Mesenosaurus

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Mesenosaurus
Temporal range: Permian, 289–265  Ma
Mesenosaurus romeri skeleton.JPG
Mesenosaurus romeri
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Family:† Varanopidae
Subfamily:† Mycterosaurinae
Genus:† Mesenosaurus
Efremov, 1938
Type species
†Mesenosaurus romeri
Efremov, 1938
Species
  • M. romeriEfremov, 1938
  • M. efremoviMaho, Gee et Reisz, 2019

Mesenosaurus is an extinct genus of synapsid belonging to the family Varanopidae. This genus includes two species: the type species Mesenosaurus romeri from the middle Permian (upper Kazanian) Mezen River Basin of northern Russia, and Mesenosaurus efremovi from the early Permian (Artinskian) Richards Spur locality (Oklahoma, United States). [1] M. romeri’s stratigraphic range is the middle to late Guadalupian while M. efremovi’s stratigraphic range is the Cisuralian. [1]

Contents

Etymology

Famous Russian paleontologist, Ivan Efremov, established Mesenosaurus as a genus. The genus name means “lizard from Mezen” while the specific epithet is given in honor of Alfred Romer.

Mesenosaurus efremovi was named in honor of Ivan Efremov, who erected the genus. [1]

Description

Mesenosaurus are small sized varanopid synapsids. They are characterized by mainly cranial features. [2] Many of the postcranial features of this genus have not been analyzed fully due to a lack of fossil evidence.

Skull

Mesenosaurus romeri skull illustration Mesenosaurus romeri skull.jpg
Mesenosaurus romeri skull illustration

The cranial features that characterize Mesenosaurus are:

  1. Slender premaxilla forms a narrow, rectangular snout (from dorsal and ventral views)
  2. Dorsal process of premaxilla is long and forms anterior half of dorsal margin of external naris
  3. Expanded narial shelf that extends almost to the snout tip
  4. Palatal process of premaxilla is long with long median suture
  5. Well-developed depression on the lateral surface of the nasal that extends posteriorly from the narial border to nearly the anterior end of the prefrontal
  6. Slight lateral swelling of the maxilla at the level of the caniniform tooth
  7. Short posterior process of the maxilla fails to reach the level of the postorbital bar
  8. First premaxillary tooth smaller than the second and third teeth. (There is a maximum of five teeth on the premaxilla. The first tooth is smaller than the second and third, but larger than the last two. These five teeth are closely spaced, strongly recurved, sharply pointed, and have a sharp, cutting edge along the distal half of the posterior edge.)
  9. Single, median vomerine tooth row
  10. Postorbital cheek region of skull unusually broad and low, with nearly vertical posterior margin
  11. Posterior edge of transverse flange of the pterygoid is angled slightly anterolaterally from basal articulation.
  12. Stapes slender, short, and rod-like, with modestly developed footplate and distally expanded quadrate process. (The stapes of Mesenosaurus more closely resembles those in early therapsids of the Permian.)
  13. A very prominent, nodular ornamentation is present on the orbital margins of the prefrontal, postorbital, and jugal. (This characteristic is strikingly pronounced in well-preserved skulls of Mesenosaurus, so much so that Efremov originally considered it to be an autapomorphy of Mesenosaurus. However, careful examination and analysis have shown that this feature is present in other varanopids.) [2]

Discovery

Mesenosaurus romeri

Mesenosaurus romeri skeleton Paleontologicheskii muzei Orlova (20221008142948).jpg
Mesenosaurus romeri skeleton

The Mezen River basin is located in northern Russia and has extensive exposures of Middle Permian sediments along the edges of affluent rivers of the Mezen River. These sediments have produced many skeletal remains of diverse amniotes, but most importantly, a partial skull of “a small synapsid of varanopseid affinities”, Mesenosaurus romeri. [2] M. romeri was discovered in 1938 by Ivan Efremov and became the first species of Mesenosaurus, due to its lack of cranial similarities to others. It was also the first synapsid described from the Russian area to be considered a “good pelycosaur”, as it possessed upper jaw teeth that were consistent with other known pelycosaurs (slender, recurved, enlarged incisors, single caniniform tooth). Romer and Price hypothesized that M. romeri belonged to Varanopseidae. [3] This hypothesis was confirmed in 2001 based on the following synapomorphies:

