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Athena Review Vol. 5, no. 1


Records of Life: Fossils as Original Sources



The Origin of Reptiles:

Amniotes vs. Anamniotes

             As currently understood, the first reptiles evolved from reptilomorph amphibians during the Paleozoic era about 335-320 million years ago (mya), in the Late Mississippian or Early Pennsylvanian (Carboniferous) period. An early, and definitive reptile innovation was the amniotic membrane, which surrounds and protects the developing embryo in all reptiles, birds, and mammals (fig.1). This provided a major advantage for reptiles over amphibians, in that the reptiles could move away from water into inland and upland environments, and exploit more food resources.

            As discussed in the section on Classification, the term Amniota was introduced by Ernst Haeckel in the 1860s and 70s as a superclass for all reptiles, birds and mammals. This remains in standard usage. Amphibians, tetrapods, and fish, on the other hand, who lack this trait, are classified as Anamniotes.  The first eggs provided with an amniotic membrane were probably small, lacked a shell or outer leather membrane, and were laid by small adult females in a transitional environment such as moist leaf litter, that stayed humid and protected both the adult and the egg from desiccation. 

            The reptile egg (fig.1), besides having the amnion or protective membrane over the embryo, has two attached sacs, one called the allantois which exchanges fluids with the embryo, and the other the yolk sac which contains food. Around these is an inner wall called a chlorion, with all being encompassed by a shell. 

   Fig.1: Diagram of the amniotic egg of a reptile.               

Sauropsid reptiles: "Lizard faces" and anapsids, diapsids, and eurapsids.    

          Primitive reptiles have long been recognized to have split into two major branches, sauropsids and therapsids (now usally called synapsids) [1] The first branch, the Sauropsids ("lizard faces")  include the forerunners of most reptiles, from dinosaurs to crocodiles and turtles. The term Sauropsid for ancestral reptiles was coined by Thomas Henry Huxley in 1863, when he grouped the vertebrate classes informally into mammals, sauroids, and ichthyoids (the latter containing the amphibians and fish). He subsequently proposed the names of Sauropsida and Ichthyopsida for reptiles (amniotes) vs. amphibians and fish (anamniotes).

                The term Sauropsida was reused later by E.S. Goodrich in 1916, in the context of much greater fossil information by then available on mammal-like reptiles (synapsids) from South Africa, allowing palaeontologists to trace synapsid evolution in much greater detail.  Goodrich used the term Sauropsida much as Huxley had first used it, to include lizards, birds and their relatives. He distinguished them from mammals and their extinct relatives, which he included in the sister group Theropsida or "beast faces,"  now spelled Therapsida. According to Goodrich, both Sauropsids and Therapsids evolved from an earlier stem group, the Protosauria ("first lizards"), which included some Paleozoic amphibians as well as early reptiles predating the sauropsid/therapsid split .

          Romer (1956) later subdivided the sauropsids into three basic groups,  recognizable by the number of openings or fenestrae (Latin for "windows") in the side of the skull, used for jaw muscle attachment (fig.1). This anatomical criteria for all reptiles remains in widespread use (Benton 2005). The anapsids, with no fenestrae, include many basal reptiles or eureptiles, some lizards, and perhaps also the turtles (molecular studies, however, indicate the placement of turtles within diapsids). The diapsids, with two fenestrae,  produced the dinosaurs, and more recently, the birds. Huxley (1876) had correctly inferred that birds, who are diapsids, evolved from lizards including dinosaurs, which are also diapsids. An offshoot group of the diapsids, called the eurapsids, with two fenestrae fused into one, produced the large marine reptiles called ichthyosaurs, including Pleisiosaurs and Mosasaurs who were common during the Cretaceous.  

            Since the advent of cladistics in the late 1960s and its strict focus on phylogeny or evolutionary links, the term Reptilia has fallen out of favor with taxonomists as too imprecise, with the term Sauropsida used in its place to include a monophyletic group containing reptiles and birds. It seems very likely, however, that common usage will long retain traditional distinctions between Reptiles, Amphibians, Fish, and Mammals (all shown as Classes in the Linnean scheme).

