Synapsida

Not to be confused with synapse, synapsis, or therapsid.

Synapsida Temporal range: Pennsylvanian–Holocene, 318 [1] –0 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Examples of synapsids (left to right, top to bottom): Cotylorhynchus , Dimetrodon , Inostrancevia , Moschops , Castorocauda , Adelobasileus , Tachyglossus aculeatus , and Panthera tigris
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Superclass:	Tetrapoda
Clade:	Reptiliomorpha
Clade:	Amniota
Clade:	Synapsida Osborn, 1903
Subgroups
† Asaphestera † Protoclepsydrops ? † Diadectomorpha? [2] [3] [4] † Caseasauria † Caseidae † Eothyrididae † Phreatophasma ? Eupelycosauria † Ophiacodontidae † Varanopidae? [5] Haptodontiformes † Edaphosauridae Sphenacodontia
Synonyms
Theropsida (Seeley, 1895) [6] "Pelycosauria" (Cladistically including therapsids)

Synapsida ^[a] is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant reptiles and birds). Unlike other amniotes, synapsids have a single temporal fenestra, an opening low in the skull roof behind each eye socket, leaving a bony arch beneath each; this accounts for the name "synapsid". ^[7] The distinctive temporal fenestra developed about 318 million years ago during the Late Carboniferous period, ^[1] when synapsids and sauropsids diverged, but was subsequently merged with the orbit in early mammals.

The basal amniotes (reptiliomorphs) from which synapsids evolved were historically simply called "reptiles". Therefore, stem group synapsids were then described as mammal-like reptiles in classical systematics, and non-therapsid synapsids were also referred to as pelycosaurs or pelycosaur-grade synapsids. These paraphyletic terms have now fallen out of favor and are only used informally (if at all) in modern literature, as it is now known that all extant reptiles are more closely related to each other and birds than to synapsids, so the word "reptile" has been re-defined to mean only members of Sauropsida or even just an under-clade thereof. In a cladistic sense, synapsids are in fact a monophyletic sister taxon of sauropsids, rather than a part of the sauropsid lineage. ^{[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, proto-mammals, paramammals or pan-mammals. ^{[12] [13] [14]} Most lineages of pelycosaur-grade synapsids were replaced by the more advanced therapsids, which evolved from sphenacodontoid pelycosaurs, at the end of the Early Permian during the so-called Olson's Extinction.

Synapsids were the largest terrestrial vertebrates in the Permian period (299 to 251  mya), rivalled only by some large pareiasaurian parareptiles such as Scutosaurus . They were the dominant land predators of the late Paleozoic and early Mesozoic, with eupelycosaurs such as Dimetrodon , Titanophoneus and Inostrancevia being the apex predators during the Permian, and theriodonts such as Moschorhinus during the Early Triassic. Synapsid population and diversity were severely reduced by the Capitanian mass extinction event and the Permian–Triassic extinction event, and only two groups of therapsids, the dicynodonts and eutheriodonts (consisting of therocephalians and cynodonts) are known to have survived into the Triassic. These therapsids rebounded as disaster taxa during the early Mesozoic, with the dicynodont Lystrosaurus making up as much as 95% of all land species at one time, ^{[15] [16]} but declined again after the Smithian–Spathian boundary event with their dominant niches largely taken over by the rise of archosaurian sauropsids, first by the pseudosuchians and then by the pterosaurs and dinosaurs. The cynodont group Probainognathia, which includes the group Mammaliaformes, were the only synapsids to survive beyond the Triassic, ^[17] and mammals are the only synapsid lineage that have survived past the Jurassic, having lived mostly nocturnally to avoid competition with dinosaurs. After the Cretaceous-Paleogene extinction wiped out all non-avian dinosaurs and pterosaurs, synapsids (as mammals) rose to dominance once again during the Cenozoic. ^[18]

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 evolving increasingly mammalian features, hence the name "mammal-like reptiles" (also known as pelycosaurs). These became the traditional terms for all Paleozoic (early) synapsids. More recent studies have debunked this notion as well, and reptiles are now classified within Sauropsida (sauropsids), the sister group to synapsids, thus making synapsids their own taxonomic group. ^{[8] [10]}

