Theropsida Seeley , 1895
SYNAPSIDS (Greek , 'fused arch'), synonymous with THEROPSIDS (Greek, 'beast-face'), are a group of animals that includes mammals and every animal more closely related to mammals than to other living amniotes . They are easily separated from other amniotes by having a temporal fenestra , an opening low in the skull roof behind each eye, leaving a bony arch beneath each; this accounts for their name. Primitive synapsids are usually called pelycosaurs or pelycosaur-grade synapsids; more advanced mammal-like ones, therapsids . The non-mammalian members are described as MAMMAL-LIKE REPTILES in classical systematics; they can also be called STEM MAMMALS or PROTO-MAMMALS. Synapsids evolved from basal amniotes and are one of the two major groups of the later amniotes; the other is the sauropsids , a group that includes modern reptiles and birds . The distinctive temporal fenestra developed in the ancestral synapsid about 312 million years ago, during the Late Carboniferous period .
Synapsids were the largest terrestrial vertebrates in the Permian
period, 299 to 251 million years ago, although some of the larger
pareiasaurs at the end of
* 1 Linnaean and cladistic classifications
* 1.1 Synapsids as a reptilian subclass * 1.2 The "mammal-like reptiles" * 1.3 Primitive and advanced synapsids
* 2 Characteristics
* 2.1 Temporal openings * 2.2 Teeth * 2.3 Jaw * 2.4 Palate
* 2.5 Skin and fur
* 2.5.1 Mammary glands
* 2.6 Metabolism
* 3 Evolutionary history * 4 Relationships * 5 See also * 6 References * 7 Further reading * 8 External links
LINNAEAN AND CLADISTIC CLASSIFICATIONS
SYNAPSIDS AS A REPTILIAN SUBCLASS
Synapsids were originally defined at the turn of the 20th century as one of the four main subclasses of reptiles , on the basis of their distinctive temporal openings . These openings in the cheek bones allowed the attachment of larger jaw muscles, hence a more efficient bite. Synapsids were considered to be the reptilian lineage that led to mammals; they gradually evolved increasingly mammalian features, hence the name "mammal-like reptiles", which became a broad, traditional description for all Paleozoic synapsids.
THE "MAMMAL-LIKE REPTILES"
The traditional classification of synapsids as reptiles is continued by some palaeontologists (Colbert "> 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.
Eothyris , an early synapsid with multiple canines
Synapsids are characterized by having differentiated teeth. These include the canines , molars , and incisors . 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 form of protocanines. This trait was subsequently lost in the sauropsid 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.
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).
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.
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
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.
The ancestral skin type of synapsids has been subject to discussion. Among the early synapsids, only two species of small varanopids have been found to possess scutes; fossilized rows of osteoderms indicate horny armour on the neck and back, and skin impressions indicate some possessed rectangular scutes on their undersides and tails. 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).
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, an animal noted as being semi-aquatic . The oldest
known fossil showing unambiguous imprints of hair is the Callovian
However, recent discoveries on Russian
Early synapsids, as far back as their known evolutionary debut in the Late Carboniferous period, may have laid parchment-shelled (leathery) eggs 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. Because they were vulnerable to desiccation, secretions from apocrine -like glands may have helped keep the eggs moist. 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, 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.
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 .
Recently it has been found that endothermy was present as far back as Ophiacodon .
The therapsids, a more advanced group of synapsids, appeared during
Only a few therapsids went on to be successful in the new early
Triassic landscape; they include
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-
Today, the 5,500 species of living synapsids, known as the mammals , include both aquatic (whales) and flying (bats) species, and the largest animal ever known to have existed (the blue whale ). Humans are synapsids, as well. Unique among the synapsids, however, most mammals are viviparous and give birth to live young rather than laying eggs, with the exception of the monotremes .
Below is a cladogram of the most commonly accepted phylogeny of
synapsids, showing a long stem lineage including
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 border:0;padding:0 0.2em;border-bottom:1px solid;vertical-align:bottom;text-align:center;">
? Elliotsmithia longiceps (BP/1/5678)
Euromycter rutenus (="Casea" rutena)
However, more recent examination of the phylogeny of basal synapsids, incorporating newly described basal caseids and eothyridids, returned Caseasauria to its position as the sister to all other synapsids. Brocklehurst et al. (2016) 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.
* ^ Seeley, Harry Govier (1895). "Researches on the Structure,
Organisation, and Classification of the Fossil Reptilia. Part X. On
the Complete Skeleton of an Anomodont
* ^ 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. PMID 8211141 . doi :10.1126/science.8211141 . * ^ Laurin, M.; Reisz, R.R. (2011). "Synapsida. Mammals and their extinct relatives".