Sauropsid

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Sauropsida
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Sauropsids
Temporal range:
Ma
Clockwise from top left:
captorhinid
eureptile)
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Superclass: Tetrapoda
Clade: Reptiliomorpha
Clade: Amniota
Clade:
Sauropsida
Watson
, 1956
Subclades

Sauropsida (

theropod dinosaurs, are nested within reptiles as more closely related to crocodilians than to lizards or turtles).[1] The most popular definition states that Sauropsida is the sibling taxon to Synapsida, the other clade of amniotes which includes mammals as its only modern representatives. Although early synapsids have historically been referred to as "mammal-like reptiles", all synapsids are more closely related to mammals than to any modern reptile. Sauropsids, on the other hand, include all amniotes more closely related to modern reptiles than to mammals. This includes Aves (birds), which are now recognized as a subgroup of archosaurian reptiles despite originally being named as a separate class in Linnaean taxonomy
.

The base of Sauropsida forks into two main groups of "reptiles": Eureptilia ("true reptiles") and Parareptilia ("next to reptiles"). Eureptilia encompasses all living reptiles (including birds), as well as various extinct groups. Parareptilia is typically considered to be an entirely extinct group, though a few hypotheses for the origin of turtles have suggested that they belong to the parareptiles. The clades Recumbirostra and Varanopidae, traditionally thought to be lepospondyls and synapsids respectively, may also be basal sauropsids. The term "Sauropsida" originated in 1864 with Thomas Henry Huxley,[2] who grouped birds with reptiles based on fossil evidence.

History of classification

Huxley and the fossil gaps

Archaeopteryx lithographica
, a historically important fossil which helped to establish birds as a component of the reptile family tree

The term Sauropsida ("lizard faces") has a long history, and hails back to

synapsids) like Dicynodon among the sauropsids. Thus, under the original definition, Sauropsida contained not only the groups usually associated with it today, but also several groups that today are known to be in the mammalian side of the tree.[4]

Sauropsids redefined (Goodrich, 1916)

By the early 20th century, the fossils of

Araeoscelida in the Theropsida. [4]

Detailing the reptile family tree

In 1956,

This classification supplemented, but was never as popular as, the classification of the reptiles (according to

monophyletic
group containing the traditional reptiles and the birds.

Cladistic definitions

Reptilia. Both are superimposed on a cladogram of Tetrapods
, showing the difference in coverage.

The class Reptilia has been known to be an

extant reptiles and birds. A number of phylogenetic stem, node and crown definitions have been published, anchored in a variety of fossil and extant organisms, thus there is currently no consensus of the actual definition (and thus content) of Sauropsida as a phylogenetic unit.[10]

Some taxonomists, such as Benton (2004), have co-opted the term to fit into traditional rank-based classifications, making Sauropsida and Synapsida class-level taxa to replace the traditional Class Reptilia, while Modesto and Anderson (2004), using the PhyloCode standard, have suggested replacing the name Sauropsida with their redefinition of Reptilia, arguing that the latter is by far better known and should have priority.[10]

Cladistic definitions of Sauropsida include:

Evolutionary history

Mesozoic sauropsids: non-avialandinosaurs (Europasaurus and iguanodonts) alongside the early bird Archaeopteryx perched on the foreground tree stump.

Sauropsids evolved from basal amniotes approximately 320 million years ago, in the Carboniferous Period of the Paleozoic Era. In the Mesozoic Era (from about 250 million years ago to about 66 million years ago), sauropsids were the largest animals on land, in the water, and in the air. The Mesozoic is sometimes called the Age of Reptiles. In the Cretaceous–Paleogene extinction event, the large-bodied sauropsids died out in the global extinction event at the end of the Mesozoic era. With the exception of a few species of birds, the entire dinosaur lineage became extinct; in the following era, the Cenozoic, the remaining birds diversified so extensively that, today, nearly one out of every three species of land vertebrate is a bird species.

Phylogeny

The cladogram presented here illustrates the "family tree" of sauropsids, and follows a simplified version of the relationships found by M.S. Lee, in 2013.[12] All genetic studies have supported the hypothesis that turtles (formerly categorized together with ancient anapsids) are diapsid reptiles, despite lacking any skull openings behind their eye sockets; some studies have even placed turtles among the archosaurs,[12][13][14][15][16][17] though a few have recovered turtles as lepidosauromorphs instead.[18] The cladogram below used a combination of genetic (molecular) and fossil (morphological) data to obtain its results.[12]

Amniota

Synapsida (mammals and their extinct relatives)

Sauropsida /
Eureptilia

Captorhinidae

Romeriida

Paleothyris

Diapsida

Araeoscelidia

Neodiapsida

Claudiosaurus

Younginiformes

Crown 
Reptilia
/
Pan-Lepidosauria
/

Kuehneosauridae

Lepidosauria

Rhynchocephalia (tuatara and their extinct relatives)

Squamata (lizards and snakes)

Lepidosauromorpha
Archelosauria/
Pan-Testudines
/

Eosauropterygia

Pantestudines
Pan-Archosauria

Choristodera

s. s.