Synapsids and "beast faces"           

            The other major branch of reptiles, the Synapsids (fig.2), have a single large fenestra on each side of the skull, located behind the eyes. The synapsids developed an aorta or arch on the left side only, and strengthened the skull by moving the quadrate bone up and back, eliminating the otic notch. This modification of the jaw (effectively, eliminating the location of the reptilian ear drum, in the otic notch)  proved essential in the later development of more complex anatomy related to mammalian hearing (Hunt 1994-7).  

        Synapsid development led to a larger brain, more developed hearing, heterodont teeth, and homeothermy or regulated body temperature.  A group of synapsids known as  the Therapsids ("beast faces; the term coined by Goodrich [1916]), equivalent to the term "mammal-like reptiles," are considered to be the ancestors of mammals, by way of the Cynodonts, mammal-like synapsids in the Early and Middle Triassic.  Evidence on transitional fossils bears out the development of these late, advanced synapsids into early mammals by the Late Triassic and Early Jurassic. Synapsids and their descendants are covered in later sections.

 Fig. 2: Skull fenestrae (after Benton 2005).  

       The following discussion presents a summary of some of the earliest types of Saurapsid reptiles.

        Romer (1956) defined the order Cotylosaura (“socket-lizards”) as a diverse assemblage of primitive anapsid reptiles containing, among other groups, the  Captorhimomorphs, Diadectics, and Pareisours. Of these, only the captorhinomorphs are recognized as being related to more modern reptilians (Heaton 1979).  Since they have been defined as being at the base of reptilian phylogeny, a number of  detailed descriptions of their anatomy have been made by Carroll (1970), Carroll and Baird (1972), Clark and Carroll (1973), Heaton (1979), and others, who probably would be classified as evolutionary systematists (see section on Classification). More recently, cladistic analyses of Captorhinomorphs have been compiled by Reisz et al (2011), shown below in fig.2.

.Eureptiles and Captorhinomorphs 

            During the Middle Carboniferous (Late Mississippian to early Pennsylvanian), according to a fairly recent  interpretation by Benton (2005), the amniote radiation gave rise to three primary clades or branches: the Eureptiles ("true reptiles"), the Anapsids, and the Synapsids. Here Eureptiles are portaryed as a "sister group" of the anapsids, both being subgroups of  Sauropsida, the most basal reptilian group, from which Synapsids also derived

            In an alternative scheme attributed to evolutionary systematists (Palaios 2014b), the most basal reptile group are the Captorhinomorpha,  thought to be the single ancestral stem group from which all other reptiles evolved; and from them, birds and mammals (Carroll 1988 ). The name Captorhinomorpha is derived from that of the type genus Captorhinus, meaning "nose capturer.".  

             In the scheme of Benton (2005), the diapsid reptiles, who were later to flourished in the age of reptiles (Triassic through Cretaceous), are classified as descending from Romeriids, one of the Eureptile groups. They and anapsids are thus  distinguished from synapsids who separated from all other early Sauropsids at some time in the Pennsylvanian. Sauropsids then developed during the Permian and Early Triassic into Therapsids and Cynodonts, the forefunners of early mammals (Benton 2005; Palaios 2014).    

             The Eureptiles were terrestrial forms who were still partly lizard-like, characterized by very primitive features, showing that they had only just diverged from their reptiliomorph ancestors (Palaios 2014b). They had rounded, amphibian-like skulls, with most taxa aretaining from fish and tetrapods an opening in the frontal bone for the pineal gland (a “third eye”). They also had amphibian-like shoulders and hips (small, relatively weak pectoral and and pelvic girdles), and limbs (i.e., fused lower leg bones). The rest of their skeleton was mainly reptilian, with spool-shaped vertebral centra. 

