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ANAPSID REPTILES

 

reptile

As was discussed in a previous chapter, the first amphibians such as Acanthostega were not well adapted to support their weight on land and probably spent most of their time in the water. Ichthyostega could support its weight on land but retained a number of adapatations for aquatic life. The Late Devonian fossil Tulerpeton possessed lengthened limbs but its hands and feet retained the primitive feature of more than 5 digits.

tulerpeton limbs

In the Early Carboniferous Period, some amphibians became better adapated for terrestrial life. Greerpeton is the first known fossil in which 5 digits had been established for the hands and feet.

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greerpeton

limbs

It possessed a few derived features of the skull (such as a longer parasphenoid and larger basipterygoid) and hip (such as the presence of a modified sacral rib to which offered stronger support than the ligaments present in more primitive amphibians). The humerus still retained the primitive "L" shape and holes passing through it (Clack, 2002)

sacral ribhumeruship

 

Casineria kiddi

The common ancestor of Casineria and amniotes had modified its vertebrae so that the pleurocentra regions were increasingly dominant over intercentra. Its ribs were longer and thinner. The arm bones slender and the curved last phalanx improved its grasp.

Anthracosaurs

pholiderpeton

Anthracosaur amphibians (such as Pholiderpeton in the above drawing) evolved a number of features shared by reptiles and are one of the candidate groups of amphibians which might have evolved into reptiles. The advanced features of anthracosaur amphibians include a braincase and otic capsule which were more attached to the dermal bones of the skull, the pleurocentrum in the vertebrae became larger, and the intercentrum became smaller although it was still present, modifications of skull bones ( a transverse phlange on the pterygoid [a component of the sphenoid] and the parashenoid grew in size), and a number of phalanges (finger bones) of the hand similar to reptiles (a count of 2,3,4,5,3). In addition to these advanced features, they retained primitive features from the first amphibians such as an intertemporal bone in the skull and a joint where skull the met cheek called kinetic line which had been inherited from the osteolepiforms. Although the skull of Pholiderpeton was more similar to that of reptiles, it still retained large fangs on the roof of the mouth as in early amphibians and fish.

pholiderpeton

palatal fangs

One late anthracosaur named Seymouria was once considered as a primitive reptile typical of the ancestors of reptiles. Now that a fossil seemingly related to Seymouria is known to have possessed an aquatic larvae, most consider Seymouria to be an amphibian whose adapatations to terrestrial living made it similar to reptiles. In the following illustrations, the occipital region of Seymouria is more similar to that of reptiles (especially in the reduction in the size of the stapes [in pink], perhaps as a modification for hearing) than that of a more basal anthracosaur Pholiderpeton (Carroll, p. 167; Clack, 2002).

occipital region of skull

occipital region

The limbs and limb girdles of Seymouria (depicted below) show modifications for terrestrial living and similarities to reptilian features.

humerus hip

sacrum

hand and foot

 

AMNIOTES

Ancestral amniotes evolved a number of derived features which better adapted them for terrestrial life. Although these first reptiles possessed a number of traits which linked them to anthracosaur amphibians, they also possessed derived features which identify them as reptiles.  There was reduction of some of the skull bones (the tabular, postparietal, supratemporal), the process from the pterygoid bone allowed for jaw muscle attachment, the braincase structure was more advanced, and the primitive palatoquadrate was reduced to small epipterygoid and quadrate bones. The frontal bones had become larger and composed part of the orbit. The parietal bone also became larger. The supraoccipital expanded to join postparietal and tabular bones to the exoccipitals and otic capsule. The bones in the back of their braincases (in the occipital region) were still not firmly attached to each other and the stapes was still a large bone, rather than the small structure in modern reptiles which is modified for hearing.  Amniotes lost the labyrinthine folds typical of the teeth of primitive amphibians (although some of the earliest amniotes still possessed these folds. After the earliest amniotes, the primitive feature of teeth from the palate was lost. Amniotes evolved teeth equivalent to canines. They could bite with greater force because of a pterygoid flange for the attachment of pterygoideus muscle for closing the jaw (Carroll, 1988; Clack, 2002).

