Synapsid
A request that this article title be changed to Synapsida is under discussion. Please do not move this article until the discussion is closed. |
Synapsids Temporal range:
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Examples of synapsids (left to right, top to bottom): Panthera tigris
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Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Superclass: | Tetrapoda |
Clade: | Reptiliomorpha |
Clade: | Amniota |
Clade: | Synapsida Osborn, 1903 |
Subgroups | |
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Synonyms | |
Synapsids
The animals (basal amniotes) from which non-mammalian synapsids evolved were traditionally called "reptiles". Therefore, synapsids were described as mammal-like reptiles in classical systematics, and non-therapsid synapsids were also referred to as
Synapsids were the largest
During the Triassic, the sauropsid
Linnaean and cladistic classifications
At the turn of the 20th century synapsids were thought to be one of the four main subclasses of reptiles. However, this notion was disproved upon closer inspection of skeletal remains, as synapsids are differentiated from reptiles by their distinctive temporal openings. These openings in the skull bones allowed the attachment of larger jaw muscles, hence a more efficient bite.
Synapsids were subsequently considered to be a later reptilian lineage that became mammals by gradually
As a result, the
Additionally,
Primitive and advanced synapsids
The synapsids are traditionally divided for convenience, into therapsids, an advanced group of synapsids and the branch within which mammals evolved, and stem mammals, (previously known as pelycosaurs), comprising the other six more primitive families of synapsids.[17] Stem mammals were all rather lizard-like, with sprawling gait and possibly horny scutes, while therapsids tended to have a more erect pose and possibly hair, at least in some forms. In traditional taxonomy, the Synapsida encompasses two distinct grades: the low-slung stem mammals have given rise to the more erect therapsids, who in their turn have given rise to the mammals. In traditional vertebrate classification, the stem mammals and therapsids were both considered orders of the subclass Synapsida.[7][8]
Practical versus phylogenetic usage of "synapsid" and "therapsid"
In phylogenetic nomenclature, the terms are used somewhat differently, as the daughter clades are included. Most papers published during the 21st century have treated "Pelycosaur" as an informal grouping of primitive members. Therapsida has remained in use as a clade containing both the traditional therapsid families and mammals.
Although Synapsida and Therapsida include modern mammals, in practical usage, those two terms are used almost exclusively when referring to the more basal members that lie outside of Mammaliaformes.
Characteristics
Temporal openings
Synapsids evolved a temporal fenestra behind each eye orbit on the lateral surface of the skull. It may have provided new attachment sites for jaw muscles. A similar development took place in the diapsids, which evolved two rather than one opening behind each eye. Originally, the openings in the skull left the inner cranium covered only by the jaw muscles, but in higher therapsids and mammals, the sphenoid bone has expanded to close the opening. This has left the lower margin of the opening as an arch extending from the lower edges of the braincase.
Teeth
Synapsids are characterized by having differentiated teeth. These include the
Jaw
The jaw transition is a good
Mammalian jaw structures are also set apart by the dentary-squamosal
Palate
Over time, as synapsids became more mammalian and less 'reptilian', they began to develop a secondary palate, separating the mouth and nasal cavity. In early synapsids, a secondary palate began to form on the sides of the maxilla, still leaving the mouth and nostril connected.
