Stereosternum

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Stereosternum
Temporal range:
Ma
Fossil in Naturmuseum Senckenberg
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: †Parareptilia
Order: †Mesosauria
Family: †Mesosauridae
Genus: †Stereosternum
Cope, 1885
Type species
†Stereosternum tumidum
Cope, 1885[1]
Synonyms

Mesosaurus tumidum (Cope, 1885)

Stereosternum tumidum (meaning "rigid chest") (Stereos, Greek: “solid, firm”; Sternon, Greek: “chest, breastbone”)

Mesosaurus tenuidens
.

For most of the 20th century, information of Stereosternum was reported as Mesosaurus.[4] Unlike previous interpretations of Mesosaurs as filter feeding animals, later studies have shown that these animals were very much active aquatic predators.[5] Stereosternum and Mesosaurus are the oldest reported reptile species to have had a range spanning two present-day continents, then joined as Gondwana and they represent the first record of reptile species shared by both Southern Africa and South America.[3]

Description

Restoration

Stereosternum was about 80 centimetres (2.6 ft) in length as an adult,

pachyostotic characteristic,[4]
which would have helped the animal to be able to dive deep in the water as a counteracting force against buoyancy.

Post-cranial skeleton

Ribs

One of the more unusual and unique characteristics about the anatomy of Stereosternum is the thickness of the dorsal ribs, which continues to increase in size until the fifth rib, which measures 47.5 millimetres (1.87 in) in width.[8] The ribs are noted here to be holocephalous.[4] With the pachyostosis of the postcrania elements and restricted movement at the appendicular joints, it has been suggested that mesosaurs could not have walked on land, but may have been able to push themselves across terrestrial substrates.[9] When studying the histology of the ribs of Stereosternum and Mesosaurus, the kind of pachyostosis of these two species has, is very much characterized by hyperostosis of the periosteal cortex, increased bone density, which was due to the bone becoming more compact, the absence of free medullary cavity and the remnants of calcified cartilage matrix. This kind of pachyostosis is called “pachyosteosclerosis”.[10]

Axial skeleton

Another characteristic of Stereosternum is the length of the tail and the amount of caudal vertebrae. As counted by Osborn, there are about 60-64

caudal vertebrae, which is very predictable for an aquatic predator and these adaptations helped individuals to push and swim gracefully in the water.[8] There are also about 34 presacral vertebrae, with 11 cervical and 23 thoracic-lumbar vertebrae.[6] The presence of accessory articulations, like the zygantra and the zygosphenes, are there to supposedly to have dampened the twisting movements of the vertebral column. On the axial pleurocentrum, there is a process that is distinctly analogous to the odontoid structure found in mammals. This probably correlates with the presence of a long skull on a long neck that is found with Stereosternum and this process could have helped to minimize the stresses and movement between the skull and the neck.[4]

Appendicular skeleton

Another characteristic of Mesosaurs was that they probably had webbing of skin on both their feet and their hands, analogous to that of a duck's foot. This paddle would have easily initiated movement within the water to swim and hunt for food.

Iliac blade has a rounded dorsal margin, and the pubic foramen is early closed.[12] The most distal end of the humerus is shown to be slightly less than 35 per cent of the total length of the humerus.[4]

Skull

Skull

Many of the modifications of the bones within the skull seem to reduce drag and help propel the animal through an aqueous environment while swimming and feeding.

homodont structures.[4]

Dentition

Stereosternum had small teeth compared to

dentary wall. These adaptations helped to increase the tooth's resistance to tension forces, especially the lateral forces.[5]

Discovery

Fossil in the Royal Ontario Museum

Stereosternum tumidum was discovered by

phalangeal formula, going from digits 2-6, as 4-4-4-3.[1]

Classification

Slab and counterslab of a specimen

Mesosaurs are a clade within the larger clade of

plesiomorphic for mesosaurs, whereas the characteristic of Mesosaurus living within a pelagic zone is derived.[clarification needed] The evolutionary history is seen as the first mesosaurs were much like Brazilosaurus in form, first conquering the aquatic environments on the coasts of the Ecca sea, and then creatures like Mesosaurus conquer life in the open ocean.[12]

Mesosauridae

Brazilosaurus

Stereosternum

Mesosaurus

Paleobiology

Diet

For many of the mesosaurid family, including Stereosternum, fossil evidence seems to suggest that their diet mainly included crustaceans. Evidence of large amounts of the

coprolites of mesosaurs have mainly come from the Mangrullo Formation of Uruguay.[citation needed] There has also been evidence of small mesosaurid bones within the coprolites of mesosaurs as well, suggesting that cannibalism within this group did occur. Capturing fish and small mesosaurs would have been easier to catch and swallow, since many of its prey would have been small or soft bodied animals. Post-molt soft bodied crustaceans would have also been prey for Stereosternum and other mesosaurs, although there is very little evidence of this in the fossil record.[5] The idea that Stereosternum used its marginal teeth to impale its prey seems very unlikely and probably used its teeth as a capturing mechanism to quickly entrap and restrain its prey.[15]

Reproduction

Fossil juvenile

There is ample evidence to suggest that mesosaurs may have been the oldest known amniotes that displayed extended embryo retention, which could have been either

Iliac blade, suggests that mesosaurs had the capability to move on dry land and to deposit their eggs on land, plausibly close to water. The females probably carried one to two embryos at a time and the hatchings are interpreted to be about 10% of the adult body length. When comparing mesosaurs and their size differences between the adults and hatchlings to how monotremes and marsupials raise their young, the idea that parental care of the offspring or of the eggs until hatching occurring in mesosaurs is up for speculation.[16]