              The remainder of this section will focus on early reptile familes from both the Eureptile and Anapsid branches of Saurapsids. In the Eureptiles, this includes the Protothyrididae family (within the Romeriida clade), plus one very early genus named Casineria, still unclassified but which is probably a Eureptile. Within the Anapsid branch, descriptions are given of a number of genera within the Captorhinidae family, also considered as very early reptiles.

 Eureptilia

  

Casineria    

 Class:  Amphibia/Reptilia (uncertain); clade:  Archosauromorpha  Order:  Eureptilia ?

             Casineria kiddi, the type species, was discovered by an amateur paleontologist in 1992 on the shore of Cheese Bay, Scotland, from formations datings from the Visean phase of the Early Carboniferous or Mississippean (about 335 mya). Casineria is probably the earliest fossil taxa yet classified as an amniote (a basal or proto-reptile). The Archosauromorpha clade which it is assigned to represents the earliest amniotes (Paton, Smithson and Clack 1999). 

            Casineria was a small insectivore with five fingers, and with claws on each hand, representing the earliest evidence of claws. The development of claws is linked to the formation of keratinous scales in reptiles; thus Casineria very likely had scaly, reptilian skin. The skull of Casineria had no otic notch, as is also the case with another early Eureptile, Brouffia, described below (thus, it probably had no tympanic membrane or eardrum, and consequently had poor hearing of high frequency sounds). Casineria had a mixture of anatomical features, some from the amphibian reptiliomorph groups such as Seymouriamorpha and Diadectomorpha, but others linking it with early reptiles, such as lighter leg bones, unfused ankles, and toes terminating in claws. 

            All this would have enabled the animal to walk more efficiently, and indicates a primarily terrestrial lifestyle. The small size of Casineria is also seen as relevant to a speeded-up evolutionary path, which has probably occurred often in vertebrate branches. Smaller animals have relatively short lifespans, but generally mature faster and thus reproduce more often, which likely had evolutionary significance in the case of Casineria (cf. Carroll  1970).

 

Romeriida

 Class:   Reptilia     Clade:  Eureptilia    Clade:  Romeriida     

Romeriida     

             Romeriida (which does not include the genus Romeria, which is a Captorhinidae),  is a clade of reptiles ancestral to diapsids, who first appear during the the Pennsylvanian period, and last into recent times (311.3–0 mya). The grouping consists of diapsids (including dinosaurs and birds), the genus Paleothyris, and the large extinct family of early reptiles named Protorothyrididae (Gauthier et al., 1988)  It is defined by Laurin & Reisz (1995) in cladistic terms as the last common ancestor of Paleothyris and diapsids, and all its descendants. The grouping is named after Alfred Romer, a prominent vertebrate paleontologist of the twentieth century.

            Several anatomical traits characterize the romeriids. Skull changes include the separation of the tabular bone from the opisthotic bone. In the post cranial skeletal, there are vertebrae changes to stronger, more reptilian forms, and a trend to longer and more slender wrist and akle bones (carpals and tarsals).

.

Protorothyrididae

 Class:   Reptilia      Clade:  Eureptilia    Clade:  Romeriida    Family:  Protorothyrididae   Price, 1937

            Protorothyrididae is a family within the Romeriida clade, representing  small, short legged, lizard-like insectivores.  They lived from the Late Carboniferous to Early Permian periods (Pennsylvanian-Asselian stage, dated at 307.1–294.6 mya),  in the midwestern region of North America. In the cladistic analysis of Müller and Reisz (2006), the Protothyrididae aree defined as the stem lineage from which the Diapsids evolved. Among the basal eureptiles in this family are Brouffia, Coelostegus, Paleothyris, and Hylonomus.

            
    
         Coelostegus prothales is from the Late Carboniferous of Nyr'any in the Czech Republic. This is the most basal Eureptilian, according to Müller & Reisz (2006).   

            Protorothyris archeri  was first named by Price (1937). Carroll & others place this as early amniote stem group with Hylonomus. 

