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Athena Review Vol. 5, no. 1


Records of Life: Fossils as Original Sources


Synapsids 1: Pelycosaurs to Therapsids

 Pelycosaurs.

            Pelycosaurs (“pelvis lizards”), who emerged during the Late Pennsylvanian period (330-310 mya), were synapsids with early versions of differentiated teeth which featured larger canines, and a developing hard palate. By the end of the Pennsylvanian, early pelycosaurs called Ophiaconodonts (“snake-teeth”) gave rise to another group of Pelycosaurs, the Sphenacodonts (“wedge-point teeth”), who lived through most of the Permian. The Sphenacodonts, in turn, gave rise to the Therapsids (“beast-faces”); the name coined by T.H. Huxley for mammal-like reptiles, as a contrast to saurapsid reptiles or  "lizard faces".

            The Therapsids, a category now seen as too heterogenous for precise usage in cladistics, were nevertheless an important group with six major families which already had various mammal-like traits, including lactation. One of these, the Cynodonts (“dog-teeth”),  were the direct ancestors of mammals. As discussed in a later section, during the late Triassic and early Jurassic periods (230-150 mya), early mammals evolved from very similar Cynodonts

            In the Late Pennsylvanian period (306 mya), an early synapsid named Archaeothyrus emerged with a thicker, more robust lower jaw attached to the skull by a diagonal muscle fastened through the fenestra or hole behind the eye.  This arrangement provided more force or leverage in chewing or biting by Archaeothyris, who was an omnivore or carnivore. Consistent with this, Archeothyrus had enlarged upper canines, which shows a further  development of this reptilian character. Thus several related reptilian traits with evolutionary significance emerge in comparing the skull and jaw configurations of both of these early reptiles, linked to their eating habits. In the case of the carnivore Archaeothyris, stronger jaws and  larger, more specialized canine teeth proved an advantage not required by Hylonomus.

Pelycosaurs 1: Ophiacodontidae

            Ophiacodonts (“snake-teeth”), dating  from the Late Pennsylvanian and early Permian periods (about 300-280 mya) were a family of a large (2-5 m long), carnivorous pelycosaurs. They represent the earliest-known family of synapsids considered direct ancestors of mammals.

            Ophiacodon retroversus who lived in the Early Permian at about 290 mya, was first defined by Cope in 1878. Ophiacodon was derived from a sister taxon of the much smaller Archaeothyris,  noted in the previous section as the oldest known synapsidOphiacodon had a mixture of traits, with many aspects of the skull, postcranial skeleton and dentition retaining amphibian features. It reached 4 m in length and had a long narrow skull, which may have been best suited for fish eating.      

            Compared to Archaeothyrus, Ophiacodon showed several advanced skeletal features related to mammals, including shorter cervical and lumbar vertebrae. It also had elongated neural spines on the vertebra, although nowhere near as long as those of the later sailbacked Dimetrodon and other Sphenacodonts.  The skull of O. retroversus, like that of the earlier Archaeothyris, had the typical Synapsid single aperture behind the eye orbit, used as a muscle attachment in conjunction with stronger chewing muscles and enlarged canine teeth in the upper jaw or maxilla. Both the canine teeth and the skull aperture were smaller than those of Archaeothyris. Also distinct from Archaeothyris, the skull of Ophiacodon was relatively larger and had a larger rostrum or frontal area, with an upwardly curving mandible, which itself developed an opening or fenestra.                                                             

            The family Ophiacondontidae gave rise to at least five major lines of synapsids in the Permian, all considered as Pelycosaurs. These included Eothyridids, Varonopsids, Caseids, Edaphosaurids such as the large sail-back herbivore Edaphasaurus boanergi, and Sphenacodonts, all well represented in the fossil record. Only the latter group, however, the Sphenacodonts, are considered ancestral to mammals; the other four families are “sister taxa” of Sphenacodonts.

           

Pelycosaurs 2: Sphenacodontidae

          Sphenocodonts were large carnivorous pelycosaurs with distinctive “sail backs,” composed of skin-covered, elongated neural spines on their vertebra. Genera which descended from the earlier Ophiocodonts included Ctenospondylus, Dimetrodon, Secodontosaurus, and Sphenacodon. These were the dominant carnivores of the Early Permian (299-270 mya).           

            Dimetrodon (“two teeth sizes”), a genus named by Cope in 1878 for outsized canines compared to its other teeth, is a sister genus to Sphenocodon and is not considered a direct ancestor of mammals. Since it is well known, however, from abundant fossils in the Permian Basin in Texas and Oklahoma, as well as in finds from Germany, it provides a useful model for sphenacodont anatomy.