As a result, the paraphyletic terms "mammal-like reptile" and "pelycosaur" are seen as outdated and disfavored in technical literature, and the term stem mammal (or sometimes protomammal or paramammal) is used instead. Phylogenetically, it is now understood that synapsids comprise an independent branch of the tree of life. ^[19] The monophyly of Synapsida is not in doubt, and the expressions such as "Synapsida contains the mammals" and "synapsids gave rise to the mammals" both express the same phylogenetic hypothesis. This terminology reflects the modern cladistic approach to animal relationships, according to which the only valid groups are those that include all of the descendants of a common ancestor: these are known as monophyletic groups, or clades.

Additionally, Reptilia (reptiles) has been revised into a monophyletic group and is considered entirely distinct from Synapsida, falling within Sauropsida, the sister group of Synapsida within Amniota. ^[20]

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. ^[21] 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 canines, molars, and incisors. ^[22] The trend towards differentiation is found in some labyrinthodonts and early anapsid reptilians in the form of enlargement of the first teeth on the maxilla, forming a sort 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 classification tool, as most other fossilized features that make a chronological progression from a reptile-like to a mammalian condition follow the progression of the jaw transition. The mandible, or lower jaw, consists of a single, tooth-bearing bone in mammals (the dentary), whereas the lower jaw of modern and prehistoric reptiles consists of a conglomeration of smaller bones (including the dentary, 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). ^[23]

Mammalian jaw structures are also set apart by the dentary-squamosal jaw joint. In this form of jaw joint, the dentary forms a connection with a depression in the squamosal known as the 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 palatine bone. The maxilla is also closed completely. In fossils of one of the first eutheriodonts, the beginnings of a palate are clearly visible. The later Thrinaxodon has a full and completely closed palate, forming a clear progression. ^[24]

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 osteoderms (bony armor embedded in the skin), scutes (protective structures of the dermis often with a horny covering), hair or fur, and 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. ^[25]

The ancestral skin type of synapsids has been subject to discussion. The type specimen of the oldest known synapsid Asaphestera preserved scales. ^[26] Among the early synapsids, only two species of small varanopids have been found to possess osteoderms; ^[27] fossilized rows of osteoderms indicate bony armour on the neck and back. However, some recent studies have cast doubt on the placement of Varanopidae in Synapsida, ^{[28] [29]} while others have countered and lean towards this traditional placement. ^{[30] [31]} Skin impressions indicate some early synapsids basal possessed rectangular scutes on their undersides and tails. ^{[32] [33]} The pelycosaur scutes probably were nonoverlapping dermal structures with a horny overlay, like those found in modern crocodiles and turtles. These differed in structure from the scales of lizards and snakes, which are an epidermal feature (like mammalian hair or avian feathers). ^[34] Recently, skin impressions from the genus Ascendonanus suggest that at least varanopsids developed scales similar to those of squamates. ^[35]

It is currently unknown exactly when mammalian characteristics such as body hair and mammary glands first appeared, as the fossils only rarely provide direct evidence for soft tissues. An exceptionally well-preserved skull of Estemmenosuchus , a therapsid from the Upper Permian, preserves smooth skin with what appear to be glandular depressions, ^[36] an animal noted as being semi-aquatic. ^[37] The oldest known fossil showing unambiguous imprints of hair is the Callovian (late middle Jurassic) Castorocauda and several contemporary haramiyidans, both non-mammalian mammaliaform ^{[38] [39]} (see below, however). More primitive members of the Cynodontia are also hypothesized to have had fur or a fur-like covering based on their inferred warm-blooded metabolism. ^[40] While more direct evidence of fur in early cynodonts has been proposed in the form of small pits on the snout possibly associated with whiskers, such pits are also found in some reptiles that lack whiskers. ^[40] There is evidence that some other non-mammalian cynodonts more basal than Castorocauda, such as Morganucodon , had 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. ^[40] 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. ^[40] The oldest examples of nocturnality in synapsids is believed to have been in species that lived more than 300 million years ago. ^[41]

