DESCRIPTION OF THE CLASS ANAPSIDA (WILLISTON 1917)

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Anapsida (an-AP-si-da) is derived from two Greek roots which mean the absence [an (άν)] of an arch [apsis (ayiV)]. This refers to the absence of temporal fenestrae in the skulls of these animals. The name was created by Williston (1917).

INTRODUCTION TO THE ANAPSIDA The origins and relationships of the anapsids are obscure. As a group, the anapsids, both extant and extinct, range from large terrestrial herbivores to carnivores that occupied both aquatic and terrestrial environments. Some even evolved a bipedal gait independently of the Archosauromorpha. They have a very long fossil history (upper Pennsylvanian – 320/286 mya- to the present) with much diversity of form; however, only the turtles survive to the present. One of the confounding aspects of the Anapsida is that the class is defined by the absence of a zygomatic arch in the skull. However, the absence of a character is a weak basis on which to delineate a group. The anapsid condition could be primitive, in which case the taxa are synplesiomorphic for that character. The anapsid condition could be due to the loss of arches from a diapsid or synapsid condition (see Figure 1A & B for illustrations of anapsid and diapsid skulls). The loss could have occurred multiple times, in which case the anapsid taxa could be paraphyletic or polyphyletic. Thus, the basal affinities that the anapsid line has with the synapsids and diapsids are not clear (Figure 2A). Clearly, the turtles appear to be monophyletic and turtles+pareisaurids likely are monophyletic (Figure 2B). However, the sister groups for the anapsids are somewhat murky.

A.	B.
FIGURE 1. Two vertebrate skulls. A. The skull on the left has an orbit, but no other holes or arches (the anapsid condition). B. The skull on the right has two different arches making what appears to be two holes behind the orbit (the diapsid condition, the skull of lizards, snakes, crocodilians, and birds). Image from Biodidac.

A.	FIGURE 2.A. Major clades and affinities of the Anapsida with the other major groups of the gnathostomes according to Benton (2005). Parareptilia includes the Mesosaurids, Procolophonids, and Pareiasaurids. At issue is the clade of amniotes with which the Anapsids might be more closely allied. Taxa in red are extinct.
B.	MAJOR CLADES OF THE ANAPSIDA 1. Protorothyrinids + Anapsids 2. Protorothyrinid Clade 3. Anapsid Clade 4. Mesosaurid Clade 5. Parareptilia Clade 6. Procolophonid Clade 7. Pareiasaurs + Turtles 8. Pareiasaur Clade 9. Turtle Clade 10. Pleurodire Clade 11. Cryptodire Clade
FIGURE 2.B. Major Clades of the Anapsida. The Protorothyrinds are the outgroup for the Anapsida, but the relationship is questionable. It is bounded by clades of the synapsids (Mammals) and Diapsids (lizards, snakes, crocs, and birds). Taxa in red are extinct. The cladogram is after Pough et al. (1998), Pough et al. (2009), and Benton (2005).

The Protorothyrinids as Basal Anapsids (Figure 2 Clades 1 – 3) The protorothyrinids, like Hyalonomus (Figure 3) were very early amniotes dating to the upper Carboniferous. They were lizard-like in their appearance, and likely in their ecology. They are placed as a sister group to the Anapsids because their skulls were anapsid in form. However, Laurin and Reisz (1995) argue that the taxa in this group are paraphyletic and basal to all other amniote taxa. See an expansion on this discussion below.

Mesosaurid Clade (Figure 2 Clade 4) Mesosaurs were small (no more than 1m long) anapsids with a suite of characters suggesting that they were fully aquatic or semi aquatic (Figure 4) in the Permian. They had a long, flattened tail, much like that of a crocodile. In addition, they had long, narrow jaws filled with needle-like teeth. Likely, the teeth formed a sieve through which they might have strained small fish and invertebrates (Modesto 1999). Alfred Wegener (1915) used the mesosaurids to support his theory of continental drift by arguing the fossil deposits on the west coast of Africa and the east coast of South America had to have been continuous and separated only when the continents moved apart. Although the mesosaurs were aquatic, they were too small to be ocean-going animals.

Parareptilian Clade (5) and Procolophonid Clade (6) The Parareptilia were a collection of paraphyletic families from which the Pareiasaurs and turtles emerged. The Procolophonids (Figure 5), a late Permian family of parareptilian animals were burrowers, suggested by their broad feet and compact bodies. Their teeth were peg-like and occluded, particularly the cheek teeth, such that they were effective at chewing tough materials, either insects or fibrous plants.

Pareiasaurid Clade (Figure 2 Clade 8) Pareiasaurs were large herbivores of the upper Permian. They were stocky animals with legs and feet like small elephants. Scutosaurus was the largest of the pareiasaurs (Figure 6). They had heavy bony projections on the skull and a muscular hump (much like a bison) on the back between the shoulders. Their teeth reveal that they ate a diet of soft vegetation.

Turtle Clade (Figure 2 Clade 9) and Pleurodire Clade (Figure 2 Clade 10) Benton (2005) suggests that turtles evolved from a line of pariesaurs which became dorsoventrally flattened, evolved a protective shell or carapace made of flattened ribs, lost their teeth, and developed a horny beak, all synapomorphoies of the turtles. Today, turtles today are divided into two major groups, each defined by how it retracts its head into the carapace. The pleurodires are called the side-necked turtles because they retract their heads sideways in a s-shape as seen from above. Mostly, the pleurodires have a southern continental distribution and inhabit freshwater habitats. One of the most spectacular pleurodires is the Matamata (Figure 7) of the Amazon and Orinoco basins. They are semiaquatic piscivores which feed by suddenly opening their mouths to create a large cavity which sucks in fish.

