DESCRIPTION OF THE CLASS EOSYNAPSIDA+[i]

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Eosynapsida (e-o-sin-AP-si-da) is formed from three Greek roots that mean “dawn animals with a single arch” [dawn- eos (έως); with- syn (συν); arch- apsida (αψίδα)]. We coined this term to refer to the pre-mammalian synapsids. The name is derived from a single temporal fenestra, also a characteristic of mammals.

INTRODUCTION TO THE EOSYNAPSIDA The synapsids, which also include the mammals, have a single zygomatic arch behind the orbit (Figure 1). This seems to have been the primitive condition for this clade unlike the euryapsids, which, although they have a single arch, evolved from the diapsid line by the loss of the lower arch. The synapsids also had taxa that were heterodont, and some of the primitive carnivores like Dimetrodon, a name that means two-measure teeth, had multiple fang-like “canine” teeth and smaller shearing teeth (Figure 2). The early synapsids, also called the Mammal-like Reptiles, formed two distinctive groups: pelycosaurs and therapsids (Benton 2005). The separation of the two groups (here the Pelycosauria and Therapsida) is artificial from a cladistic perspective because the pelycosaurs are nested in the clade that includes the therapsids and the mammals (see Figure 3B.). However, to make this group consistent from a cladistic perspective would require a series of nested taxa with the mammals, a sister group to the cynodonts (a suborder in this system). Laurin and Reisz (2007) address the issue by separating the Pelycosaurs and lumping the Therapsids into the mammals.

FIGURE 1. A generalized “reptilian” synapsid skull that shows the single zygomatic arch behind the orbit. Note also that the lower jaw is made of multiple bones. Image by: © Biodidac	FIGURE 2. The skull of Dimetrodon, a carnivorous pelycosaur, has the typical architecture of a synapsid skull with a single zygomatic arch. The temporal fenestra, the depression that underlies the arch is fairly modest on this animal. Note also that it has a pronounced heterodont condition. Image from: http://fossils.valdosta.edu/fossil_pages/fossils_per/images/t4_L.gif

FIGURE 3. A. The relationship of the Eosynapsida, an unnamed Class of Benton (2005), with the rest of the tetrapods. In this cladogram, the relationships appear to be simple and the Eosynapsids are in the clade which contains mammals as a sister group.

FIGURE 3. B. This cladogram, with the reptilomorph proamniotes as the outgroup, displays a much more complex view of the early synapsids. Benton (2005) describes two major groups: Pelycosaurs (P) and Therapsids (T). However, these groups, considered subclasses here, are nested from the Caesauria to the Cynodonts, which are the likely sister group to the mammals. Note that Tetraceratops is a pivotal taxon and does not fit well into either of the lizard-like pelycosaurs or the mammal-like therapsids.

THE PELYCOSAURS The pelycosaurs (from two Greek roots meaning “pelvis lizards”) were among the earliest of the amniotes, animals that produced eggs surrounded by an amnion, a special membrane that surrounds the egg and allows for the exchange of gasses (mainly carbon dioxide and oxygen) while retaining water (Figure 4). Thus, amniotes could lay eggs in environments that were terrestrial. The selective pressure for such a character may have been the reduction in predation. Eggs in the water were subject to predation by fish and aquatic tetrapods while those on land were exposed to a lower density of predators. So, the amniotic egg with a supporting shell may have preceded a fully terrestrial existence. Also, the amniotic synapsids probably did not have a larval stage, which was characteristic of pre-amniotic tetrapods. The pelycosaurs appeared in the lower Pennsylvanian and diversified into carnivores (see Eothyris, Figure 5) and herbivores (see Casea, Figure 6) that occupied terrestrial and semi-aquatic environments (Hopson 1969, Falcon-Lang et al. 2007). In general, they were low with splayed legs. Also, the temporal fenestra was relatively small. Furthermore, the lower jaw was very primitive or reptilian in that the dentary, the tooth-bearing bone, occupied only about half of the lower jaw. The earliest pelycosaurs were very lizard-like and seem to have coexisted with the early precursors of the diapsids. Archaeothyris (Figure 7) was a lizard-like insectivore of the Pennsylvanian period. However, others were carnivores that may have eaten early tetrapods and fish. The most notable of the pelycosaurs were the animals that evolved elongated neural spines that supported a sail. The most well-known taxa were Dimetrodon (Figure 2) and Edaphosaurus (Figure 8). That these animals were in different families and their sibling taxa did not have the sails suggests that the sail structure evolved independently. The neural spines of the sail-backed taxa had a high density of blood vessels, which likely were involved in thermal regulation. Bramwell and Fellgett (1973) demonstrated through a series of elegant experiments that the sails of Dimetrodon would cut the time the animal would need to sun itself by one-third over an animal without such a sail. Furthermore, the sail could also be an efficient structure for cooling the animal.