.             Cephalerpeton is a protorothyridid found at the Mazon Creek site in Illinois, in the Francis Creek Shale Member of the Carbondale Formation. These levels date from the late Westphalian stage of the Pennsylvanian (Late Carboniferous, 306 mya).  Its holotype at the Yale Peabody Museum (YPM 796) is a partial skeleton.  The type species, Cephalerpeton ventriarmatum, was first named by R. L. Moodie in 1912 as an amphibamid amphibian [3]. The genus was later assigned to the basal reptilian  Protorothyrididae family by Robert L. Carroll and Donald Baird in 1972, and this placement has been widely accepted.

            Cephalerpeton shows many primitive traits. Cephalerpeton ventriamatum, measuring 10 cm in length, has similarities with the earlier Reptiliform amphibian, GephyrostegusThe skull of Cephalerpeton was relatively large with a large orbit,  traits typically associated with juveniles. The quadrate was aligned vertically, and the otic notch was greatly reduced, with relevance to the development of the reptilian middle ear in this taxa.

        Its upper (maxillary) teeth were very large. The mandible was concave on its upper (dorsal) surface, in order to accommodate the upper teeth. The maxilla was slightly raised to just above the lower rim of the orbit. the premaxilla was bent downwaard, with the longest premaxillary teeth located toward the center. The palate was relatively shorter. The pterygoid bone in the skull also had a single transverse row of teeth. 

         In the post-cranial skeleton, the cervical vertebrae were elongated, and numbered two more than in Gephyrostegus. The scapula and coracoid were unfused and as tall as the neural spines. The humerus was longer, more slender and hourglass-shaped. The radius and ulna were likewise more slender and relatively longer than in the earlier reptilomorph. Regarding the hands, while only the metacarpal or wrist bones are preserved, they appear to be assymmetrical as in Gephrostegus, with the fourth metacarpal the longest.

           Brouffia orientalis is a species of basal reptile dating from the Late Carboniferous, and found at Nyr'any in the Czech Republic.  According to Müller & Reisz (2006), they are even more basal than Paleothyris, in the Captorhinidae. 

            Brouffia is represented by a skull and partial skeleton, which Brough and Brough (1967) originally identified as the Reptiloform amphibian, Gephyrostegus. Features, which are otherwise only known in such advanced amphibians, include an ilium with two tops, and an intertemporal bone in the skull, both identified by Brough and Brough (1967).  Later analysis by Carroll and Baird (1972) , however, interpreted the intertemporal bone as being absent, and having fused to the parietal, a trait of early reptiles. A lack of complete ossification in the pelvis and pectoral girdle suggests the specimen represents a juvenile, while presence of complete ossification in the carpus argues for a mature status. Brouffia is thought to be related to both Cephalerpeton and Casineria, the latter probably among the most primitive of reptiles.


Anapsida

Captorhinidae   

Class:  Sauropsida : Clade: Anapsida : family: Captorhinidae

             Captorhinidae is a family of Anapsids dating from the Late Carboniferous (Pennsylvanian) and Early Permian. It was first defined as a family by Case (1911), based on examples of Captorhinus aguti found by Cope (1882) from the Arroyo Formation of the Leonardian phase of the Early Permian (280-270 mya) in northern Texas. The name, Captorhini or "capture-nose", refers to a view that the hook-like premaxilla was used to capture prey (Cope 1882; Palaois 2014b). The family was further defined in numerous studies, including those of Romer (1933, 1956), Fox and Bowman (1957), Clark and Caroll (1973), Hatton (1979), and Kissel et al. (2002). Its taxa have been recently studied by Reisz et al. (2011), whose analysis of their phylogeny or ancestral relations is presented in a cladogram  (fig.3).

            Captorhinids inhabited an area of Pangea called the Wichita uplift, now part of the Permian Basin in Oklahoma and north Texas. They have broad, robust skulls that are generally triangular or heart- shaped when seen from above (i.e., in dorsal view),  caused by a swollen cheek region. The premaxillae are characteristically hooked or downturned. Early, smaller forms including Protocaptorhinus and Romeria possessed single rows of teeth, while larger, more derived forms such as Captorhinus aguti have two or three rows of teeth on both the maxillary and dentary bones. The teeth are short, with chisel-like tips.