            Of the twelve recognized species of Dimetrodon the largest, D. grandis, was 3.2 m long (10 ft), and the smallest, D. Teutonis, 60 cm (2 ft) long. Its most distinctive feature is the “sailback” composed of very long neural spines on its vertebra, an area which some have suggested was used for body temperature regulation. If so, this represented an early form of homeostasis.  Other research, however,  suggests a display role for the tall sails in Dimetrodon and Edaphosaurus (Huttenlocker et al. 2011)  Compared to earlier pelycosaurs, Dimetrodon also had longer transverse spines as attachments for locomotion or walking muscles.

                The skull of Dimetrodon had medium-sized skull apertures or fenestra. Compared with Archaeothyris or Ophiocodon, its teeth were further differentiated along mammalian lines, with small incisors, two huge deep- rooted upper canines on each side, and smaller cheek teeth, all replaced continuously in the reptilian manner. Its jaw hinge, comprised of the quadrate and articular bones, was fully reptilian (contrasted with the eventual mammalian jaw hinge of the dentary and squamosal bones).

     Dimetrodon also had a primitive reptilian middle ear, hearing only ground-borne vibrations. Its lower jaw, however, included the first signs of a bony prong later to become part of the reptilian eardrum as the columella or stapes, which sensed air-borne sound vibrations.  Reptiles retained the stapes as a single ossicle in the middle ear, while early mammals developed three ossicles, the same as today: stapes,incus, and malleus, the latter two being reformed and miniaturized reptile jaw bones. (As the middle ear development is considered highly diagnostic for identifying early mammal fossils, the subject will be discussed in more detail in a later section.)

            Sphenacodon (class Synapsida; family Sphenacodontidae) the type genus for the Sphenacodontidae family, was a synapsid carnivore between 2 and 3 m in length. Sphenacodon and its sister genus Dimetrodon were contemporary predators from the Early Permian, whose fossils have typically have been found in different but adjacent geographical areas, separated during that period by an inland sea called the ancient Hueco Seaway that covered parts of New Mexico and west Texas (Lucas 2011). The American paleontologist O.C. Marsh (1878) first named Sphenacodon feroz (“wedge-point teeth”, “fierce”) for its back teeth with sharp cutting edges, based on part of a lower jaw or dentary bone found in the redbeds of northern New Mexico. All examples of S. feroz have since been found in New Mexico or the Utah-Arizona border region. 

            In 1937, Alfred S. Romer described a second species from New Mexico named Sphenacodon ferocior ("fiercer") that was larger and more robust, with proportionately longer neural spines. Romer and Price (1940) provided detailed descriptions of both S. ferox and S. ferocior with skeletal reconstructions.

            Dimetrodon is mainly known from Texas and Oklahoma in ancient deltaic and riverine environments, although the species Dimetrodon occidentalis is found in New Mexico (Berman 1977; Lucas et al 2009) During the Early Permian, Dimetrodon and Sphenocodon were mainly separated by the ancient Hueco Seaway. Each of the two genera, in its respective region, would have been a major terrestial predator, probably hunting amphibians, diadectids, and early synapsid and diapsid reptiles. Sphenacodon appears to have died out before about 280 mya during the Wolfcampian phase of the Permian, while Dimetrodon survived until about 270 mya.

            The skull of Sphenacodon is very similar to that of Dimetrodon, with the same array of skull bones (Romer and Prince 1940) It is narrow from side to side and vertically deep, with an indented notch at the front of the maxillary bone in the upper jaw. The upper and lower jaws are equipped with an array of powerful teeth, divided into large “canines" (called caniniforms), with sharp pointed "incisors" (precaniniforms) in front, and  smaller slicing back teeth (postcaniniforms). The eye orbit is set high and far back, with a single aperture or temporal fenestra behind and partly below the eye.

            Body proportions are also similar to Dimetrodon, with a very large head, short neck, robust trunk, relatively short front and hind limbs, and a tapering tail that makes up about half the animal's entire length. However, the tops of the neural spines along the back bone are strikingly shorter than in the sailback Dimetrodon. In Sphenacodon, the neural spines are enlarged but retain a flat-tipped, blade-like shape along the back and tail, and form a crest rather than a tall sail. In both Sphenacodon and Dimetrodon, there is evidence for strong epaxial muscles along the base of the raised neural spine, which likely helping to stiffen and strengthen the backbone for walking and for lunging at prey by restricting side-to-side flexing motion.

