Synapsida

From Palaeos.org

Revision as of 10:10, 24 April 2007

o Cotylosauria
|--Diadectomorpha
`--+?--Casineria
   `--o AMNIOTA
      |--o Sauropsida
      |  |--Anapsida/Parareptilia
      |  `--Eureptilia 
      `--Synapsida

The Early Permian apex predator Dimetrodon. (Eupelycosauria,Sphenacodontia,Sphenacodontidae,)

The Synapsida ('fused arch') are an important group of amniotes that appeared and diversified very early, becoming the dominant life-form on land for the entire Permian period. Following the P-T mass extinction they wer much reduced in numbers and diversity, and found themselves progressively supplanted by the Archosaurian reptiles. Synapsids then became more and more mammal-like throughout the Triassic, finally becoming successful is small mammals near the end of the period. They remained under the feet of the dinosaurs until another great extinction event gave them the opportunity to once again inherit the Earth.

Two streams of evolution - Synapsid and Sauropsid

The Synapsids are one of two major evolutionary lineages of amniotes, the other being the sauropsids. The two clades diverged very early in the history of reptilian evolution, some time during the early part of the late Carboniferous.

Essentially, if very simplistically, the Sauropsids or "lizard faces" constitute an evolutionary sequence starting from a primitive anapsid condition (e.g. the parareptiles and the captorhinids) with a simple skull with no openings in the side for attachment of jaw muscles. The sequence continues through to various archosaurs, including dinosaurs, pterosaurs, and diverse other forms (most of which have various temporal skull openings for muscle attachment); and from archosaurs finally to birds -- thus far the culmination of Sauropsid evolution.

The Synapsida, or to give them their alternative name, Theropsida or "beast (mammal) faces" constitute a different evolutionary lineage that developed a single opening, the synapsid arch, behind each eye, for attachment of jaw muscles, giving a superior bite and permitting adaptive radiation during the late Carboniferous. These basal forms evolved through the primitive pelycosaur stage, to the therapsids or mammal-like reptiles, and finally the mammals themselves. Pelycosaur, therapsid, and mammal represent three evolutionary grades in a single progressive evolutionary axis (see the cladogram, below). The therapsids, as forms transitional between basal amniote and mammal, can be thought of as occupying the same evolutionary space as the dinosaurs, which are transitional between reptiles and birds, do.

o Amniota
|--Sauropsida
`--o SYNAPSIDA
   `==Pelycosauria
      `==Therapsida
         `==Mammaliaformes
            `--Mammalia 

Evolutionary history

Archaeothyris and Clepsydrops were the earliest known synapsids. They belonged to a group called pelycosaurs and they lived in Pennsylvanian time of the Carboniferous Period. 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 are currently divided into two clades, the Caseasauria and the Eupelycosauria. 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.

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 were by far the most diverse and abundant animals of the Middle and Late Permian, including a diverse range of 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 no pelycosaurs, survived the Permian extinction and went on to be successful in the new early Triassic landscape; they include Lystrosaurus and (later in the early Triassic) Cynognathus. Now, however, they were accompanied by the early archosaurs (formerly known as thecodonts, although this term is not 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, the specialised, beaked herbivores known as dicynodonts (such as Lystrosaurus and its descendants, the Kannemeyeriidae), some of which reached large size (up to a tonne or more); the increasingly mammal-like carnivorous, herbivorous, and insectivorous cynodonts (including, from the Olenekian age, the eucynodonts, an early representative of which was Cynognathus) and 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 (only the size of a shrew, came the first mammal precursors, during the Carnian age of the Late Triassic, about 220 million years ago (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, so that 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 or 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 all of the large non-dinosaurian archosaurs. Their places were taken by the diapsid 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 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, and 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 is evidence that this group survived. New fossil finds have been found in the Cretaceous rocks of Gondwana. This is an example of Lazarus taxon.

Today, the 4,500 species of living synapsids are currently the dominant land animals and include both aquatic (whales) and flying (bats) species, including the largest animal ever known to have existed (the blue whale).

The synapsids have dominated the world twice, once in the Permian and once in the Cenozoic (today).

