Everything about Synapsid totally explained
Late Carboniferous - Middle
Cretaceous(exluding mammalia)
| image = Dimetrodon.jpg
| image_width = 210px
| image_caption =
Dimetrodon grandis skeleton at the
National Museum of Natural History of U.S.A.
| regnum =
Animalia
| phylum =
Chordata
| subphylum =
Vertebrata
| infraphylum =
Gnathostomata
| superclassis =
Tetrapoda
| classis =
Synapsida *
| classis_authority =
Osborn, 1903
| synonyms =
Synapsids ('fused arch'), also known as
theropsids ('beast face'), are a class of
animals that includes
mammals and everything closer to mammals than to other living
amniotes. The non-mammalian members were traditionally described as
mammal-like reptiles, and are sometimes referred to as "stem-mammals". Synapsids are one of the two major groups of
amniote, the other being the
sauropsids (or
reptiles in the proper sense). They are distinguished from the latter by a single opening (
temporal fenestra) in their
skull behind each eye, which developed in the ancestral synapsid about 324
million years ago (
mya) during the
late Carboniferous Period.
Synapsids were the dominant
terrestrial animals in the middle to late
Permian period. As with almost all life forms then extant, their numbers and variety were severely reduced by the
Permian extinction. Some species survived into the
Triassic period, but
saurischian dinosaurs quickly became the dominant animals and few of the non-mammalian synapsids outlasted the Triassic, although survivors persisted into the
Cretaceous. However, they included the prehistoric ancestors of mammals and in this sense, synapsids are still very much a living class of
vertebrates.
Changing classifications
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 attachment of larger jaw muscles, hence a more efficient bite. Synapsids were considered to be the reptilian lineage that led to mammals via gradually
evolved, increasingly mammalian features, hence the name "mammal-like reptiles" which became a broad, traditional description for all non-mammalian synapsids. They are currently divided between two primary synapsid clades, the
Caseasauria and the
Eupelycosauria, the latter of which also includes all the more advanced synapsids and therefore the mammals. That is to say, therapsids make up a well-defined clade within the eupelycosaurs, as long as mammals are included in the therapsids.
Characteristics
Synapsids evolved a
temporal fenestra behind each eye
orbit on the lateral surface of the
skull. It may have evolved to provide new attachment sites for
jaw muscles.
Some synapsids (including mammals) also have a
warm-blooded metabolism, even though early synapsids, such as pelycosaurs were most certainly
cold-blooded.
Like mammals, the nonmammalian synapsids possessed
glandular
skin that lacked
scales, though at least the
pelycosaurs retained the "scales" of more primitive
tetrapods on their undersides. These scales differed in structure from reptilian scales, an epidermal feature like mammalian hair or avian feathers. It is currently unknown at what stage they acquired mammalian characteristics like
body hair and
mammary glands, as the
fossils only rarely provide direct evidence for soft tissues. Much, however, can be inferred from differences in
skeletal structure. Thus the more primitive synapsids can be better visualized as being "naked
lizards", both
furless and scaleless, as their overall aspect was more like a modern lizard than a modern mammal, and the distinguishing features are relatively fine ones of internal structure. On the other hand, the presence of a
secondary palate, erect posture and other indicators of high metabolic rate among the advanced
cynodonts strongly suggests that many mammalian features, including an effective insulating layer of body hair, had evolved by this stage. This is now confirmed by impressions of fur in rocks directly underlying some fossil therapsids.
Synapsids are the first tetrapods to have differentiated teeth. These include the canines, molars, and incisors. Early synapsids had multiple jaw bones. As they evolved, these jaw bones were reduced in size and gradually moved into the ear, forming the middle ear bones.
Most
paleontologists hold fossilized
jaw remains to be the distinguishing feature used to classify synapsids and reptiles. 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
dentary, or lower jaw, consists of a single bone in mammals, where the lower jaw of modern and prehistoric reptiles consists of a conglomeration of smaller bones.
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
skull called the
quadrate 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 doesn't 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
Thrinaxodon has a full and completely closed palate, forming a clear progression.
Evolutionary history
Archaeothyris and
Clepsydrops are the earliest known synapsids. They lived in the
Pennsylvanian subperiod of the
Carboniferous Period and belonged to the series of primitive synapsids which are conventionally grouped as 'pelycosaurs'. The 'pelycosaurs' were the first successful group of
amniotes, spreading and diversifying until they became the dominant large terrestrial animals in the latest Carboniferous and Early
Permian Periods. They were sprawling, bulky,
cold-blooded and had small brains. They were the largest land animals of their time, ranging up to 3 m (10 ft) in length. Many, like
Dimetrodon, had large sails that may have helped
raise their body temperature. A few
relict groups lasted into the later Permian, but most of the 'pelycosaurs' became extinct before the
end of Permian.
