Synapsid

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Synapsids
Temporal range: PennsylvanianHolocene, 318[1]–0 Ma
Examples of synapsids (left to right, top to bottom):
Panthera tigris
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Superclass: Tetrapoda
Clade: Reptiliomorpha
Clade: Amniota
Clade: Synapsida
Osborn, 1903
Subgroups
Synonyms

Theropsida (Seeley, 1895)[5]

Synapsids

Late Carboniferous period,[1]
when synapsids and sauropsids diverged, but was subsequently merged with the orbit in early mammals.

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

cladistical sense.[8][9][10][11] Therefore, calling synapsids "mammal-like reptiles" is incorrect under the new definition of "reptile", so they are now referred to as stem mammals, and sometimes as proto-mammals, or paramammals.[12][13]

Synapsids were the largest

eutheriodonts (consisting of Therocephalia and Cynodontia) are known to have continued into the Triassic period. The cynodont group Probainognathia, which includes Mammaliaformes (mammals and their closer ancestors), were the only synapsids to survive beyond the Triassic.[14]

During the Triassic, the sauropsid

dinosaurs. When all non-avian dinosaurs were wiped out by the Cretaceous–Paleogene extinction event
, the mammalian synapsids diversified again to become the largest land and marine animals on Earth.

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

Sauropsida (sauropsids), the sister group to synapsids, thus making synapsids their own taxonomic group.[8][10]

As a result, the

monophyletic groups, or clades
.

Additionally,

Sauropsida, the sister group of Synapsida within Amniota.[16]

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

The synapsids are distinguished by a single hole, known as the temporal fenestra, in the skull behind each eye. This schematic shows the skull viewed from the left side. The middle opening is the orbit of the eye; the opening to the right of it is the temporal fenestra.

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

Eothyris, an early synapsid with multiple canines

Synapsids are characterized by having differentiated teeth. These include the

anapsid reptilians in the form of enlargement of the first teeth on the maxilla, forming a form of protocanines. This trait was subsequently lost in the diapsid
line, but developed further in the synapsids. Early synapsids could have two or even three enlarged "canines", but in the therapsids, the pattern had settled to one canine in each upper jaw half. The lower canines developed later.

Jaw

The jaw transition is a good

articular, and others). As they evolved in synapsids, these jaw bones were reduced in size and either lost or, in the case of the articular, gradually moved into the ear, forming one of the middle ear bones: while modern mammals possess the malleus, incus and stapes, basal synapsids (like all other tetrapods) possess only a stapes. The malleus is derived from the articular (a lower jaw bone), while the incus is derived from the quadrate (a cranial bone).[19]

Mammalian jaw structures are also set apart by the dentary-squamosal

glenoid cavity. In contrast, all other jawed vertebrates, including reptiles and nonmammalian synapsids, possess a jaw joint in which one of the smaller bones of the lower jaw, the articular, makes a connection with a bone of the cranium called the quadrate bone
to form the articular-quadrate jaw joint. In forms transitional to mammals, the jaw joint is composed of a large, lower jaw bone (similar to the dentary found in mammals) that does not connect to the squamosal, but connects to the quadrate with a receding articular bone.

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

eutheriodonts, the beginnings of a palate are clearly visible. The later Thrinaxodon has a full and completely closed palate, forming a clear progression.[20]

Skin and fur

The sea otter has the densest fur of modern mammals.

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

scale-like structures (often formed from modified hair, as in pangolins and some rodents). While the skin of reptiles is rather thin, that of mammals has a thick dermal layer.[21]

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

Harderian glands, which are associated with the grooming and maintenance of fur. The apparent absence of these glands in non-mammaliaformes may suggest that fur did not originate until that point in synapsid evolution.[35] It is possible that fur and associated features of true warm-bloodedness did not appear until some synapsids became extremely small and nocturnal, necessitating a higher metabolism.[35] The oldest examples of nocturnality in synapsids is believed to have been in species that lived more than 300 million years ago.[36]

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 (

anurans can carry eggs or tadpoles attached to the skin, or embedded within cutaneous "pouches" and how most salamanders curl around their eggs to keep them moist, both groups also having glandular skin.[40]

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

eutriconodonts. A fossil Volaticotherium has an exquisitely preserved furry patagium with delicate wrinkles and that is very extensive, "sandwiching" the poorly preserved hands and feet and extending to the base of the tail.[44] Argentoconodon, a close relative, shares a similar femur adapted for flight stresses, indicating a similar lifestyle.[45]

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

endothermy was developed as early as Ophiacodon in the late Carboniferous. The presence of fibrolamellar, a specialised type of bone that can grow quickly while maintaining a stable structure, shows that Ophiacodon would have used its high internal body temperature to fuel a fast growth comparable to modern endotherms.[47]

Evolutionary history

Archaeothyris, one of the oldest synapsids found
Cotylorhynchus (background), Ophiacodon and Varanops were early synapsids that lived until the Early Permian.