  1. Dorsoventral expanded temporal fenestra occupies most of the height of temporal region, resulting in narrow subtemporal bar
  2. Marginal dentition is composed of strongly recurved, laterally compressed, sharply pointed teeth front and back cutting edges restricted to the distal half of the tooth
  3. Well-developed premaxillary subnarial shelf whose external surface is broadly rounded in transverse section
  4. Anterior median process of parietal extends into the supraorbital region of skull table
  5. Small postfrontal medially borders narrow, posterior process of the frontal
  6. Tabular shrinks in size that become small, narrow elements that contacts medial margin of posterolateral wing of parietal
  7. Absence of medial, occipital flange of posterior margin of squamosal that covers posterior margin of quadrate
  8. Parasphenoid plate is broad and basisphenoidal tubera are wing like and extend far laterally and posteriorly from the base of cultriform process
  9. Prominent, nodular ornamentation is present on orbital margins of prefrontal, postorbital, and jugal [2]

Mesenosaurus efremovi

The second species of this genus is Mesenosaurus efremovi. Its nearly complete skull and mandible was discovered at Richards Spur locality within a series of infilled karst fissures in the Ordovician Arbuckle limestone in Oklahoma, which is one of the most plentiful sites for early Permian tetrapod fossils. [4]

In terms of classifying M. efremovi, it shares distinct cranial features with mycterosaurines (stem based group that includes Mycterosaurus longiceps and all varanopseids related more closely to it than to Varanodon agilis), such as the “exclusion of the lacrimal from the external naris and an anteroposteriorly broad dorsal lamina of the maxilla that underlies the nasal and contacts the prefrontal”. However, M. efremovi shares more features with M. romeri from Russia. Some of these shared features include relative size and shape of the temporal fenestra, lateral swelling of the maxilla in the caniniform region and five premaxillary tooth positions (not reported in other mycterosaurines). [1]

Though M. efremovi and M. romeri share many distinct features, there are four main morphological differences between these specimens that deem a taxonomic distinction at the species level (differences insufficient for distinction above species level):

  1. The presence of short dorsal premaxillary processes (that do not extend to either the posterior narial margin or the posterior separation of the premaxillae by the nasals)
  2. More posteriorly extensive maxilla
  3. Fewer tooth positions on the maxilla
  4. Contact between the postorbital and supratemporal bones

M. efremovi is also larger than the largest known specimen of M. romeri. [1]

Phylogeny

Below is a cladogram modified from the analysis of Benson, after the exclusion of Basicranodon: [5]

Paleoenvironment

There is a 20 Myr gap between these two species, which exceeds the temporal range of most extinct tetrapods. Though this gap is questionable, the radioisotopic dating of speleothems, recovered for Richards Spur, used to identify this 20 Myr gap is a reliable technique that has been used to identify other large gaps/long temporal ranges. [6] This gap is significant, as it indicates evolutionary stasis (persisting throughout different environments across Pangea as well as faunal turnovers throughout the Permian-specifically Olson’s gap). This evolutionary stasis may be attributed to a conserved niche occupation throughout their temporal and geographical ranges. Further research on postcranial features is required in order to determine if this hypothesis is correct, as we would need to observe a similar degree of stasis throughout the entire skeleton. [1]

Paleobiology

Mesenosaurus was a small mobile creature, capable of climbing rocks/trees. [7] Their small size allowed them to occupy and thrive in niches as small faunivores while therapsids dominated most terrestrial environments. It was not until the late Permian when small diapsids appeared [8] and provided competition, leading to a decline in Mesenosaurus. Mesenosaurus represents a guild of highly agile subordinate predators [1] [9] in their communities due to their large, slender, curved teeth, which could cause severe wounds when piercing its prey. [10] Mesenosaurus was also characterised by extremely rapid rates of tooth development and greatly reduced tooth longevity compared to almost every other land-dwelling amniote. [11]