The therapsids, a more advanced group of synapsids, appeared during the first half of the Permian and went on to become the dominant large terrestrial animals during the latter half. They have dominated the world twice: once in the
Permian and once in the
Cenozoic, the current era. They were by far the most diverse and abundant animals of the Middle and Late Permian and included herbivores and carnivores, ranging from small animals the size of a rat (e.g:
Robertia), to large bulky herbivores a tonne 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
extinction in
Earth's history, which may have been related to the
Siberian Traps volcanic event.
Only a few therapsids (and some relict 'pelycosaur' taxa) survived the Permian extinction and 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. Now, however, they were accompanied by the early
archosaurs (formerly known as
thecodonts; this term isn't used in modern classifications). Some of these, like
Euparkeria, were small and lightly built, while others, like
Erythrosuchus, were as big as or bigger than the largest therapsids.
Triassic therapsids included three groups. Specialised, beaked herbivores known as
dicynodonts (such as
Lystrosaurus and its descendants, the
Kannemeyeriidae), contained some members which reached large size (up to a tonne or more). The increasingly mammal-like carnivorous, herbivorous, and insectivorous
cynodonts included the
eucynodonts from the
Olenekian age, an early representative of which was
Cynognathus. Finally, there were the
therocephalians, which only lasted into the early part of the Triassic.
Unlike the dicynodonts, which remained large, the cynodonts became progressively smaller and more mammal-like as the Triassic progressed. From the most advanced and tiny cynodonts, which were only the size of a
shrew, came the first
mammal precursors, during the
Carnian age of the Late Triassic, about 220 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
Norian age, even before the
Triassic-Jurassic extinction event that killed off most of the large
non-dinosaurian archosaurs. Their places were taken by the archosaurs known as
dinosaurs, which dominated the terrestrial ecosystem for the rest of the
Mesozoic Era. 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
Tritylodon. No cynodont grew larger than a
cat. Most Jurassic and Cretaceous cynodonts were
herbivorous, though some were carnivorous. The family
Trithelodontidae first appeared near the end of the Triassic. They were
carnivorous and persisted well into the
Middle Jurassic. The other,
Tritylodontidae, first appeared at the same time as the trithelodonts, but they were herbivorous. This group became extinct at the end of the Early Cretaceous epoch. Dicynodonts are thought to have become extinct near the end of the Triassic period, but there's evidence that this group survived. New fossil finds have been found in the
Cretaceous rocks of
Gondwana. This is an example of a
Lazarus taxon.
Today, there are 4,500
species of living synapsids known as the
mammals, including both aquatic (
whales) and flying (
bats) species, and the largest animal ever known to have existed (the
blue whale).
Humans are synapsids as well. In fact, there remains one group of mammals that lays eggs like their ancestors. These are known as
monotremes.
Synapsids' evolution into mammals is believed to have been triggered by moving to a
nocturnal niche, one of the few niches that the increasingly prominent
dinosaurs didn't dominate.
Proto-mammals with higher metabolic rates were able to keep their bodies warm at night, and were more likely to survive. This meant consuming food (generally thought to be insects) in much greater quantity. To facilitate rapid
digestion, proto-mammals evolved
mastication (chewing) and specialized teeth that aided chewing. Limbs also evolved to move under the body instead of to the side, allowing proto-mammals to breathe more efficiently during locomotion and also to be able to change direction more quickly in order to catch small
prey at a faster rate. This helped make it possible to support their higher metabolic demands. It is believed that, rather than out-running
predators, proto-mammals adapted the strategy of outmaneuvering predators using their improved locomotor capabilities.
[Taxonomy
]Classification
Series Amniota
Phylogeny
Synapsida
|-Caseasauria
`-Eupelycosauria
|-Varanopseidae
`-+-Ophiacodontidae
`-+-Edaphosauridae
`-Sphenacodontia
|-Sphenacodontidae
`-Therapsida
|-Biarmosuchia
| `-Eotitanosuchus
`-Eutherapsida
|-Dinocephalia
`-Neotherapsida
|-Anomodontia
`-Theriodontia
|-Gorgonopsia
`-Eutheriodontia
|-Therocephalia
`-Cynodontia
|-+-Dvinia
| `--Procynosuchidae
`-Epicynodontia
|-Thrinaxodon
`-Eucynodontia
|-+-Cynognathus
| `-+-Tritylodontidae
| `- Traversodontidae
`-Probainognathia
|-+- Trithelodontidae
| `--Chiniquodontidae
`-+- Prozostrodon
`- Mammaliaformes
`-MammaliaFurther Information
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