FIGURE 6. Casea was an early Caseasaurian synapsid from the lower Permian of Europe and North America. The ungainly, small-headed animal was only about a meter long and one of the earliest herbivores. It had adaptations for feeding that seemed to have been specialized for feeding on small tough plants like ferns and fern-allies. The lower jaw was toothless, and the upper jaw had peg-like teeth. The most striking aspect was the large ribcage which suggests the presence of a digestive fermentation chamber to process the tough plant material. Image by Nobu Tamura; Creative Commons.	FIGURE 7. Archaeothyris was one of the earliest pelycosaurs from the Pennsylvanian Period. It was lizard-like with both front and hind legs held in a sprawling stance. Image by: Arthur Weasley; Creative Commons	FIGURE 8. Edaphosaurus was a herbivorous pelycosaur in which the neural spine of the vertebrae became very elongate and supported a sail, which possible served a thermoregulatory function. Image by: Dimitry Bogdanov; Creative Commons

THE THERAPSIDS The therapsids (the name comes from two Greek roots meaning “beast arches”, a reference to the mammal-like zygomatic arch) were more mammal-like than the pelycosaurs. Their legs were pillar-like legs and their skulls had the long snouts, saggital crests, single sets of canines, and larger zygomatic arches more typical of the mammals. Also, the lower jaw was almost entirely taken up by the dentary bone. Some of them may even have had hair. The therapsids included both herbivores and carnivores and persisted from the lower Permian to the lower Cretaceous (Hopson and Barghusen 1986). Tetraceratops (a name that means four horned head, Figure 9) of the lower Permian was a transitional animal between the more reptilian pelycosaurs and mammal-like therapsids. Although it had a long, lizard-like body with splayed legs, its skull was very therapsid in its construction, including a large temporal fenestra. Laurin and Reisz (1996) interpreted the skull of the animal as being transitional. However, Conrad and Sidor (2001) reexamined the fossil and concluded that it was an unusual Sphenacodont (relative of Dimetrodon). The Permian therapsids diversified such that all seven suborders were represented (Modesto and Rybczynski 2000). They included small animals and some that were quite large. One of the most distinctive was a dinocephalian therapsid called Moschops (a name that means calf-face, Figure 10) of the middle Permian (Olson 1962, Battail and Surkov 2000). It had a stocky body that was more than five meters long, a short tail, and splayed legs (Gregory 1926). The teeth indicate that Moschops was a herbivore. It did have a well-reinforced skull, a condition that suggests they engaged in head-butting, a ritualized form of combat for territory and mates (Barghusen 1975). Only three suborders of the therapsids survived the great Permian mass extinction. However, those that did survive were very mammal-like (Kemp 2005). Whereas most of the reconstructions of Permian therapsids make the animals look reptilian, the reconstructions of the Triassic therapsids look like mammals. This distinction is more than superficial. The Triassic animals had a lower jaw that was almost entirely made up of the dentary bone, and the teeth looked very mammalian with well-differentiated incisors, canines and molars. The secondary palate developed a shelf that separated the mouth cavity from the nasal cavity more completely thus allowing the animals to chew and breath at the same time. Many of them had a skeleton with splayed ribs that suggested the operation of an efficient diaphragm. Lystrosaurus (a name that means shovel lizard, Figure 11), was a typical dicnodont: a herbivore with a stocky body, a beak, and only two teeth that grew as tusks. Furthermore, it was truly transitional as a therapsid. Lystrosaurus flourished in Gondwanaland during the late Permian (Modesto et al. 1999) and survived the Permian mass extinction to thrive in the absence of competitors and reradiate during the lower Triassic (Surkov et al. 2005). It then moved throughout the world to populate most major land masses. Most of them were moderate in size, somewhat comparable to pigs, and the heavily-built forelimbs are consistent with digging, and perhaps even burrowing. Cynognathus (a name that means dog jaw, Figure 12) of the lower Triassic had a somewhat mammal-like body and skull (Luo et al. 2002). The genus was very successful and was found on almost all of the land masses. The animal was only about a meter long, but its head was about one-fourth of the that length. Pits in the bone of the snout suggest that it had a collection of nerves where modern-day mammals have sensory whiskers. This secondary evidence implies that the animals had hair, a form of insulation that is most useful if the animal is endothermic. Still, there were some primitive features, which included a very large zygomatic arch and slightly splayed front legs. The last of the therapsids included Tritylodon (Figure 13) of the late Triassic and early Jurassic. It looked like a rat. Indeed, it was long with both sets of legs placed under the body, just like modern mammals (Luo 1994, Luo et al. 2002). The incisors were enlarged, characteristic of mammals that chew tough plant material and the structure of the shoulder and legs together with the tubular body all imply that it was a burrower. The three-cusped teeth and other features allied them with the early multituberculate mammals.

FIGURE 9. Tetraceratops, a name that means four-horned head, was an animal from the lower Permian that had characters which seem to make it transitional between the Pelycosaurs and Therapsids. In this system it is placed in the Therapsids because it has a very large temporal fenestra. Image by: Dimitry Bogdanov; Creative Commons	FIGURE 10. Moschops was the largest of the Permian therapsids. It was large and herbivorous, which made it like a splay-legged cow. The dome of the skull was reinforced which suggests that they engaged in head-butting. Image by: Dimitry Bogdanov; Creative Commons	FIGURE 11. Lystrosaurus had only two tusk-like teeth on either side of a horny beak. They were about the size of a pig and may have been diggers and burrowers. This genus survived the Permian mass extinction to thrive in the lower Triassic. Image by: Dimitry Bogdanov; Creative Commons	FIGURE 12. Cynognathus was a carnivore with a relatively large head, stocky body and clear mammal-like features in the skull and postcranial skeleton. It was about as big as a medium-sized dog. Image by: Dimitry Bogdanov; Creative Commons	FIGURE 13. Tritylodon was a burrowing animal with a body that resembled a weasel but with rodent-like features in the skull. Its cheek teeth had three-cusps and most other features are mammal-like. Indeed, it seems to belong to the sister group of the Mammals. Image by: Arthur Weasley; Creative Commons

[i] Eosynapsida is a name that literally means the dawn synapsids. We chose to keep them separate rather than create even more problems by lumping them into the Mammalia.

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By Jack R. Holt and Carlos A. Iudica. Last revised: 08/09/2016