Amniote ribs became long and thin with modified ribs in the neck (cervical) region.   The reptilian vertebral column was much more advanced and adapted for supporting an animal’s weight on land. The first two vertebrae (the atlas and axis) retained the primitive amphibian condition, the vertebrae retained intercentra components which would be lost in later reptiles, and the notochord still passed unrestricted through the vertebral column. Early amniotes possessed small intercentra and retained the suture uniting the neural arch and pleurocentrum. In the shoulder, the size of the clavicle and cleithrum was decreased and the shoulder became more agile. The humerus became more narrow, developed a supinator process, and lost several primitive features. In the foot, small bones consolidated to form larger ankle bones (the os fibulare and 1-2 os centrale fused to form the calcaneus while the os tibiale, os intercentrum, and a variable number of os centrale fused to form the talus/astragalus). Primitive amniotes possessed 11 wrist bones (Isidro, 2002; Carroll, 1988; Clack, 2002)

There were still small dermal scales on their undersides between their arms and legs and the number of bones in the digits were the same as those of anthracosaurs. 

   The first reptiles are known from the Carboniferous Period: Westlothiana and another fossil are known from the early Carboniferous (Gee, 1988). Westlothiana possessed thicker vertebrae in which the centrum fused to the neural arch, slender limb bones, and reduced the sizes of bones in the temporal region of the skull (Clack, 2002).

humerus

 

The primitive reptiles Hylonomus and Paleothyris are known from the Middle Carboniferous.  These earliest reptiles possessed a number of primitive features in common with anthracosaur amphibians which were not present in later reptiles.

hylonomus

skull of Paleothyris
SKULL BONES  

Paleothyris 1 paleothyris 2

HOLES IN THEIR HEADS

      Most amniotes have holes in the temporal region of the skull, which increase the area for jaw attachment and give room for bulging jaw muscles.  These holes are called temporal openings (or temporal fenestrae) because they occur in the region of the skull near the temporal bone.  The first reptiles lacked these holes and are classified as anapsid reptiles.  Although anapsids were the first reptiles, they were quickly replaced by two higher groups of reptiles which did possess openings in the temporal region of the skull.  These openings occur where sarcopterygian fish possessed some mobility between skull bones and perhaps the sarcopterygian condition made the development of these openings more likely.  Synapsid reptiles possessed a single temporal opening.  They were enormously successful in the remainder of the Paleozoic and were the dominant terrestrial vertebrates of the Triassic.  Mass extinctions and the rise of dinosaurs limited their numbers although one group, the cynodonts, evolved into mammals.  Modern mammals (including ourselves) possess the synapsid opening in the skull through which jaw muscles pass.

temporal
alligator
Diapsid reptiles possess two openings: upper and lower temporal fenestrae.

 

   Thus, the earliest anapsid reptiles quickly produced two descendant lineages, the diapsids and synapsids, which would produce the dominant land and air vertebrates (and even a diversity aquatic vertebrates) from the end of the Paleozoic to the modern day.

different holes
chart
bolosaurus
     Is the number temporal openings the only important characteristic used to classify reptilian groups?  No.  Actually, there are a few minor groups of “anapsids” that actually do have temporal fenestrae (such as Bolosaurus; note the shaded area in the side of the skull) but they evolved these fenestrae separately from the synapsids and diapsids (and thus cannot be classified with them).  Although the first turtles lacked temporal openings, later ones developed them. 

     A few groups of diapsids (such as the armored dinosaur group of ankylosaurs) would later lose their temporal openings as their skull bones fused.  Thus, although the number of temporal openings is an important feature in the classification of fossil reptiles, it should be considered in conjunction with other features as well.     

    Eudibamus was the first bipedal vertebrate known (Early Permian) and was a relative of Bolosaurus.  It ran on its toes, had knees modified to be under its body, and was about 25 cm long (Berman, 2000; Stokstad,2000).

A number of early anapsid groups (pareiasaurs, mesosaurs, millersaurs, and captorhinids) appeared and diversified in the Permian but became extinct at the end of the Permian.  The mesosaurs became adapted to aquatic habitats.  They possessed an elongated snout with many teeth for grabbing fish and a large paddle-shaped rear foot.  They were the first amniotes to become adapted to aquatic life.

mesosaurus

anapsid mesosaurus
eocaptorhinus
    One group of anapsids, the procolophonoids, survived through the Permian but became extinct in the Early Triassic.

pareiasau 

 