Synapsids evolution into mammals is believed to be triggered by moving to a nocturnal (night) niche, one of the few niches that the increasing dinosaurs didn't dominate. In order to survive at night, proto-mammals had to increase their metabolic rate to keep their body warm. This meant consuming food (generally thought to be insects) more rapidly. 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. This allowed the proto-mammals to be able to change direction quicker in order to catch small prey at a faster rate. Rather than out-running predators, instead proto-mammals adapted the strategy of outmaneuvering predators using this same ability, it is believed.

Information

Cladistically, the Synapsida are a vertical clade, not a horizontal class or grade. Hence the term Synapsida refers to a major monophyletic group in the present cladistic interpretation of tetrapod evolution. But confusion can come about because the term Synapsida is also used in the Linnean/Evolutionary system to refer to all theropsids except mammals.

Stratigraphic Range: as paraphyletic Linnaean taxon (subclass of Reptilia) Carboniferous to Cretaceous ; as monophyletic clade Carboniferous to Recent

Original Author: Osborn, 1903

Changing classifications

The Synapsids were originally defined, at the turn of the 20th Century, as one of the five main subclasses of reptiles, on the basis of their distinctive [[temporal openings. These openings in the cheek bones were for the purpose of allowing attachment for larger jaw muscles, hence a more efficient bite. The synapsids represented the reptilian lineage that led to the mammals and gradually evolved increasingly mammalian features, hence the term "mammal-like reptiles".

The traditional classification continued through to the late 1980s (see e.g. Carroll 1988). In the 1990s this approach was replaced by a cladistic one, according to which the only valid groups are those that include common ancestors and all their descendants. Because Synapsids evolved into mammals, the mammals therefore are included under the Clade Synapsida.

A recent, compromise position (see Benton 2004) has the class Synapsida as intentionally paraphyletic, constituting a grade of animals from the earliest split with sauropsids to the arbitrary division with its daughter class, Mammalia.

Taxonomy

Classification

• Series Amniota
  • CLASS SYNAPSIDA *
    • Order Pelycosauria *
      • Suborder Caseasauria
      • Suborder Eupelycosauria *
        • Family Varanopseidae
        • Family Ophiacodontidae
        • Family Edaphosauridae
        • Family Sphenacodontidae *
    • Order Therapsida *
      • Suborder Biarmosuchia *
      • Suborder Dinocephalia
      • Suborder Anomodontia
      • Suborder Gorgonopsia
      • Suborder Therocephalia
      • Suborder Cynodontia *
        • Family Probainognathidae
        • Superfamily Chiniquodontoidea *
  • CLASS MAMMALIA

Phylogeny

Synapsida
 |-Caseasauria
 `-Eupelycosauria
    |-Varanopseidae
    `-+-Ophiacodontidae
      `-+-Edaphosauridae
        `-Sphenacodontia
           |-Sphenacodontidae
           `-Therapsida
              |-Biarmosuchia
              |   `-Eotitanosuchus
              `-Eutherapsida
                 |-Dinocephalia
                 `-Neotherapsida
                   |-Anomodontia
                   `-Theriodontia
                       |-Gorgonopsia
                       `-Eutheriodontia
                         |-Therocephalia
                         `-Cynodontia
                            |- + -Dvinia
                            |  `--Procynosuchus
                            `- Epicynodontia
                               |-Thrinaxodon
                               `-Eucynodontia
                                  |- + -Cynognathus
                                  |  `- + -Tritylodontidae
                                  |      `- Traversodontidae 
                                  `-Probainognathia
                                     |- + - Trithelodontidae
                                     |  `--Chiniquodontidae 
                                     `- + - Prozostrodon
                                        `- Mammaliaformes
                                            `-Mammalia

References

• Benton, M. J. (2004), Vertebrate Paleontology, 3rd ed. Blackwell Science Ltd
• Carroll, R. L. (1988), Vertebrate Paleontology and Evolution, WH Freeman & Co.
• Colbert, E. H., (1969), Evolution of the Vertebrates, John Wiley & Sons Inc (2nd ed.)
• Laurin, M. and Reisz, R. R., (1997), Tree of Life - Synapsida - Tree of Life Web Project

Links

• Transitional Vertebrate Fossils - includes description of important transitional genera from reptile to mammal
• [1]

Credits

Kheper MAK(date), Palaeos com - Synapsida, Palaeos org MAK061001

wikipedia

• Evolutionary history, Taxonomy, References http://en.wikipedia.org/wiki/Synapsid (MAK060501 co-author), copied to Palaeos org MAK061018
• Phylogeny