They possessed unbent and flattened unguals, suggesting they were diggers. It is uncertain whether their digging unguals were adapted for burrowing or solely digging for food. [10] Many varanopids were arboreal, however the well-developed olecranon(bony prominence of the elbow) of Mesenosaurus indicates the presence of triceps and anconeus muscle, [12] [13] [14] both of which would provide powerful forearm extension. This forearm extension strength combined with its somewhat small/medium body size supports the idea of a burrowing lifestyle. [10] It is also proposed that facultative bipedalism occurred in Mesenosaurus. This is based on the presence of a rearward shift of center of body mass (slender trunks, elongated hindlimbs, and short forelimbs) that is necessary for facultative bipedalism. [10]

See also

Related Research Articles

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<span class="mw-page-title-main">Sphenacodontia</span> Clade of synapsids

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<span class="mw-page-title-main">Eupelycosauria</span> Clade of synapsids

Eupelycosauria is a large clade of animals characterized by the unique shape of their skull, encompassing all mammals and their closest extinct relatives. They first appeared 308 million years ago during the Early Pennsylvanian epoch, with the fossils of Echinerpeton and perhaps an even earlier genus, Protoclepsydrops, representing just one of the many stages in the evolution of mammals, in contrast to their earlier amniote ancestors.

<span class="mw-page-title-main">Caseasauria</span> Extinct clade of synapsids

Caseasauria is one of the two main clades of early synapsids, the other being the Eupelycosauria. Caseasaurs are currently known only from the Late Carboniferous and the Permian, and include two superficially different families, the small insectivorous or carnivorous Eothyrididae, and the large, herbivorous Caseidae. These two groups share a number of specialised features associated with the morphology of the snout and external naris.

Varanopidae is an extinct family of amniotes known from the Late Carboniferous to Middle Permian that resembled monitor lizards and may have filled a similar niche. Typically, they are considered to be relatively basal synapsids, although some studies from the late 2010s recovered them being taxonomically closer to diapsid reptiles, recent studies from the early 2020s support their traditional placement as synapsids on the basis of high degree of bone labyrinth ossification, maxillary canal morphology and phylogenetic analyses. A varanopid from the latest Middle Permian Pristerognathus Assemblage Zone is the youngest known varanopid and the last member of the "pelycosaur" group of synapsids.

<i>Tetraceratops</i> Extinct genus of synapsids

Tetraceratops insignis is an extinct synapsid from the Early Permian that was formerly considered the earliest known representative of Therapsida, a group that includes mammals and their close extinct relatives. It is known from a single 90-millimetre-long (3.5 in) skull, discovered in Texas in 1908. According to a 2020 study, it should be classified as a primitive non-therapsid sphenacodont rather than a genuine basal therapsid.

<i>Mycterosaurus</i> Extinct genus of tetrapods

Mycterosaurus is an extinct genus of amniotes belonging to the family Varanopidae. It is classified in the varanopid subfamily Mycterosaurinae. Mycterosaurus is the most primitive member of its family, existing from 290.1 to 272.5 MYA, known to Texas and Oklahoma. It lacks some features that its advanced relatives have.

<i>Eothyris</i> Extinct genus of synapsids

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<i>Cotylorhynchus</i> Extinct genus of synapsids

Cotylorhynchus is an extinct genus of herbivorous caseid synapsids that lived during the late Lower Permian (Kungurian) and possibly the early Middle Permian (Roadian) in what is now Texas and Oklahoma. The large number of specimens found make it the best-known caseid. Like all large herbivorous caseids, Cotylorhynchus had a short snout sloping forward and very large external nares. The head was very small compared to the size of the body. The latter was massive, barrel-shaped, and ended with a long tail. The limbs were short and robust. The hands and feet had short, broad fingers with powerful claws. The barrel-shaped body must have housed large intestines, suggesting that the animal had to feed on a large quantity of plants of low nutritional value. Caseids are generally considered to be terrestrial, though a semi-aquatic lifestyle has been proposed by some authors. The genus Cotylorhynchus is represented by three species, the largest of which could reach more than 6 m in length. However, a study published in 2022 suggests that the genus may be paraphyletic, with two of the three species possibly belonging to separate genera.