  Concordia, dating from the Late Pennsylvanian, is  the earliest known captorhinid; all other taxa from this family are known only from Permian deposits. In anatomical terms, the following traits are observed by Müller and Reisz (2005), in their study of Eureptilia: The premaxilla has no curvature on its bottom (ventral) side, and the lower jaw has a dorsal ridge. Caniniform teeth are absent.  Toward the rear of the skull, the supratemporal bone is small. The parietal and squamosal bones are broadly in contact, so that the postorbital does not reach the supratemporal. 

        The stapes or columella,  a hearing ossicle, is short. The jugal bone has a sharply pointed anterior process, and lacks a medial process, with distinct anterior and posterior ventral margins.   (Müller & Reisz 2005)

Fig.3: Cladogram of Captorhinidae (after Reisz et al. 2011).

            Hylonomus (“forest-dweller”) was an early Captorhinidae from the Pennsylvanian period (312 mya). Like  the related Paleothyris, Hylonomus was a small, lizard-sized reptile about 20 cm (8 inches) long.   Hylonomus was an insectivore, with small, pointed teeth, but also with the Sauropsid trait of larger canines in the upper jaw. The muscle attaching the lower jaw of Hylonomus to its skull was of the primitive reptilian type, a simple vertical band used for opening and shutting the jaw, without much required force.       

            Paleothyris acadiana was found in Middle Pennsylvanian deposits in Nova Scotia (Carroll 1969). It had a number of features that were probably typical for both protorothyrids and basal eureptiles as a whole. These included an inncrease in the strength of the jaws, when compared with Hylonomus. A muscle attached to the pterygoid bone of the skull (the Pterygoideus), supplemented the adductor chewing muscles in pulling the jaw up and forward. 

            The rear upper (palatial) teeth were small, possibly designed to hold  food rather than chew it. Its stapes was relatively heavy as in basal tetrapods, so its hearing probably limited to low frequency vibrations. There was no otic notch, and hence no tympanium or ear drum (Benton, 2000, pp.104-5) 

             Romeria texana dates from the Asselian stage of the Cisuralian epoch of the Early Permian (299-294 mya). It was first found by Llewellyn Ivor Price (1937) at the Cottonwood Creek site, in the Archer City Formation in northern Texas (holotype MCZ 1480). A second species, Romeria prima, from the same location, was found by Clark and Carroll in 1973 (holotype MCZ 1963; Heaton 1979) Both species were represented by skulls .
            Romeria has a single set of teeth in its upper and lawer jaws (maxilla and dentary), which Heaton (1979) regarded as a primitive trait, also shared by Protocaptorhinus, Captorhinus laticeps, and Captorhinus major. This contrasts to the dentition of Captorhinus aguti, which is found in later stratigraphic levels, and has two or three rows of teeth in both the maxilla and dentary.

             Protocaptorhinus pricei  was first found in the  Admiral Formation of Texas (Late Wolfcampian stage of the Permian, 290-280 mya), which predates the Arroyo formation of the Clear Fork group (Leonardian stage; 280-270 mya) by  5-10 million years. It was described by the discoverers, Clark and Carroll (1973), as a "Romeriid".  According to Heaton (1979), it is a primitive form of Captorhinid which is found consistenly in Early Permian layers below the Arroyo formation in Texas, and the equivalent Wellington layers of Oklahoma.

           
  Captorhinus aguti , the type genus for the Captohinidae family, was first found by Cope (1882) in the Clear Fork group of the Permian Basin of northern Texas (280-270 mya). It was a small herbivorous anapsid with a body length about of 40 cm, and a skull length of 5.5 - 8.0 cm. C. aguti is well represented in the Permian basins of Oklahoma (Garber Formation, Sumner Group), and Texas (Arroyo Formation). 