However, Late Permian coprolites from Russia and possibly South Africa showcase that at least some synapsids did already have pre-mammalian hair in this epoch. These are the oldest impressions of hair-like structures on synapsids. ^{[42] [43]}

Mammary glands

Early synapsids, as far back as their known evolutionary debut in the Late Carboniferous period, ^[44] may have laid parchment-shelled (leathery) eggs, ^[45] 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. ^[46] Because they were vulnerable to desiccation, secretions from apocrine-like glands may have helped keep the eggs moist. ^[44]

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 (echidnas carry their eggs and offspring via a temporary pouch ^{[47] [48]} ), though this would limit the mobility of the parent. The latter may have been the primitive form of egg care in synapsids rather than simply burying the eggs, and the constraint on the parent's mobility would have been solved by having the eggs "parked" in nests during foraging or other activities and periodically be hydrated, allowing higher clutch sizes than could fit inside a pouch (or pouches) at once, and large eggs, which would be cumbersome to carry in a pouch, would be easier to care for. The basis of Oftedal's speculation is the fact that many species of 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. ^[46]

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 sauropsid lines). Cynodonts were almost certainly able to produce this, which allowed a progressive decline of yolk mass and thus egg size, resulting in increasingly altricial hatchlings as milk became the primary source of nutrition, which is all evidenced by the small body size, the presence of epipubic bones, and limited tooth replacement in advanced cynodonts, as well as in mammaliaforms. ^{[44] [45]}

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. ^[49]

Within true mammals, aerial locomotion first occurs in volaticotherian 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. ^[50] Argentoconodon , a close relative, shares a similar femur adapted for flight stresses, indicating a similar lifestyle. ^[51]

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. ^[52]

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. ^[53]

Evolutionary history

Main article: Evolution of mammals

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. ^[54]

The earliest known synapsid Asaphestera coexisted with the earliest known sauropsid Hylonomus which lived during the Bashkirian age of the Late Carboniferous. ^{[26] [14]} It was 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. ^[55]

Moschops was a tapinocephalian from the Middle Permian of South Africa.

The therapsids, a more advanced group of synapsids, appeared during the Middle Permian and included the largest terrestrial animals in the Middle and 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 therocephalians, which only lasted the first 20 million years of the Triassic period. The second were specialised, beaked herbivores known as dicynodonts (such as the Kannemeyeriidae), which contained some members that reached large size (up to a tonne or more). And finally there were the increasingly mammal-like carnivorous, herbivorous, and insectivorous cynodonts, including 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 Tritylodon . No cynodont grew larger than a cat. Most Jurassic and Cretaceous cynodonts were herbivorous, though some were 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. ^[56] 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. ^[57]

Today, the 5,500 species of living synapsids, known as the mammals, include both aquatic (cetaceans) and flying (bats) species, and the largest animal ever known to have existed (the blue whale). Humans are synapsids, as well. Most mammals are 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 digestion, these synapsids evolved 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. ^[58] This helped make it possible to support their higher metabolic demands.

Relationships

Below is a cladogram of the most commonly accepted phylogeny of synapsids, showing a long stem lineage including Mammalia and successively more basal clades such as Theriodontia, Therapsida and Sphenacodontia: ^{[59] [60]}

Synapsida	†Caseasauria Eupelycosauria †Varanopidae †Ophiacodontidae †Edaphosauridae Sphenacodontia †Sphenacodontidae Therapsida †Biarmosuchia Eutherapsida †Dinocephalia Neotherapsida †Anomodontia Theriodontia †Gorgonopsia Eutheriodontia †Therocephalia Cynodontia † Cynognathia Probainognathia Mammalia

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): ^[61]

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 Dimetrodon spp. Sphenacodon spp.