Angelosaurus is an extinct genus of herbivorous caseid synapsids that lived during the late Lower Permian (Kungurian) and early Middle Permian (Roadian) in what is now Texas and Oklahoma. Like other herbivorous caseids, it had a small head, large barrel-shaped body, long tail, and massive limbs. Angelosaurus differs from other caseids by the extreme massiveness of its bones, particularly those of the limbs, which show a strong development of ridges, processes, and rugosities for the attachment of muscles and tendons. Relative to its body size, the limbs of Angelosaurus were shorter and wider than those of other caseids. The ungual phalanges looked more like hooves than claws. The few known cranial elements show that the skull was short and more robust than that of the other representatives of the group. Angelosaurus is also distinguished by its bulbous teeth with shorter and wider crowns than those of other caseids. Their morphology and the high rate of wear they exhibit suggests a diet quite different from that of other large herbivorous caseids, and must have been based on particularly tough plants. A study published in 2022 suggests that the genus may be paraphyletic, with Angelosaurus possibly only represented by its type species A. dolani.

<i>Ennatosaurus</i> Extinct genus of synapsids

Ennatosaurus is an extinct genus of caseid synapsid that lived during the Middle Permian in northern European Russia. The genus is only represented by its type species, Ennatosaurus tecton, which was named in 1956 by Ivan Antonovich Efremov. The species is known from at least six skulls associated with their lower jaws, as well as from the postcranial bones of several juvenile individuals. Ennatosaurus has the typical caseid skull with a short snout tilted forward and very large external nares. However, it differs from other derived caseids by its postcranial skeleton with smaller proportions compared to the size of the skull. As with other advanced caseids, the teeth of Ennatosaurus were well suited for slicing and cutting vegetation. The presence of a highly developed hyoid apparatus indicates the presence of a massive and mobile tongue, which had to work in collaboration with the palatal teeth during swallowing. With a late Roadian - early Wordian age, Ennatosaurus is one of the last known caseids.

<i>Limnoscelis</i> Genus of diadectomorphs

Limnoscelis was a genus of large diadectomorph tetrapods from the Late Carboniferous to early Permian of western North America. It includes two species: the type species Limnoscelis paludis from New Mexico, and Limnoscelis dynatis from Colorado, both of which are thought to have lived concurrently. No specimens of Limnoscelis are known from outside of North America. Limnoscelis was carnivorous, and likely semiaquatic, though it may have spent a significant portion of its life on land. Limnoscelis had a combination of derived amphibian and primitive reptilian features, and its placement relative to Amniota has significant implications regarding the origins of the first amniotes.

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<i>Colobomycter</i> Extinct genus of reptiles

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<i>Aerosaurus</i> Extinct genus of tetrapod

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<i>Elliotsmithia</i> Extinct genus of tetrapods

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<i>Acleistorhinus</i> Extinct genus of reptiles

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Microleter is an extinct genus of basal procolophonomorph parareptiles which lived in Oklahoma during the Early Permian period. The type and only known species is Microleter mckinzieorum. Microleter is one of several parareptile taxa described from the Richards Spur fissure fills, and can be characterized from its high tooth count, lacrimal/narial contact, short postfrontal, and slit-like temporal emargination edged by the postorbital, jugal, squamosal, and quadratojugal. Contrary to Australothyris, which had a similar phylogenetic position as a basal procolophonomorph, Microleter suggests that early parareptile evolution occurred in Laurasia and that multiple lineages developed openings or emarginations in the temporal region.

<i>Kenomagnathus</i> Extinct genus of synapsids

Kenomagnathus is a genus of synapsid belonging to the Sphenacodontia, which lived during the Pennsylvanian subperiod of the Carboniferous in what is now Garnett, Kansas, United States. It contains one species, Kenomagnathus scottae, based on a specimen consisting of the maxilla and lacrimal bones of the skull, which was catalogued as ROM 43608 and originally classified as belonging to "Haptodus" garnettensis. Frederik Spindler named it as a new genus in 2020.