Paleoecology

Mesosaurs have been found in the

Palaeoniscoid fishes, with the bottom half of the water column that could not support a benthic zone due to the toxic bottom brines. The lithology representing that bottom half of the water column are black shales and also carbonaceous oil is found in the same area.[citation needed
]

Stereosternum tumidum and Brazilosaurus sanpauloensis have been recently considered as nomina dubia and synonymized to Mesosaurus by Graciela Piñeiro, Jorge Ferigolo, Alvaro Mones and Pablo Nuñez Demarco (2021), after a detailed anatomical and taphonomic study of more than 500 specimens from palaeontological collections of several countries, which demonstrated that most of the characters used to distinguish three mesosaur taxa are indeed derived of the preservation of specimens in different ontogenetic stages and others are influenced by taphonomy. These results were also supported by morphometric analyses applied to specific region of the skeleton such as the vertebral column and limb bones by Pablo Nuñez Demarco, Jorge Ferigolo and Graciela Piñeiro (2022) and by an incredible similar article by Antoine Verrière and Jörg Fröbisch (2022), which was submitted posteriorly to the acceptation of the Nuñez Demarco et al. (2022) paper. Nuñez Demarco et al. (2022) found that juvenile and adult mesosaurs do not display appreciable change in bone proportions during growth, suggesting a surprising isometric pattern in mesosaurs which is a rare condition for reptiles. The isometry, and particularly, the high interrelation between metatarsals and phalanges permit us to suggest that the mesosaur hind limb is subject to notable modularity. This evidence strongly argues that the differences previously described to support three mesosaur species in Western Gondwana, might instead reflect natural intraspecific variability, taphonomic features or even possible sexual dimorphism, as recently suggested by Piñeiro et al. (2021).

References

  1. ^ a b COPE, E.D. 1886. A contribution to the vertebrate paleontology of Brasil. Proceedings of the American Philosophical Society 25, 7-15.
  2. ^ "Untitled Document." Untitled Document. N.p., n.d. Web. 04 Mar. 2017. http://www.paleofile.com/Anapsidalist/Stereosternum.asp
  3. ^ a b c Oelofsen, B. W. and D. C. Araujo. "Mesosaurus Tenuidens and Stereosternum Tumidum from the Permian Gondwana of Both Southern Africa and South America." South African Journal of Science, vol. 83, no. 6, 01 June 1987, pp. 370-372.
  4. ^ a b c d e f g h i Modesto, S.P. 1999. Observations on the structure of the Early Permian reptile Stereosternum tumidum Cope. Paleontologia Africana 35: 7–19.
  5. ^
    doi:10.4202/app.2011.0121
    .
  6. ^ a b Shikama, T. and Ozaki, H. 1966. On a reptilian skeleton from the Palaeozoic formation of San Paulo, Brazil. Transactions and Proceedings of the Palaeontological Society of Japan 64: 351–358.
  7. ^ Vieira, P.C., Mezzalira, S., and Ferreira, F.J.F. 1991. Mesossaurídeo (Stereosternum tumidum) e crustáceo (Liocaris huenei) no Membro Assistência da Formação Irati (P) nos municípios de Jataí e Montevidiu, Estado de Goiás. Revista Brasileira de Geociências 21: 224–235.
  8. ^ a b Osborn, Henry Fairfield. The reptilian subclasses Diapsida and Synapsida and the early history of the Diaptosauria. Vol. 1. Knickerbocker Press, 1903.
  9. ^ a b c Modesto, S. P. "The anatomy and historical ecology of Mesosaurs, the oldest amniotes known from Gondwana." Journal of African Earth Sciences 27.1 (1998): 138-138.
  10. ^ Timm, L. L., & Araujo Barberena, D. C. (1996). Preliminary observations on the pachyostosis of the ribs of the mesosaurs (Proganosauria). Anais Da Academia Brasileira De Ciencias, 68(2), 288.
  11. ^ ROSLER, O. & TATIZANA, C. 1989. As membranas natatorias em Stereosternum tumidum. In: DE ALMEIDA CAMPOS, D. (Ed.), Coletanea de Trabalhos Paleontologicos, Serie Geologia (Brazil) 27,129-131.
  12. ^ a b Pineiro, G.; Ferigolo, J. (2004). "The Presence of Mesosaurus Tenuidens Gervais and Stereosternum Tumidum Cope in the Lower Permian Mangrullo Member (Melo Formation) of Uruguay". Ameghiniana. 41 (4, Suppl. 1): 59R.
  13. ^ Araújo, D.C. 1976. Taxonomia e Relações dos Proganosauria da Bacia do Paraná. Anais da Academia Brasileira de Ciências 48: 91–116.
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  15. S2CID 52036785
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  16. S2CID 59475679
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  17. doi:10.25249/0375-7536.19831310106
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  18. ^ Oelofsen B.W. (1981). An anatomical and systematic study of the family Mesosauridae (Reptilia, Proganosauria) with special reference to its associated fauna and palaeoecological environment in the Whitehill sea. PhD thesis, University of Stellenbosch.

Piñeiro, G., Ferigolo, J., Mones, A. & Núñez Demarco, P. 2021. Mesosaur taxonomy: Are Stereosternum and Brazilosaurus valid taxa?. Revista Brasileira de Paleontologia, 24(3):205–235. Núñez Demarco, P., Ferigolo, J., and Piñeiro, G. 2022. Isometry in mesosaurs: Implications for growth patterns in early amniotes. Acta Palaeontologica Polonica 67 (2): 509–542. https://doi.org/10.4202/app.00931.2021 Verrière, A. and Fröbisch, J. Ontogenetic, dietary, and environmental shifts in Mesosauridae. PeerJ 10:e13866. DOI: 10.7717/peerj.13866

External links

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