However, more recent examination of the phylogeny of basal synapsids, incorporating newly described basal caseids and eothyridids, ^[62] returned Caseasauria to its position as the sister to all other synapsids. Brocklehurst et al. (2016) ^[62] 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
• List of prehistoric mammals
• Lists of synapsids
• Mammal classification
• Timeline of the evolutionary history of life
• Vertebrate paleontology

Notes

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

References

1. Steen, Margaret C. (1934). "The amphibian fauna from the South Joggins. Nova Scotia". Journal of Zoology. 104 (3): 465–504. doi:10.1111/j.1096-3642.1934.tb01644.x.
2. David S. Berman (2013). "Diadectomorphs, amniotes or not?". New Mexico Museum of Natural History and Science Bulletin. 60: 22–35.
3. Klembara, Jozef; Hain, Miroslav; Ruta, Marcello; Berman, David S.; Pierce, Stephanie E.; Henrici, Amy C. (2019). "Inner ear morphology of diadectomorphs and seymouriamorphs (Tetrapoda) uncovered by high‐resolution x‐ray microcomputed tomography, and the origin of the amniote crown group". Palaeontology. 63: 131–154. doi: 10.1111/pala.12448 .
4. Brocklehurst, N. (2021). "The first age of reptiles? Comparing reptile and synapsid diversity, and the influence of lagerstätten, during the Carboniferous and early Permian". Frontiers in Ecology and Evolution. 9: 669765. doi: 10.3389/fevo.2021.669765 .
5. Simões, T.; Kammerer, C. (August 2022). "Successive climate crises in the deep past drove the early evolution and radiation of reptiles". Science Advances. 08 (33): eabq1898. Bibcode:2022SciA....8.1898S. doi: 10.1126/sciadv.abq1898 . PMC   9390993 . PMID   35984885. S2CID   251694019.
6. Seeley, Harry Govier (1895). "Researches on the structure, organisation, and classification of the fossil reptilia. Part X. On the complete skeleton of an anomodont reptile (Aristodesmus rutimeyeri, Wiedersheim), from the Bunter sandstone of Reihen, near Basel, giving new evidence of the relation of the Anomodontia to the Monotremata". Proceedings of the Royal Society of London. 59: 167–169. doi: 10.1098/rspl.1895.0070 .
7. Romer, A.S; Parsons, T.S. (1985). The Vertebrate Body (6th ed.). Philadelphia, PA: Saunders.
8. Carroll, Robert L. (1988). Vertebrate Paleontology and Evolution. New York, NY: W.H. Freeman & Co. p. 397. ISBN   0-7167-1822-7.
9. D. W. Dilkes, R. R. Reisz (1996). "First Record of a Basal Synapsid ('Mammal-Like Reptile') in Gondwana". Proceedings of the Royal Society B. 263 (1374): 1165–1170. doi:10.1098/rspb.1996.0170.
10. Benton, Michael J. (2005). Vertebrate Palaeontology (3rd ed.). Oxford, UK: Blackwell Science Ltd. p. 122. ISBN   0-632-05637-1.
11. "Jaws to ears in the ancestors of mammals". evolution.berkeley.edu. Retrieved 2020-02-20.
12. "New proto-mammal fossil sheds light on evolution of earliest mammals". University of Chicago. August 7, 2013.
13. Naish, Darren. "The Stem-Mammals--a Brief Primer". Scientific American Blog Network. Retrieved 2022-02-27.
14. Marjanović, D. (2021). "The Making of Calibration Sausage Exemplified by Recalibrating the Transcriptomic Timetree of Jawed Vertebrates". Frontiers in Genetics. 12. 521693. doi: 10.3389/fgene.2021.521693 . PMC   8149952 . PMID   34054911.
15. Benton, M.J. (2006). When Life Nearly Died: The greatest mass extinction of all time. London, UK: Thames & Hudson. ISBN   978-0-500-28573-2.