               Captorhinus laticeps ("broad headed Captorhinus") was first discovered in north Texas as a poorly preserved sample by Cope (1882), who named it Ectocynadon. It was later renamed Patriatus laticeps by Williston (1909). After a number of subequent name permutations, on the basis of well-preserved samples from Oklahoma, it was renamed Eocaptorhinus laticeps  ("dawn  Captorhinus") by Heaton (1979, p.11).  Eocaptorhinus, in Heaton;s words (1979, p.5) showed  "an astonishing correlation of similar anatomical characters with Sphenadon and primitive iguana lizards, leading to the use of these modern forms as the major source of information."   Liike the more primitive forms, Romeria and Protocaptorhinus, C. laticeps has only a single rows of teeth in its dentary and maxilla.   The skull of C. laticeps, from 6.5-8.0 cm  in length, is larger and wider than those of either Romeria or Proocaptorhinus. Like them, it also had a pineal foramen (third eye) in the top of the skull, a primitive trait.

             Heaton (1979) considered Eocaptorhinus as an intermediate form between Protocaptorhinus and Captorhinus.  On the basis of stratigraphic finds of Captorhinidae with single rows of teeth (i.e., lower down and considered earlier) vs. those whith multiple rows (i.e., higher up and considered as later), Heaton proposed the evolutionary sequence of 1) Romeria  to  2) Protocaptorhinus to  3) Eocaptorhinus to 4) Captorhinus. 

              More recently, E. laticeps has been assigned to Captorhinus by several authors who, on the basis of cladistic analysis, perceived a general similarity to C. aguti, the type species of Captorhinida (Kissel, Dilkes, and Reisz 2002; Reisz et al. 2011). The former E. laticeps  is now "officially" named Captorhinus laticeps (as reflected on the chart by Reisz et al. 2011 [fig.2 in this article], which does not list Eocaptorhinus.)

  

             Captorhinus magnus , the most recently discovered species in the captorhinidae family, and by far the largest, was found in Permian levels at Fort Sill, Oklahoma (Kissel, Dilkes, and Reisz 2002). C. magnus  has only single rows of teeth, and thus (according Heaton's evolutionary scheme) would lie somewhere between Protocaptorhinus and Captorhinus in the scale of advanced  or derived traits of this family. According to cladistic analysis, however, it has been placed as a sister clade to C. aguti , and both are seen as further removed from C. laticeps (Palaios 2014b). 

            This attribution does not seem perfectly clear. C. magnus is distinct from both C. aguti and C. laticeps in being twice as large as either. It also contrasts with both, in having ogival-shaped teeth in the posterior portion of both maxillary and dentary (the upper and lower jaw bones). Another difference is in the distal articulation surface of the femur of C. magnus, which is concave, whereas in both C. aguti and C. laticeps it is convex.  Finally, some collections at the Ft. Sill site in Oklahoma showed that specimens of C. magnus are more abundant in the lower parts of the section,  whereas C. aguti is more abundant in the higher part.  All of this would seem to fit into the evolutionary, stratigraphically-based view of Heaton (1979) that C. magnus may be earlier and more primitive than C. aguti. .

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Moradisaurinae

Class:  Sauropsida : Clade: Anapsida      family: Captorhinidae   subfamily: Moradisaurinae

                The Moradisaurinae, a subfamily of the Captorhinidae, had a widespread distribution in China, Morocco, Niger, Russia, Texas and Oklahoma. The Moradisaurines were insectivores and herbivores.  

 
     

Lepidosauromorpha vs Archosauromorpha. 

            This is another pair of classes that, for those considering modern reptiles, are of basic significance, but here, where the concern is primarily with the emergence and development of Synapsids (the fossil taxa leading to the evolution of mammals), these two groups are necessarily relegated to a lower level of priority.  Lepidosauromorpha include all the reptilian taxa sharing a more recent common ancestor with living lizards (called the Lepidosauria), than with Archosauria (crocodiles, and birds). Conversely, Archosauromorpha includes all reptilian taxa sharing a more recent common ancestor with Archosauria than with Lepidosauria (Gauthier 1986). Since Synapsids appear by the end of the Pennsylvanian and early Permian (about 310-280 mya), these more recent reptilian taxa (including dinosaurs) will be largely bypassed in this discussion.