Eventually, the two sides of the palate began to curve together, forming a U shape instead of a C shape. The palate also began to extend back toward the throat, securing the entire mouth and creating a full
Skin and fur
In addition to the glandular skin covered in fur found in most modern mammals, modern and extinct synapsids possess a variety of modified skin coverings, including
The ancestral skin type of synapsids has been subject to discussion. Among the early synapsids, only two species of small
It is currently unknown exactly when mammalian characteristics such as
However, Permian coprolites from Russia showcase that at least some synapsids did already have fur in this epoch. These are the oldest impressions of hair on synapsids.[37]
Mammary glands
Early synapsids, as far back as their known evolutionary debut in the Late Carboniferous period,[38] may have laid parchment-shelled (leathery) eggs,[39] which lacked a calcified layer, as most modern reptiles and monotremes do. This may also explain why there is no fossil evidence for synapsid eggs to date.[40] Because they were vulnerable to desiccation, secretions from apocrine-like glands may have helped keep the eggs moist.[38]
According to Oftedal, early synapsids may have buried the eggs into moisture laden soil, hydrating them with contact with the moist skin, or may have carried them in a moist pouch, similar to that of monotremes (
The glands involved in this mechanism would later evolve into true mammary glands with multiple modes of secretion in association with hair follicles. Comparative analyses of the evolutionary origin of milk constituents support a scenario in which the secretions from these glands evolved into a complex, nutrient-rich milk long before true mammals arose (with some of the constituents possibly predating the split between the synapsid and
Patagia
Aerial locomotion first began in non-mammalian haramiyidan cynodonts, with Arboroharamiya, Xianshou, Maiopatagium and Vilevolodon bearing exquisitely preserved, fur-covered wing membranes that stretch across the limbs and tail. Their fingers are elongated, similar to those of bats and colugos and likely sharing similar roles both as wing supports and to hang on tree branches.[43]
Within true mammals, aerial locomotion first occurs in
Therian mammals would only achieve powered flight and gliding long after these early aeronauts became extinct, with the earliest-known gliding metatherians and bats evolving in the Paleocene.[46]
Metabolism
Recently, it has been found that
Evolutionary history
Over the course of synapsid evolution, progenitor taxa at the start of adaptive radiations have tended to be derived carnivores. Synapsid adaptive radiations have generally occurred after extinction events that depleted the biosphere and left vacant niches open to be filled by newly evolved taxa. In non-mammaliaform synapsids, those taxa that gave rise to rapidly diversifying lineages have been both small and large in body size, although after the Late Triassic, progenitors of new synapsid lineages have generally been small, unspecialised generalists.[48]
Asaphestera, Archaeothyris and Clepsydrops, the earliest-known synapsids,[49][50] lived in the Pennsylvanian subperiod (323–299 mya) of the Carboniferous period and were one of many types of primitive synapsids that are now informally grouped together as stem mammals or sometimes as protomammals (previously known as pelycosaurs). The early synapsids spread and diversified, becoming the largest terrestrial animals in the latest Carboniferous and Early Permian periods, ranging up to 6 metres (20 ft) in length. They were sprawling, bulky, possibly cold-blooded, and had small brains. Some, such as Dimetrodon, had large sails that might have helped raise their body temperature. A few relict groups lasted into the later Permian but, by the middle of the Late Permian, all had either died off or evolved into their successors, the therapsids.[51]
The therapsids, a more advanced group of synapsids, appeared during the
Only a few therapsids went on to be successful in the new early Triassic landscape; they include Lystrosaurus and Cynognathus, the latter of which appeared later in the early Triassic. However, they were accompanied by the early archosaurs (soon to give rise to the dinosaurs). Some of these archosaurs, such as Euparkeria, were small and lightly built, while others, such as Erythrosuchus, were as big as or bigger than the largest therapsids.
After the Permian extinction, the synapsids did not count more than three surviving clades. The first comprised the
Unlike the dicynodonts, which were large, the cynodonts became progressively smaller and more mammal-like as the Triassic progressed, though some forms like Trucidocynodon remained large. The first mammaliaforms evolved from the cynodonts during the early Norian age of the Late Triassic, about 225 mya.
During the evolutionary succession from early therapsid to cynodont to eucynodont to mammal, the main lower jaw bone, the dentary, replaced the adjacent bones. Thus, the lower jaw gradually became just one large bone, with several of the smaller jaw bones migrating into the inner ear and allowing sophisticated hearing.
Whether through climate change, vegetation change, ecological competition, or a combination of factors, most of the remaining large cynodonts (belonging to the Traversodontidae) and dicynodonts (of the family Kannemeyeriidae) had disappeared by the Rhaetian age, even before the Triassic–Jurassic extinction event that killed off most of the large non-dinosaurian archosaurs. The remaining Mesozoic synapsids were small, ranging from the size of a shrew to the badger-like mammal Repenomamus.