16. Cokinos, Christopher (12 October 2007) [May-June 2007]. "The consolations of extinction". Orion . Archived from the original on 2007-10-12.
17. "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.
18. Laurin, Michel; Reisz, Robert R. (2011). "Synapsida: Mammals and their extinct relatives" (Version 14 ed.). The Tree of Life Web Project.
19. Angielczyk, Kenneth D. (2009). "Dimetrodon is not a dinosaur: Using tree thinking to understand the ancient relatives of mammals and their evolution". Evolution: Education and Outreach. 2 (2): 257–271. doi: 10.1007/s12052-009-0117-4 . S2CID   24110810.
20. Modesto, S.P.; Anderson, J.S. (2004). "The phylogenetic definition of Reptilia". Systematic Biology. 53 (5): 815–821. doi: 10.1080/10635150490503026 . PMID   15545258.
21. Benton, Michael J. (2005). Vertebrate Paleontology , 3rd ed. Oxford: Blackwell Science Ltd. ISBN   0-632-05637-1. p. 120.
22. Angielczch, Kennenth; Kammerer, Christian F.; Frobisch, Jorg. (2013). Early Evolutionary History of Synapsida. Springer Science & Business Media. ISBN   978-94-007-6841-3, p. 11
23. Salentijn, L. Biology of Mineralized Tissues: Prenatal Skull Development, Columbia University College of Dental Medicine post-graduate dental lecture series, 2007
24. Hopson, James A. (1987). "The Mammal-Like Reptiles: A Study of Transitional Fossils". The American Biology Teacher. 49 (1): 16–26. doi:10.2307/4448410. JSTOR   4448410.
25. Hildebran, M.; Goslow, G. (2001). Analysis of Vertebrate Structure (5th ed.). New York: John Wiley & Sons. ISBN   0-471-29505-1.
26. Mann, Arjan; Gee, Bryan M.; Pardo, Jason D.; Marjanović, David; Adams, Gabrielle R.; Calthorpe, Ami S.; Maddin, Hillary C.; Anderson, Jason S. (5 May 2020). Sansom, Robert (ed.). "Reassessment of historic 'microsaurs' from Joggins, Nova Scotia, reveals hidden diversity in the earliest amniote ecosystem". Papers in Palaeontology. Wiley. doi:10.1002/spp2.1316. ISSN   2056-2802.
27. Vickaryous, Matthew K. & Sire, Jean-Yves (2009). "The integumentary skeleton of tetrapods: origin, evolution, and development". Journal of Anatomy. 214 (4): 441–464. doi:10.1111/j.1469-7580.2008.01043.x. PMC   2736118 . PMID   19422424.
28. Ford, David P.; Benson, Roger B. J. (May 2019). Mannion, Philip (ed.). "A redescription of Orovenator mayorum (Sauropsida, Diapsida) using high‐resolution μ CT , and the consequences for early amniote phylogeny". Papers in Palaeontology. 5 (2): 197–239. doi: 10.1002/spp2.1236 . ISSN   2056-2802.
29. Ford, David P.; Benson, Roger B. J. (January 2020). "The phylogeny of early amniotes and the affinities of Parareptilia and Varanopidae". Nature Ecology & Evolution. 4 (1): 57–65. doi:10.1038/s41559-019-1047-3. ISSN   2397-334X.
30. Maddin, Hillary C.; Mann, Arjan; Hebert, Brian (January 2020). "Varanopid from the Carboniferous of Nova Scotia reveals evidence of parental care in amniotes". Nature Ecology & Evolution. 4 (1): 50–56. doi:10.1038/s41559-019-1030-z. ISSN   2397-334X.
31. Benoit, Julien; Ford, David; Miyamae, Juri; Ruf, Irina (2021). "Can maxillary canal morphology inform varanopid phylogenetic affinities?". Acta Palaeontologica Polonica. 66. doi: 10.4202/app.00816.2020 . ISSN   0567-7920.
32. Reisz, Robert (1975). "Pennsylvanian Pelycosaurs from Linton, Ohio and Nýřany, Czechoslovakia". Journal of Paleontology. 49 (3): 522–527. ISSN   0022-3360.