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Therapsids

            Sphenacodonts were ancestral to the order of Therapsids (“beast-faces”), a term considered synonymous with “mammal-like reptiles.” The Therapsids comprised six synapsid families or sister taxawhich had evolved during the middle and Late Permian, after the decline of the Sphenacodonts and the large “sailback” carnivore taxa of the Early Permian. Three of these, the Dinocephalia, Eotitanosuchids and Gorgonopsids (“gorgon-faces”), did not survive the massive end-of Permian extinction; a fourth, the Therocephalids, lasted into the Early Triassic. Only two of the Therapsid groups, the Dicynodonts and the Cynodonts, survived through the Triassic.

            One of these, the Cynodonts (“dog-teeth”), provided the next, and final, ancestral link between synapsids and mammals during the Late Triassic and early Jurassic The Dicynodonts, meanwhile, seem to have become extinct by the end of the Triassic.

 

Biarmosuchia (class  Synapsida; order Therapsida; suborder Biarmosuchia)

            Biarmosuchians, considered as one of the earliest, most basal groups of therapsids, lived during the Middle and Late Permian.  Biarmosuchians were moderately sized, lightly built carnivores, intermediate in form between the sphendacodont pelycosaurs, and more advanced therapsids. The type species, Biarmosuchus tener, was found in the Russian Urals.  Biarmosuchia is known from both Russia (as Phthinosuchus, Biarmosuchus, and Eotitanosuchus) and South Africa (as Ictidorhinidae, Burnetiidae, and related forms).

            The biarmosuchian skull is very similar to the sphenacodont skull. Among the retained, primitive, sphenacodontid features were jaw muscles inside the skull, and a slightly backward-sloping, platelike occiput at the base of the skull, the reverse of the pelycosaur condition where the occipital plate slanted forwards. Here it is attached still more strongly to the braincase. New features include larger temporal fenestra which occupy virtually all of the cheek, with the supratemporal bone completely gone (although these fenestra are still small relative to later therapsids). There is a reduced number of teeth, and single large canine teeth in both upper and lower jaws. In later specialised Biarmosuchia, these resemble the enlarged canines of the Gorgonopsia (Carroll 1988, p.370, Benton 2000, p.114). Tooth replacement was variable: some therocephalians (e.g Scylacosaurus) had just one canine, like mammals, and stopped replacing the canine after reaching adult size.

            The jaw hinge is more mammalian in position and shape, with mammalian-like jaw-closing muscle stronger, resulting in a stronger bite, as indicated by the flaring of the rear of the skull where these muscles were attached. The old amphibian-like hinged upper jaw has now been replaced by a fixed or immovable maxilla. Related new features include expansion of the maxilla to separate lacrymal from nasal bones, an intermediate trait between early reptiles and later mammals. There is still no secondary palate, but the vomer bones of the palate have developed a backward extension below the palatine bones. This is the first step toward a secondary palate, with exactly the same pattern seen in cynodonts. 

            The vertebrae of Biarmosuchus remained like those of sphenacodontid, although they lacked the long neural spines that distinguish Dimetrodon and its kin. Alterations in the shoulder and pelvic girdles and the limbs, meanwhile, are highly significant. All indicate a much more advanced posture and a radical alteration in the method of locomotion, with a much more mobile forelimb, more upright hindlimb, and more mammalian femur & pelvis. Primitive sphenacodontid humerus. The feet are more symmetrical, indicating that they faced forward throughout the stride, and the phalanges (toes) are reduced in length so that they are more like that of later synapsids, including both therapsids and mammals (Carroll 1988, pp. 370–1).The toes were approaching equal length, as in mammals, with the number of toe bones varying from reptilian to mammalian. The neck and tail vertebrae became distinctly different from trunk vertebrae. Finally, Biarmasuchia probably had a primitive eardrum in the lower jaw, located by the jaw hinge (the quadrate bone, which contained the stapes or columella).

Gorgonopsidae (class  Synapsida; order Therapsida; family Gorgonopsidae)  

            Gorgonopsidae are medium to large-size therapsid carnivores from the Late Permian with many advanced, mammal-like features. A number of genera have been found in the Karoo Basin of South Africa, where the group was first identified by Owen in 1876. Eva Gebauer (2007) reevaluated the relationships between various Gorgonopsid taxa, based on finds in the Ruhuhu Valley of Tanzania in the 1930s by Nowack (Von Huene 1950). A main concentration is on Sauroctonus parringtoni, which shows close similarities the Russian genus Sauroctonus, establishing for the first time a link between Tanzania and Russia in the Late Permian Pangaea.  Sauroctonus appears to have been a highly efficient predator, with surprisingly close analogies to the Pleistocene saber-toothed cat, Smilodon (who postdated Sauroctonus by 250 million years.)

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Glossary 
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References


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