During the Jurassic and Cretaceous, the remaining non-mammalian cynodonts were small, such as
Today, the 5,500 species of living synapsids, known as the
Triassic and Jurassic ancestors of living mammals, along with their close relatives, had high metabolic rates. This meant consuming food (generally thought to be insects) in much greater quantity. To facilitate rapid
Relationships
Below is a
Synapsida |
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Most uncertainty in the phylogeny of synapsids lies among the earliest members of the group, including forms traditionally placed within Pelycosauria. As one of the earliest phylogenetic analyses, Brinkman & Eberth (1983) placed the family Varanopidae with Caseasauria as the most basal offshoot of the synapsid lineage. Reisz (1986) removed Varanopidae from Caseasauria, placing it in a more derived position on the stem. While most analyses find Caseasauria to be the most basal synapsid clade, Benson's analysis (2012) placed a clade containing Ophiacodontidae and Varanopidae as the most basal synapsids, with Caseasauria occupying a more derived position. Benson attributed this revised phylogeny to the inclusion of postcranial characteristics, or features of the skeleton other than the skull, in his analysis. When only cranial or skull features were included, Caseasauria remained the most basal synapsid clade. Below is a cladogram modified from Benson's analysis (2012):[57]
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However, more recent examination of the phylogeny of basal synapsids, incorporating newly described basal caseids and eothyridids,[58] returned Caseasauria to its position as the sister to all other synapsids. Brocklehurst et al. (2016)[58] demonstrated that many of the postcranial characters used by Benson (2012) to unite Caseasauria with Sphenacodontidae and Edaphosauridae were absent in the newly discovered postcranial material of eothyridids, and were therefore acquired convergently.
See also
- Anapsid
- Diapsid
- Euryapsida
- Lists of synapsids
- Mammal classification
- List of prehistoric mammals
- Timeline of the evolutionary history of life
- Vertebrate paleontology
Notes
References
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- ^ David S. Berman (2013). "Diadectomorphs, amniotes or not?". New Mexico Museum of Natural History and Science Bulletin. 60: 22–35.
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- ^ Laurin, Michel; Reisz, Robert R. (2011). "Synapsida: Mammals and their extinct relatives" (Version 14 ed.). The Tree of Life Web Project.
- ^ a b Romer, A.S; Parsons, T.S. (1985). The Vertebrate Body (6th ed.). Philadelphia, PA: Saunders.
- ^ ISBN 0-7167-1822-7.
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- ^ ISBN 0-632-05637-1.
- ^ "Jaws to ears in the ancestors of mammals". evolution.berkeley.edu. Retrieved 2020-02-20.
- ^ "New proto-mammal fossil sheds light on evolution of earliest mammals". University of Chicago. August 7, 2013.
- ^ Naish, Darren. "The Stem-Mammals--a Brief Primer". Scientific American Blog Network. Retrieved 2022-02-27.
- ^ "Greatest mass extinction responsible for the making of modern mammals" (research publ. ann.). Bloemfontein, ZA: The National Museum [of South Africa]. 19 September 2013. Archived from the original on 2019-03-28. Retrieved 2015-08-22.
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- ^ Gaare, Megan (7 October 2014). "An Early Nocturnal Ancestor". Field Museum of Natural History. Retrieved 11 March 2022.
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- ^ S2CID 25806501.
- ^ PMID 22436214.
- ^ S2CID 8319185.
- ^ "Monotremes and marsupials". www.life.umd.edu. Retrieved 2018-08-23.
- ^ "Life History and Ecology of the Monotremata". www.ucmp.berkeley.edu. Retrieved 2018-08-23.
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- ^ Szalay, FS; Sargis, EJ; Stafford, BJ (2000). Small marsupial glider from the Paleocene of Itaboraí, Brazil. Meeting of the Society of Vertebrate Paleontology. Journal of Vertebrate Paleontology. Supplement 73A. Vol. 20.
- ^ "Ancestry of mammalian 'warm-bloodedness' revealed". www.sciencedaily.com. Society of Vertebrate Paleontology. October 29, 2015. Retrieved October 29, 2015.
- ^ Grossnickle, David; Hellert, Spencer; Kammerer, Christian; Angielczyk, Kenneth D.; Lloyd, Graeme (1 October 2022). "Survival of the novel: derived faunivores are the forerunners of major synapsid radiations". Journal of Vertebrate Paleontology (Program and Abstracts, 2022). Retrieved 30 September 2023.
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- ^ Laurin, M.; Reisz, R.R. (2011). "Synapsida. Mammals and their extinct relatives". The Tree of Life Web Project. Retrieved 26 April 2012.
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Further reading
- ISBN 0-471-16466-6.
External links
- Synapsida - Pelycosauria - at Palaeos
- Transitional Vertebrate Fossils - includes description of important transitionalgenera in the evolutionary sequence linking primitive synapsids to mammals