33. Niedźwiedzki, G.; Bojanowski, M. (2012). "A supposed eupelycosaur body impression from the early Permian of the Intra-Sudetic basin, Poland". Ichnos. 19 (3): 150–155. doi:10.1080/10420940.2012.702549. S2CID   129567176.
34. Carroll, R.L. (1969). "Problems of the origin of reptiles". Biological Reviews. 44 (3): 393–432. doi:10.1111/j.1469-185X.1969.tb01218.x. S2CID   84302993.
35. Spindler, Frederik; Werneburg, Ralf; Schneider, Joerg W.; Luthardt, Ludwig; Annacker, Volker; Rößler, Ronny (2018). "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. S2CID   133846070.
36. Kardong, K.V. (2002). Vertebrates: Comparative Anatomy, Function, Evolution (3rd ed.). Boston: McGraw-Hill. ISBN   0-07-112235-4.
37. Kemp, T.S. (2006). "The origin and early radiation of the therapsid mammal-like reptiles: A palaeobiological hypothesis". Journal of Evolutionary Biology . 19 (4): 1231–1247. doi: 10.1111/j.1420-9101.2005.01076.x . PMID   16780524. S2CID   3184629.
38. Ji, Q.; Luo, Z-X; Yuan, Chong-Xi; Tabrum, Alan R. (February 2006). "A swimming mammaliaform from the middle Jurassic and ecomorphological diversification of early mammals". Science. 311 (5764): 1123–7. Bibcode:2006Sci...311.1123J. doi:10.1126/science.1123026. PMID   16497926. S2CID   46067702.
    See also the news item at "Jurassic "beaver" found; rewrites history of mammals". National Geographic . Feb 2006.
39. Meng, Qing-Jin; Grossnickle, David M.; Di, Liu; Zhang, Yu-Guang; Neander, April I.; Ji, Qiang; Luo, Zhe-Xi (2017). "New gliding mammaliaforms from the Jurassic". Nature. 548 (7667): 291–296. Bibcode:2017Natur.548..291M. doi:10.1038/nature23476. PMID   28792929. S2CID   205259206.
40. Ruben, J.A.; Jones, T.D. (2000). "Selective factors associated with the origin of fur and feathers". Am. Zool. 40 (4): 585–596. doi: 10.1093/icb/40.4.585 .
41. Gaare, Megan (7 October 2014). "An Early Nocturnal Ancestor". Field Museum of Natural History. Retrieved 11 March 2022.
42. Bajdek, Piotr; Qvarnström, Martin; Owocki, Krzysztof; Sulej, Tomasz; Sennikov, Andrey G.; Golubev, Valeriy K.; Niedźwiedzki, Grzegorz (2016). "Microbiota and food residues including possible evidence of pre-mammalian hair in Upper Permian coprolites from Russia". Lethaia . 49 (4): 455–477. doi:10.1111/let.12156.
43. Smith, Roger M.H.; Botha-Brink, Jennifer (2011). "Morphology and composition of bone-bearing coprolites from the Late Permian Beaufort Group, Karoo Basin, South Africa". Palaeogeography, Palaeoclimatology, Palaeoecology. 312 (1–2): 40–53. Bibcode:2011PPP...312...40S. doi:10.1016/j.palaeo.2011.09.006. ISSN   0031-0182.
44. Oftedal, Olav T. (2002-07-01). "The mammary gland and its origin during synapsid evolution". Journal of Mammary Gland Biology and Neoplasia. 7 (3): 225–252. doi:10.1023/a:1022896515287. ISSN   1083-3021. PMID   12751889. S2CID   25806501.
45. Oftedal, O.T. (2012-03-01). "The evolution of milk secretion and its ancient origins". Animal. 6 (3): 355–368. doi: 10.1017/S1751731111001935 . ISSN   1751-732X. PMID   22436214.
46. Oftedal, Olav T. (2002-07-01). "The origin of lactation as a water source for parchment-shelled eggs". Journal of Mammary Gland Biology and Neoplasia. 7 (3): 253–266. doi:10.1023/A:1022848632125. ISSN   1083-3021. PMID   12751890. S2CID   8319185.
47. "Monotremes and marsupials". www.life.umd.edu. Retrieved 2018-08-23.
