Mycterosaurus
Mycterosaurus | |
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Life restoration of Mycterosaurus longcipes | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Family: | †Varanopidae |
Subfamily: | † Mycterosaurinae
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Genus: | †Mycterosaurus Williston, 1915 |
Type species | |
†Mycterosaurus longiceps Williston, 1915
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Mycterosaurus (Greek as mykter/mykteros meaning nose/snout, sauros meaning “lizard” It lacks some features that its advanced relatives have.
Mycterosaurus is a relatively small carnivore, estimated to be around 60 cm (23 inches) long with synonyms of Eumatthevia bolli, and possibly Basicranodon fortsillensis.[2] Restored, Mycterosaurus appears spindly and grotesque in contrast to the majority of "pelycosaurs" in its proportions and especially unlike edaphosaurids, which are commonly stocky in build.[5]
The number of valid Mycterosaurus species have varied over the years, with a total of two classifications of Mycterosaurus longiceps[3] and Mycterosaurus smithae. [6] However, recent analysis has led to a re-description of Mycterosaurus smithae.[7]
Description
Skull
Both the
Dentition
Mycterosaurus possesses 18-20 maxillary teeth
Post-cranial Skeleton
The vertebrae are similar to that of Varanops, with a thin spine that was no more than 2-3 times in height the centrum.[3] While the cervicals are unknown due to poor fossil records, details from the posterior aspect are somewhat preserved.[5] The neural spine is low and broad in both anterior and posterior direction.[5] The centra has rounded ends and no ventral keel, possessing a characteristic edaphosaurioid spool shape.[5] Additionally, the dorsal centra is moderately elongate (5 units in length). In contrast, the lumbar centras are much shorter.[5]
The scapula is short and not particularly broad, in contrast to the exceptionally broad procoracoid plate, a feature of edaphosaurids.[5] The glenoid surface is short, and the supraglenoid foramen is absent.[5] The pelvis has extreme elongation at the anterior end of the iliac blade with the anterior expansion being greater than the posterior.[5] The humerus and hind leg bones[7] are slender, with no ectepicondylar foramen.[5]
The astragalus is L-shaped and the centrum is circular when viewed in ventral view. These observations are consistent with most
Discovery
The first Mycterosaurus skull ever discovered was that of Mycterosaurus longiceps. The holotype (FMNH-UC 692) was discovered by Mr. Herman Douthitt in 1915 at a deposit of the[3] Lower Permian (Leonardian) Clyde Formation of north-central Texas.[11] Samuel Wendell Williston analyzed the holotype, describing the skull and other fragmented portions of the skeleton in his publication A New Genus and Species of American Theromorpha.[3]
In 1930, R Broom identified an unstudied fossil collected by Jacob Boll at the American Museum that he believed had been wrongly labeled by collectors as a small labyrinthodont.[8] The fossil, AMNH 7002, consisted of a fragmentary skull and partially crushed skeleton.[8][5] Broom named the fossil Eumatthevia Bolli after the late American paleontologist Professor W.D. Matthew. Broom noted that the skull of the fossil appeared similar to that of other primitive theromorphs such as Glaucosaurus and Mycterosaurus, but that it appeared more slenderly built and presented a flatter skull compared to Mycterosaurus.[8] Despite this difference, an independent junior author’s restoration differed in no aspects from Mycterosaurus except that the skull was lower, a difference attributed to crushing.[5] As such, Romer concluded that Eumatthevia bolli was surely a synonym of Mycterosaurus longiceps.[5][11]
In 1940, Romer and Price reviewed both aforementioned fossil records in their review of pelycosaurs.[5] The authors note both specimens were affected by different types of crushing, making it difficult to accurately assess the true nature of the skull.[5] However, Romer and Price estimate the true proportions were likely an intermediate between the narrow shape Williston observed[3] and the broad low type restored by Broom.[8][5] The authors failed to observe contacts between the lacrimal and jugal, ventral of the orbit, as described by Williston and Broom.[5] Additionally, the authors believe defining features of height, pineal size, and teeth differentiated “Eumatthevia” and “Mycterosaurus” were inaccurate. Instead, these differences were likely due to crushing and inaccuracies of measurement by Williston.[5] There is, however, agreement on the large size of the quadratojugal and orbits through all reports.
In 1957, Peter Paul Vaughn published a paper describing the features of a pelycosaur named Basicranodon fortsillensis that he believed carried very similar features to the Caseidae. However, Romer had previously established that Mycterosaurus should be classified as an edaphosaur. In 1966, the US Geographic Survey published a paper stating that Basicranodon fortsillensis could well belong to Mycterosaurus if better preserved specimens were ever discovered.[6]
In 1953, a new fossil (MCZ 2985), was discovered in Colorado and in 1964 named by Lewis and Vaughn as a new species that they called Mycterosaurus smithae, after Mrs Stockton Smith.[6] Features on MCZ 2985 such as the measurements of the orbit, temporal region, interorbital width, parietal region, and posteroventral corner of the cheek that matched that of Mycterosaurus longiceps led Lewis and Vaughn to their designation of a new Mycterosaurus species.[6]
However, a reexamination conducted by Brocklehurst et al (2016) using synchrotron radiation micro-computed tomography revealed observations that prompted the authors to reclassify Mycterosaurus smithae into genus Vaughnictis.[7] The additional preparation and synchrotron scanning showed a lack of slender femur, serrated lateral dentition, teeth present on the coronoid, or a lateral boss on the postorbital, these being the most unambiguous varanopid and Mycterosaurine synapomorphies.[7]
Classification
At the time of discovery of the holotype Mycterosaurus longiceps in 1915, it was believed by Mr. Herman Douthitt that the holotype belonged to the genus
Romer and Price, via their observations of Eumatthevia Bolli and the holotype, concluded that Mycterosaurus longiceps belonged as a primitive form of edaphosaurs.[5] Romer and Price believed that while M. longiceps did not present some features characteristic of advanced edaphosaurs, M. longiceps most certainly exhibited a number of key characteristics that point to its classification as a edaphosaur.[5] Specifically, Romer and Price created the Edaphosauria suborder family Nitosauridae within which genus Mycterosaurus and Nitosaurus reside.[5]
In 1982, Berman and Reisz rejected the family Nitosauride and suggested Mycterosaurus longiceps as a primitive member of the family Varanopidae.[11] Berman and Reisz believed the most important edaphosaur features cited by Romer and Price were mistakenly recorded from an isolated specimen piece that had been confused and misidentified.[11] Rather, Berm and Reisz concluded the misidentified specimen was not a pelycosaur as Romer and Price had believed, but rather a temnospondyl amphibian.[11] As a result, Berman and Reisz re-examined all known M. longiceps specimens which led them to the conclusion that Mycterosaurus is best interpreted as a member of the family Varanopidae.
Phylogenetic analysis of varanopoid interrelationships in 2006 placed Mycterosaurus under Mycterosaurinae with a sister genus of Mesenosaurus.[4][12]
Below is a cladogram modified from the analysis of Benson (in press), after the exclusion of Basicranodon:[13]
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