fossils that were initially identified asGlochinodon detinens in 1916 andGlochinodontoides gracilis in 1924 were synonymized subjectively withGalesaurus planiceps in 1972.[9][10]
Galesaurus is a member of the cladeEpicynodontia, which is within the infraorder Cynodontia, the ancestor group of all mammals.Galesaurus is also a member of the familyGalesauridae, which includes the closest relatives ofGalesaurus, Cynosaurus andProgalesaurus.[11]
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 have moved bylateral undulation, the typical axial movement of reptiles. The imbricating coastal plates in cynodonts may be analogous to the expanded ribs in certainedentates, 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.[3]
Galesaurus is often compared withThrinaxodon, a more derived basal cynodont, becauseThrinaxodon is the best known of all theEpicynodonts.[4]Galesaurus andThrinaxodon are also very similar in morphology, are both from the early Triassic, and are both found in the Karoo Basin in South Africa. ThoughGalesaurus andThrinaxodon are similar in appearance, they have a number of differences in their skulls. InGalesaurus, the zygomatic arch height has positive allometry, which indicates that older individuals ofGalesaurus had larger and more developed masseter muscles than in Thrinaxodon. The development of the angulation of the zygomatic arch in the adultGalesaurus indicates that the superficial masseter muscle also became more developed in comparison toThrinaxodon. Timing of the development of the posterior sagittal crest occurs later inGalesaurus than it does inThrinaxodon. The posterior sagittal crest develops inThrinaxodon in the late juvenile stage, while it only appears in the adult stage ofGalesaurus. While all adultThrinaxodon develop an anterior sagittal crest, the structure is absent in mostGalesaurus specimens. The absence of the anterior sagittal crest indicates that the anterior fibers of the temporalis muscle not as developed inGalesaurus as they were inThrinaxodon. Skull width, which indicates lateral expansion of the zygomatic arches, varies betweenGalesaurus andThrinaxodon. Galesaurus has a positively allometric skull width, while skull width in isometric inThrinaxodon. This indicates thatGalesaurus 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 thatGalesaurus had highly developed masseters. The external occipital crest ofGalesaurus increased during growth, though it was absent in juvenileThrinaxodon, and poorly developed in the adults. The size of the external occipital crest indicates enlarged, stronger nuchal muscles inGalesaurus, but relatively weaker nuchal muscles inThrinaxodon. Galesaurus also had a larger maximum skull size thanThrinaxodon. The presence of comparatively thicker peripheral lamellar tissue inThrinaxodon and excelerated ontogenetic development of the posterior sagittal crest suggests thatGalesaurus reached sexual maturity later thanThrinaxodon.[12]
During ontogeny, bothGalesaurus andThrinaxodon undergo changes in posterior projection of the postorbital, posterior sagittal crest, and external occipital crest. Ontogenetic changes that were unique toGalesaurus 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 toGalesaurus 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.[12]
{{cite journal}}:Cite journal requires|journal= (help){{cite journal}}: CS1 maint: multiple names: authors list (link)
| |||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||