48. "Life History and Ecology of the Monotremata". www.ucmp.berkeley.edu. Retrieved 2018-08-23.
49. Luo, Zhe-Xi; Meng, Qing-Jin; Grossnickle, David M.; Di, Liu; Neander, April I.; Zhang, Yu-Guang; Ji, Qiang (2017). "New evidence for mammaliaform ear evolution and feeding adaptation in a Jurassic ecosystem". Nature. 548 (7667): 326–329. Bibcode:2017Natur.548..326L. doi:10.1038/nature23483. PMID   28792934. S2CID   4463476.
50. Meng, J.; Hu, Y.-M.; Wang, Y.-Q.; Wang, X.-L.; Li, C.-K. (2007). "Corrigendum: A Mesozoic gliding mammal from northeastern China". Nature. 446 (7131): 102. Bibcode:2007Natur.446Q.102M. doi: 10.1038/nature05639 .
51. Gaetano, L.C.; Rougier, G.W. (2011). "New materials of Argentoconodon fariasorum (Mammaliaformes, Triconodontidae) from the Jurassic of Argentina and its bearing on triconodont phylogeny". Journal of Vertebrate Paleontology. 31 (4): 829–843. doi:10.1080/02724634.2011.589877. hdl: 11336/68497 . S2CID   85069761.
52. 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.
53. "Ancestry of mammalian 'warm-bloodedness' revealed". www.sciencedaily.com. Society of Vertebrate Paleontology. October 29, 2015. Retrieved October 29, 2015.
54. 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.
55. Modesto, Sean P.; Smith, Roger M. H.; Campione, Nicolás E.; Reisz, Robert R. (2011). "The last 'pelycosaur': a varanopid synapsid from the Pristerognathus Assemblage Zone, Middle Permian of South Africa". Naturwissenschaften. 98 (12): 1027–34. Bibcode:2011NW.....98.1027M. doi:10.1007/s00114-011-0856-2. PMID   22009069. S2CID   27865550.
56. Thulborn, T.; Turner, S. (2003). "The last dicynodont: an Australian Cretaceous relict". Proceedings of the Royal Society B: Biological Sciences. 270 (1518): 985–993. doi:10.1098/rspb.2002.2296. JSTOR   3558635. PMC   1691326 . PMID   12803915.
57. Espen M. Knutsen; Emma Oerlemans (2019). "The last dicynodont? Re-assessing the taxonomic and temporal relationships of a contentious Australian fossil". Gondwana Research. 77: 184–203. doi:10.1016/j.gr.2019.07.011. S2CID   202908716.
58. Bramble, D. M.; Jenkins, F. A. (1993). "Mammalian locomotor-respiratory integration: Implications for diaphragmatic and pulmonary design". Science . 262 (5131): 235–240. Bibcode:1993Sci...262..235B. doi:10.1126/science.8211141. PMID   8211141.
59. Laurin, M.; Reisz, R.R. (2011). "Synapsida. Mammals and their extinct relatives". The Tree of Life Web Project. Retrieved 26 April 2012.
60. Kemp, T.S. (2011). "The origin and radiation of therapsids". In Chinsamy-Turan, A. (ed.). Forerunners of Mammals. Bloomington: Indiana University Press. pp. 3–30. ISBN   978-0-253-35697-0.
61. 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.
62. Neil Brocklehurst; Robert Reisz; Vincent Fernandez; Jörg Fröbisch (2016). "A Re-Description of 'Mycterosaurus' smithae, an Early Permian Eothyridid, and Its Impact on the Phylogeny of Pelycosaurian-Grade Synapsids". PLOS ONE. 11 (6): e0156810. Bibcode:2016PLoSO..1156810B. doi: 10.1371/journal.pone.0156810 . PMC   4917111 . PMID   27333277.

Further reading

• Colbert, E. H. (1969). Evolution of the Vertebrates (2nd ed.). New York: John Wiley & Sons Inc. ISBN   0-471-16466-6.

External links

• Synapsida - Pelycosauria - at Palaeos
• Transitional Vertebrate Fossils - includes description of important transitional genera in the evolutionary sequence linking primitive synapsids to mammals