Prolacertiformes

Rhynchosauria

Trilophosaurus

Archosauriformes (crocodiles, birds, dinosaurs and extinct relatives)

s. l.
Sauria
Reptilia
 (total group)

Laurin & Piñeiro (2017) and Modesto (2019) proposed an alternate phylogeny of basal sauropsids. In this tree, parareptiles include turtles and are closely related to non-araeoscelidian diapsids. The family Varanopidae, otherwise included in Synapsida, is considered by Modesto a sauropsid group.[19][20]

Synapsida

(mammals and allies)

Sauropsida

In recent studies, the "

microsaur" clade Recumbirostra, historically considered lepospondyl reptiliomorphs, have been recovered as early sauropsids.[21][22]

Structure difference with synapsids

The last common ancestor of synapsids and Sauropsida lived at around 320mya during Carboniferous, known as Reptiliomorpha.

Thermal and secretion

The early synapsids inherited abundant glands on their skins from their amphibian ancestors. Those glands evolved into sweat glands in synapsids, which granted them the ability to maintain constant body temperature but made them unable to save water from evaporation. Moreover, the way synapsids discharge nitrogenous waste is through urea, which is toxic and must be dissolved in water to be secreted. Unfortunately, the upcoming Permian and Triassic periods were arid periods. As a result, only a small percent of early synapsids survived in the land from South Africa to Antarctica in today's geography. Unlike synapsids, sauropsids do not have those glands on the skin; their way of nitrogenous waste emission is through uric acid which does not require water and can be excreted with feces. As a result, sauropsids were able to expand to all environments and reach their pinnacle. Even today, most vertebrates that live in arid environments are sauropsids, snakes and desert lizards for example.

Brain structure

Different from how synapsids have their cortex in six different layers of neurons which is called neocortex, the cerebrum of Sauropsida has a completely different structure. For the corresponding structure of the cerebrum in the classic view, the neocortex of synapsids is homology with only the Archicortex of the avian brain. However, in the modern view appeared since the 1960s, behavioral studies suggested that avian neostriatum and hyperstriatum can receive signals of vision, hearing, and body sensations, which means they act just like the neocortex. Comparing an avian brain to that to a mammal, nuclear-to-layered hypothesis proposed by Karten (1969), suggested that the cells which form layers in synapsids' neocortex, gather individually by type and form several nuclei. For synapsids, when one new function is adapted in evolution it will be assigned to a separate area of cortex, so for each function, synapsids will have to develop a separate area of cortex, and damage to that specific cortex may cause disability.[23] However, for Sauropsida functions are disassembled and assigned to all nuclei. In this case, brain function is highly flexible for Sauropsida, even with a small brain, many Sauropsida can still have a relatively high intelligence compared to mammals, for example, birds in the family Corvidae. So, it is possible that some non-avian dinosaurs, like Tyrannosaurus, which had tiny brains compared to their enormous body size, were more intelligent than previously thought.[24]

References

  1. ^ a b Gauthier J.A. (1994): The diversification of the amniotes. In: D.R. Prothero and R.M. Schoch (ed.) Major Features of Vertebrate Evolution: 129–159. Knoxville, Tennessee: The Paleontological Society.
  2. ^ a b Huxley, Thomas Henry (1864). "The Structure and Classification of the Mammalia". Medical Times and Gazette. Huxley Archives. Retrieved 2023-03-16.
  3. ^ Huxley, Thomas Henry (1877). "Lectures on Evolution". Collected Essays IV. Retrieved 2023-03-16. {{cite book}}: |website= ignored (help)
  4. ^ .
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  6. ^ Romer, A.S. (1933). Vertebrate Paleontology. University of Chicago Press., 3rd ed., 1966.
  7. ^ Gauthier, .A., Kluge, A.G & Rowe, T. (1988). The early evolution of the Amniota. Pages 103–155 in Michael J. Benton (ed.): The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds. Syst. Ass. Spec. Vol. 35A. Clarendon Press, Oxford.
  8. ^ Laurin, Michel; Gauthier, Jacques (January 1996). "Amniota. Mammals, reptiles (turtles, lizards, Sphenodon, crocodiles, birds) and their extinct relatives". Tree of Life Web Project. Archived from the original on 10 April 2006. Retrieved 2023-03-16.{{cite web}}: CS1 maint: unfit URL (link)
  9. ^ Pearse, A.S. (ed, 1947): Zoological Names: a List of Phyla, Classes, and Orders. Prepared for Section F, American Association for the Advancement of Science. Second edition. Durham, North Carolina, U.S.A., pp. 1-22
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    PMID 15545258
    .
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  20. . Retrieved 29 December 2020.
  21. .
  22. .
  23. ^ Karten, H. J. in Comparative and Evolutionary Aspects of the Vertebrate Central Nervous System (ed. Pertras, J.) 164–179 (1969).
  24. .