   The pareiasaurs (such as Scutosaurus below) were stocky herbivores that could approach 3 m in length and were the largest Permian herbivores.  Pareiasaurs evolved osteoderms which formed dermal armor in their skin.  While the earlier forms possessed osteoderms only on the dorsal midline, later species possessed these osteoderms covering much of the back.  In a later group of dwarf pareiasaurs which seem to be more closely related to turtles, the osteoderms covered the entire dorsal region.  In addition, these dwarf pareiasaurs shared other skeletal features with early turtles such as expanded and flattened ribs and features of the scapula, femur, humerus, and tibia.  One study concluded that not only do pareiasaurs appear to be closely related to turtles, skeletal analysis indicates that they are a paraphyletic group which gave rise to the turtles (Lee, 1997).  Other authors feel that turtles were actually modified diapsids rather than anapsids and that pareiasaurs are unrelated to them. 

scutosaurus

pareiasau

eunotosaurus

placodont

TURTLES

      How did turtles evolve?  Turtles are currently classified as the only group of anapsid reptiles which has survived to the modern day.  Skeletal analysis suggests that they evolved from the dwarf pareiasaurs of the Permian which had already developed dermal armor and broader ribs.  (Recently, some have questioned whether the lack of temporal openings in the first turtles was due to a reduction of a primitive diapsid condition.  If the first turtles were actually modified diapsids, then another group of armored reptiles, the aetosaurs, might be the sister group of turtles.)  Eunotosurus is a Mid-Permian reptile that had a greatly expanded rib cage, which led some to believe that it was related to turtles although it is now thought that Euontosaurus was a synapsid unrelated to turtles (DeBraga, 1997).

The orientation of the shoulder to the carapace of the diapsid group of placodonts is more similar to turtles than that of pareiasaurs (DeBraga, 1997). Placodonts gradually evolved a turtle like shell within the group. Some feel that placodonts represent relatives of early turtles.

placodont

placodont

turtle
      Bony plates (scutes) serving as protection are not unique to turtles: they existed in pareisaurs, aetosaurs, crocodiles, and several Mesozoic groups of reptiles.  The rib cage and vertebral column help form the shell.  There are about 50 bones which compose the carapace and 23 in the plastron.  These bones are covered by horny scutes whose margins tend to lie over the sutures between shell bones.  Turtles are unique among vertebrates in the position of the limb girdles inside the rib cage rather than outside it (Ernst, 1994).
proganochelys

     The first turtle, Proganochelys, is known from the Triassic.  Its shell was about a meter long and it was a terrestrial herbivore.  It possessed a horny beak at the tip of its mouth rather than teeth, as in modern turtles.  Proganochelys  possessed a number of primitive features not found in modern turtles such as teeth on the palate and jaw margins, and processes on the basioccipital and parasphenoid bones.   There were no supratemporal or lacrimal bones (or lacrimal ducts) as in modern turtles but it still retained the primitive tabular and postparietal bones that modern turtles have lost.  There was an extra row of plates in the back of its shell, which are lost in modern turtles.  Neck retraction was not possible and bony plates in the skin protected neck.  The plastron (underside of the shell) was similar to the plastron of modern turtles but it was comprised of many more bony elements.  The base of the braincase was only loosely attached to the roof of the mouth (palate) compared to modern turtles. There was one pair of sacral ribs, compared to 2 pair in modern turtles (Carroll, 1988; Ernst, 1994).

kayentachelys

 

      By the end of the Jurassic, early representatives of both groups of modern turtles are known.  Pleurodires (the more primitive of the two groups of turtles which cannot retract their necks) lived in North America in the Cretaceous but the modern survivors of this group are limited to the Southern Hemisphere.    Kayentachelys is the oldest known cryptodire (the group of turtles which can retract their necks).  It had both primitive features (such as teeth on the roof of its mouth and a 9th costal bone in shell) and advanced characteristics (it was the first turtle with specializations associated with aquatic life) (Gaffney, 1987).

skull

archelon
 Extinct groups of turtles (especially the extinct cryptodires) retained a number of more primitive features.  Later turtles lost the cleithrum as part of the pectoral girdle.  Anatomical changes associated with the retraction of the neck are first seen in the Cretaceous.  The shell of the Cenozoic Stupendenys was more than 2 meters long and it was the largest turtle known to have lived.  Archelon reached a similar size. 

The earliest turtles which can be classified in modern groups date from the Early Cretaceous (a trionychoid, a chelonioid, and a pelmedusoid) (Krenz, 2005). The oldest fossil tortoises date from the Early Paleocene (Le, 2006). From 2-3 million years ago, a fossil tortoise of the genus Geochelone lived in North America and could reach twice the size of the modern Galapagos tortoise (Kurten, 1988).

A primitive sea turtle is pictured below.

Below are photos of modern turtles.
spotted turtle snapping turtle