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]

Middle Permian
of South Africa.

The therapsids, a more advanced group of synapsids, appeared during the

Late Permian. They included herbivores and carnivores, ranging from small animals the size of a rat (e.g.: Robertia), to large, bulky herbivores a ton or more in weight (e.g.: Moschops). After flourishing for many millions of years, these successful animals were all but wiped out by the Permian–Triassic mass extinction about 250 mya, the largest known extinction in Earth's history, possibly related to the Siberian Traps
volcanic event.

Nikkasaurus was an enigmatic synapsid from the Middle Permian of Russia.
Lystrosaurus was the most common synapsid shortly after the Permian–Triassic extinction event.

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

eucynodonts from the Olenekian
age, an early representative of which was Cynognathus.

Cynognathus was the largest predatory cynodont of the Triassic.

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.

Repenomamus was the largest mammal of the Mesozoic.

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.

Tritylodon was a cynodont that lived in the Early Jurassic.

During the Jurassic and Cretaceous, the remaining non-mammalian cynodonts were small, such as

carnivorous. The family Tritheledontidae, which first appeared near the end of the Triassic, was carnivorous and persisted well into the Middle Jurassic. The other, Tritylodontidae, first appeared at the same time as the tritheledonts, but was herbivorous. This group became extinct at the end of the Early Cretaceous epoch. Dicynodonts are generally thought to have become extinct near the end of the Triassic period, but there was evidence this group survived, in the form of six fragments of fossil bone that were found in Cretaceous rocks of Queensland, Australia.[52] If true, it would mean there is a significant ghost lineage of Dicynodonts in Gondwana. However, these fossils were re-described in 2019 as being Pleistocene in age, and possibly belonging to a diprotodontid marsupial.[53]

Today, the 5,500 species of living synapsids, known as the

viviparous and give birth to live young rather than laying eggs with the exception being the monotremes
.

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

mastication (chewing) and specialized teeth that aided chewing. Limbs also evolved to move under the body instead of to the side, allowing them to breathe more efficiently during locomotion.[54]
This helped make it possible to support their higher metabolic demands.

Relationships

Below is a

phylogeny of synapsids, showing a long stem lineage including Mammalia and successively more basal clades such as Theriodontia, Therapsida and Sphenacodontia:[55][56]

Synapsida

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]

Tseajaia campi

Limnoscelis paludis

Amniota

Captorhinus spp.

Protorothyris archeri

Synapsida
Ophiacodontidae

Archaeothyris florensis

Varanosaurus acutirostris

Ophiacodon spp.

Stereophallodon ciscoensis

Varanopidae

Archaeovenator hamiltonensis

Pyozia mesenensis

Mycterosaurus longiceps

?

Elliotsmithia longiceps
(BP/1/5678)

Heleosaurus scholtzi

Mesenosaurus romeri

Varanops brevirostris

Watongia meieri

Varanodon agilis

Ruthiromia elcobriensis

Aerosaurus wellesi

Aerosaurus greenleorum

Caseasauria
Eothyrididae

Eothyris parkeyi

Oedaleops campi

Caseidae

Oromycter dolesorum

Casea broilii

Trichasaurus texensis

Euromycter rutenus
(="Casea" rutena)

Ennatosaurus tecton

Angelosaurus romeri

Cotylorhynchus romeri

Cotylorhynchus bransoni

Cotylorhynchus hancocki

Ianthodon schultzei

Edaphosauridae

Ianthasaurus hardestiorum

Glaucosaurus megalops

Lupeosaurus kayi

Edaphosaurus boanerges

Edaphosaurus novomexicanus

Sphenacodontia

Haptodus garnettensis

Pantelosaurus saxonicus

Therapsida

Raranimus dashankouensis

Biarmosuchus tener

Biseridens qilianicus

Titanophoneus potens

Sphenacodontidae

Cutleria wilmarthi

Secodontosaurus obtusidens

Cryptovenator hirschbergeri

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

Notes

  1. ^ Greek: συν-, romanizedsyn-, lit.'together' + ἁψίς (apsís, 'arch') > *συναψίδης (synapsídes) "having a fused arch"; synonymous with theropsids (Greek, "beast-face")

References

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Further reading

External links