Cryptodire Clade (Figure 2 Clade 11) Cryptodires retract their heads straight back so that the neck forms an s-shape as seen from the side. They include three major groups: the sea turtles and leatherbacks, the soft-shell turtles, and the tortoises and pond/box turtles. The sea turtles and leatherbacks (Chelonoidea) can grow to be more than 2 meters long (Figure 8). Although they are obligate marine animals with their limbs modified as flippers, they do come ashore on certain sandy beaches to lay eggs. The leatherbacks range throughout the oceans and feed almost exclusively on jellyfish. Softshell turtles (Trionychoidea, Figure 9) occupy temperate and tropical freshwater environments throughout much of the world. They have a reduced carapace skeleton, thus the name softshell and the covering is leathery. They are mainly carnivores. The tortoises (Testudinoidea, Figure 10) and pond turtles form the third great group of living cryptodires. Tortoises usually are large terrestrial herbivores of warm, dry environments. These may be among the longest-lived vertebrates on earth. For example, a Madagascar Radiated Tortoise presented to the royal family of Tongo in 1777 by Captain Cook lived to be more than 188 years old. Pond turtles and box turtles range from mostly aquatic to terrestrial animals of the tropics and temperate areas of the Americas, Eurasia, and Africa. By and large, the box turtles tend to be herbivores or omnivores while the pond turtles tend to be carnivores. The alligator snapping turtle is the largest freshwater chelonian in North America. It feeds by opening its mouth and wiggling an extension of its tongue as a lure. When a fish enters the mouth to investigate the movement, the turtle slams its jaws shut, instantly killing or disabling its victim.

Turtles and Amniote Systematics The systematics of the turtles is murky (see Figure 2.A). Are they sisters to the mammals? Are they sisters to the diapsid groups (lizards, snakes, crocodiles, and birds)? Are they from a basal line of amniotes? The answers to these questions determine the structure of the amniotes. If turtles are sisters to the diapsids, then they are somewhat equivalent to the mammals in rank. If so, they should be given class status (as they are in this system; see Figure 11-A). However, if the anapsid condition arose from within the diapsids (see Figure 11-B) as suggested by Rieppel (2000) and Hill (2005), all of the non-mammalian amniotes become monophyletic and form a class Sauropsida, which includes all “reptiles” + birds. If the turtles arose from a basal line of amniotes (see Figure 11-C), then they are sisters to the Synapsids and Diapsids; in which case, they should be given class-level status. So, the taxonomic fate of the typically “reptilian” groups rests on a much better, more subtle understanding of the turtles and their origins.

FIGURE 3. Hylonomus, a protorothyrinid from the Permian may have been a sister to the Anapsida. Image by Arthur Weasley, Wikipedia	FIGURE 4. Image of Mesosaurus, an extinct basal anapsid that was aquatic. Image from: http://www-geology.ucdavis.edu/~cowen/HistoryofLife/mesosaurus.gif	FIGURE 5. Procolophon, broad and flattened burrowing animals from the upper Permian to the Triassic. Image by Arthur Weasley, Wikipedia	FIGURE 6. Scutosaurus, a pareisaur, the group from which the turtles likely emerged. Image by Arthur Weasley, Wikipedia
FIGURE 7. Matamata, a pleurodire or side-neck turtle from South America. Image from: http://en.wikipedia.org/wiki/File:Chelus_fimbriatus_2005.jpg	FIGURE 8. A sea turtle, a chelonoidean cryptodire. Image from the Systematic Biology Biodiversity Archive.	FIGURE 9. A softshell turtle, a trinochoidean cryptodire. Image from: http://upload.wikimedia.org/wikipedia/commons/3/39/Apalone_spinifera.jpg	FIGURE 10. An Aldabra Tortoise, a testudoidean cryptodire. Image by Arpingstone, Wikipedia

A.	B.	C.
FIGURE 11. Three different cladograms regarding the relationships of the living amniotes. Cladogram A assumes that the turtles are sisters to the diapsids. Cladogram B assumes that the turtles arose from within the diapsids. Cladogram C assumes that the turtles arose from a basal line of amniotes. A=Anapsid; AM=Amniote; D=Diapsid; S=Synapsid.

LITERATURE CITED Benton, M. J. 2005. Vertebrate Paleontology. Third Edition. Blackwell Publishing, Malden, MA. Hill. R. V. 2005. Integration of morphological data sets for phylogenetic analysis of Amniota: The importance of integumentary characters and increased taxonomic sampling. Systematic Biology. 54: 530-547. Modesto, S. P. 1999. Observations on the structure of the early Permian reptile Stereosternum tumidum Cope. Paleontologia Africana. 35: 7-19. Pough, F. H., R. M. Andrews, J. E. Cadle, M. L. Crump, A. H. Savitzsky, and K. D. Wells. 1998. Herpetology. Prentice Hall. Upper Saddle River, NJ. Pough, F. H., C. M. Janis, and J. B. Heiser. 2009. Vertebrate Life. 8th ed. Benjamin Cummings. New York. pp. 688. Rieppel, O. 2000. Turtles as diapsid reptiles. Zoologica Scripta. 29: 199-212. Wegener, A. 1915 (reprinted 1929 and 1966). Die Entstehung der Kontinente und Ozeane [The Origin of Continents and Oceans]. Friedr. Vieweg and Sohn. Braunschweig. Translated and reprinted by Dover Publications, Inc. New York. Williston, S. W. 1917. The phylogeny and classification of reptiles. The Journal of Geology. 25(5): 411-421.

By Jack R. Holt and Carlos A. Iudica. Last revised: 11/12/2014