Notes:

[1] Therapsids: now the term Synapsids is used as an overall counterpart of Sauropsids.

[2] As discussed in Palaios (2014) on the Eureptiles, Captorhinidae, and Romeridae.

[3]  Amphibamidae is an extinct family of temnospondyl amphibians, with the earliest examples found in Late Carboniferous strata in the United States and the Czech Republic. Some groups 
in the Karoo Basin of South Africa lasted  into the Early Triassic.  Various studies have shown that  that modern amphibians,  including frogs and salamanders, may have descended from a common amphibamid ancestor (Wikipedia 2014).

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Glossary 
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References:

Benton, M., 2005, pp.104-5

Brough, M.C. and J. Brough, 1967. The Genus Gephyrostegus. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 252 (776): pp. 147–165.

Carroll R.L. 1970. The Ancestry of Reptiles. Philosophical Transactions of the Royal Society B 257: pp. 267–308.

Carroll R.L. 1988

Carroll R.L. and Baird D. 1972. Carboniferous Stem-Reptiles of the Family Romeriidae. Bulletin of the Museum of Comparative Zoology 143(5): pp. 321-363

Case, E.C. (1911): A revision of the Cotylosauria of North America. Carnegie Institution of Washington Publication vol. 145, Washington, D.C., 122 pp.

Clark, J. and R. L. Carroll  1973.  Romeriid Reptiles from the Lower Permian . Bulletin of the Museum  of Comparative Zoology 144 (5): pp. 353–407.

Cope, E. D. 1882  Third contribution to the history of the Vertebrata of the Permian formation of Texas. Proceedings of the American Philosophical Society 20: pp. 447-461.

Gauthier J.A. 1986.  Saurischian monophyly and the origin of birds, . In K. Padian (ed.) The Origin of Birds and the Evolution of Flight. Memoirs of the California Academy of Science 8, Berkeley, California, pp. 1-55.

 Gauthier, A., A.G. Kluge, and T.  Rowe 1988  The early evolution of the Amniota. in Michael J. Benton (ed.): The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds. Syst. Ass. Spec. Vol. 35A, Clarendon Press, Oxford, pp. 108-155.

Goodrich, E.S.  1916 . "On the classification of the Reptilia". Proceedings of the Royal Society of London 89B: pp. 261–276.

Gregory J. T. 1948. The structure of Cephalerpeton and affinities of the Microsauria. American Journal of Science, 246: pp. 550-568

 Heaton, M.J.  1979. Cranial Anatomy of Primitive Captorhinid Reptiles from the Late Pennsylvanian and Early Permian, Oklahoma and Texas. Oklahoma Geological Survey, Bulletin vol. 127. The University of Oklahoma, Norman, 83 p.

Heaton, M.J. and R.R. Reisz. A skeletal reconstruction of the early Permian captorhinid reptile Eocaptorhinus laticeps (Williston). Journal of Paleontology 54: 136-143.

Hunt, K. 1994-1997

Haeckel, E. 1866  Generelle Morphologie der Organismen : allgemeine Grundzüge der organischen Formen-Wissenschaft, mechanisch begründet durch die von C. Darwin reformirte Decendenz-Theorie. ) Berlin

Huxley, T.H.  1863.  The Structure and Classification of the Mammalia. Hunterian lectures, presented in Medical Times and Gazette.

Huxley, T.H.  1876. Lectures on Evolution. New York Tribune. Extra. no 36. In Collected Essays IV: pp 46-138

Kissel, R.A., D.W.  Dilkes,  and R.R. Reisz, 2002. "Captorhinus magnus, a new captorhinid (Amniota: Eureptilia) from the Lower Permian of Oklahoma, with new evidence on the homology of the astragalus." Canadian Journal of Earth Science, 39 (9), pp. 1363-1372

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