References

  1. 1 2 3 4 5 6 7 Sigi Maho; Bryan M. Gee; Robert R. Reisz (2019). "A new varanopid synapsid from the early Permian of Oklahoma and the evolutionary stasis in this clade". Royal Society Open Science. 6 (10): Article ID 191297. doi:10.1098/rsos.191297. PMC   6837192 . PMID   31824730.
  2. 1 2 3 4 Reisz, Robert R. (2001). The skull of Mesenosaurus romeri, a small varanopseid (Synapsida: Eupelycosauria) from the Upper Permian of the Mezen River Basin, northern Russia. OCLC   633882190.
  3. ↑ ROMER, A. S.; PRICE, L. W. (1940), "Review of the Pelycosauria", 28 : Review of the Pelycosauria, Geological Society of America Special Papers, vol. 28, Geological Society of America, pp. 1–534, doi:10.1130/spe28-p1
  4. ↑ MacDougall, Mark J.; Tabor, Neil J.; Woodhead, Jon; Daoust, Andrew R.; Reisz, Robert R. (June 2017). "The unique preservational environment of the Early Permian (Cisuralian) fossiliferous cave deposits of the Richards Spur locality, Oklahoma". Palaeogeography, Palaeoclimatology, Palaeoecology. 475: 1–11. Bibcode:2017PPP...475....1M. doi:10.1016/j.palaeo.2017.02.019. ISSN   0031-0182.
  5. ↑ Benson, R.J. (2012). "Interrelationships of basal synapsids: cranial and postcranial morphological partitions suggest different topologies". Journal of Systematic Palaeontology. 10 (4): 601–624. doi:10.1080/14772019.2011.631042. S2CID   84706899.
  6. ↑ Woodhead, Jon; Reisz, Robert; Fox, David; Drysdale, Russell; Hellstrom, John; Maas, Roland; Cheng, Hai; Edwards, R. Lawrence (May 2010). "Speleothem climate records from deep time? Exploring the potential with an example from the Permian". Geology. 38 (5): 455–458. Bibcode:2010Geo....38..455W. doi:10.1130/g30354.1. hdl: 1959.13/931960 . ISSN   1943-2682.
  7. ↑ Spindler, Frederik; Werneburg, Ralf; Schneider, Joerg W.; Luthardt, Ludwig; Annacker, Volker; Rößler, Ronny (2018-03-15). "First arboreal 'pelycosaurs' (Synapsida: Varanopidae) from the early Permian Chemnitz Fossil Lagerstätte, SE Germany, with a review of varanopid phylogeny". PalZ. 92 (2): 315–364. doi:10.1007/s12542-018-0405-9. ISSN   0031-0220. S2CID   133846070.
  8. ↑ Anderson, Jason S.; Reisz, Robert R. (2004-03-25). "Pyozia mesenensis, a new, small varanopid (Synapsida, Eupelycosauria) from Russia: "pelycosaur" diversity in the Middle Permian". Journal of Vertebrate Paleontology. 24 (1): 173–179. Bibcode:2004JVPal..24..173A. doi:10.1671/1940-13. ISSN   0272-4634. S2CID   59023934.
  9. ↑ Modesto, Christian A. Sidor, Bruce, Sean (2001-12-01). "A second varanopseid skull from the Upper Permian of South Africa: implications for Late Permian 'pelycosaur' evolution". Lethaia. 34 (4): 249–259. doi:10.1080/002411601753292971. ISSN   0024-1164.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  10. 1 2 3 4 Spindler, Frederik; Werneburg, Ralf; Schneider, Jörg W. (2019-01-19). "A new mesenosaurine from the lower Permian of Germany and the postcrania of Mesenosaurus: implications for early amniote comparative osteology". PalZ. 93 (2): 303–344. doi:10.1007/s12542-018-0439-z. ISSN   0031-0220. S2CID   91871872.
  11. ↑ Maho, Tea; Maho, Sigi; Scott, Diane; Reisz, Robert R. (19 August 2022). "Permian hypercarnivore suggests dental complexity among early amniotes". Nature Communications . 13 (1): 4882. doi:10.1038/s41467-022-32621-5. ISSN   2041-1723. PMC   9391490 . Retrieved 18 November 2024.
  12. ↑ Haines, R.W. (1950). "The flexor muscles of the forearm and hand in lizards and mammals". Journal of Anatomy. 84(Pt 1) (Pt 1): 13–29. PMC   1273351 . PMID   17105089.
  13. ↑ Haines, R.W. (1939). "A revision of the extensor muscles in the forearm in tetrapods". Journal of Anatomy. 73(2) (Pt 1): 211–233. PMC   1273351 . PMID   17105089.
  14. ↑ Savage, Jay M.; Romer, Alfred Sherwood (1957-07-15). "Osteology of the Reptiles". Copeia. 1957 (2): 162. doi:10.2307/1439431. ISSN   0045-8511. JSTOR   1439431.
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