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Thread: Mammal-like reptiles

  1. #1

    Post Mammal-like reptiles

    the early mammal-Like reptiles

    The Dinocephalia, mentioned earlier, were among the most primitive of the mammal-like reptiles, but some of their features, particularly in the skull, serve to illustrate the fundamental characteristics of mammal-like reptiles. The skull itself is, in contrast to that of most other reptiles, a firmly-knit structure with no internal movement between the constituent bones. A large opening in the temporal region, immediately behind the eye-socket, permitted bulging of the jaw muscles during chewing and biting, and is characteristic of all mammal-like reptiles. The tooth row, which in other reptiles consists of a line of simple, undifferentiated teeth, is in the Dinocephalia divided into functionally distinct incisors, canines and post-canines, the latter foreshadowing the mammalian premolars and molars.

    The Dinocephalia, although primitive members of the mammal-like reptile stock, were therefore altogether distinct from other reptile groups, but it is most unlikely that such mammalian characteristics as hair, mammary glands and 'warm-bloodedness' were present among them. These early animals, which did not survive the end of the Tapinocephalus Zone, were in effect a sterile offshoot of the mammal-like reptiles.

    Another sterile group, the Dicynodontia, were well removed from the evolutionary pathway leading to mammals. The dicynodonts, with such characters as the temporal opening in the side of the skull, were clearly mammal-like reptiles, but with specializations like a horny beak and highly modified chewing action, they merely represent a very successful herbivorous branch of the mammal-like reptiles persisting until late in the Karoo period.

    The carnivorous Gorgonopsia, after a humble beginning in Tapinocephalus Zone times, flourished in the succeeding Endothiodon and Cistecephalus Zones, but did not survive the end of the Permian period. However, the other early mammal-like reptile group, the Therocephalia, were from the beginning a progressive and diversified group, and it is probably through them and their descendants, the cynodonts, that the line leading to mammals ran.

    The end of the Tapinocephalus Zone is marked by the total extinction of the Dinocephalia and a reduction in the number of Therocephalia. In the ensuing Upper Permian Endothiodon and Cistecephalus Zones, dicynodonts are particularly common, and the zones derive their names from two of them. Endothiodon was a large dicynodont, possessing a row of powerful cheek teeth in addition to its horny beak, while Cistecephalus was a small animal with a box-like skull adapted, perhaps, for a burrowing existence. Besides these, a considerable number of other dicynodonts, large and small, were abundant in the marshy lowlands. The large Daptocephalus was common in late Cistecephalus Zone times, and it has in fact recently been proposed that the present
    Cistecephalus Zone be instead referred to as the Daptocephalus Zone.

    Advanced pareiasaurs were present in the Cistecephalus Zone in limited numbers only, while gorgonopsians such as Rubidgea were relatively plentiful. The latter were large and powerful carnivores, with greatly enlarged incisors and canines but with few or no post-canine teeth. Chewing or crushing of food had not yet developed in these animals, and flesh, ripped from the prey by the front teeth, was most likely gulped down in large pieces.

    Together with these gorgonopsians occurred a number of advanced therocephalians. Small, possibly insect-eating, forms such as Scaloposaurus were present, as well as the specialized Whaitsia, in which, as in the contemporary Gorgonopsia, the rear of the tooth row was greatly reduced. Whaitsids and scaloposaurids were both highly evolved therocephalian groups, and important new features seen in the tooth row of scaloposaurids are the tiny cusps on the post-canine teeth. Cusped teeth, of which these were early forerunners, are an important characteristic of mammals, and much of early mammal classification revolves around the nature of complicated tooth cusp patterns.

    Descendants of Cistecephalus Zone scaloposaurids persisted into early Triassic times and a related form Bauria, was a specialized herbivore with flat-crowned crushing teeth. Once considered to be close to mammal ancestry, Bauria is, in fact, an advanced member of the therocephalian stock.

    The end of the Cistecephalus Zone also marked the end of Permian (and Palaeozoic) sedimentation in the Karoo. The next zone, the lower Triassic Lystrosaurus Zone, reflects a Karoo very different from that of Permian times. The rocks of the Lystrosaurus Zone consist of purple or red shales and extensive bands of sandstone, and indicate a much wetter climate with periodic flooding. Gone are the Gorgonopsia which flourished in the Cistecephalus Zone, and gone also are the last remaining pareiasaurs. Absent too are the host of dicynodont genera and species, and in their place is a single genus, Lystrosaurus, from which the zone takes ts name.

    Lystrosaurus, equipped with two tusks and the standard dicynodont horny beak, was a semi-aquatic mammal-like reptile that spent much of its time in the numerous pools and ponds of its watery environment. Obviously well adapted to its surroundings, Lystrosaurus radiated into several species, and fossils of the genus are today extremely abundant.

  2. #2

    Post Re: Mammal-like reptiles


    1.Skulls of two primitive mammal-like reptiles, the dinocephalians Titanosuchus (top) and
    Tapinocephalus (below). As in all mammal-like reptiles, there is a single opening in the skull behind the eye socket. In Tapinocephalus the bones of the skull are greatly thickened and firmly fused with each other. The Dinocephalia were generally large animals, with body lengths of up to 3,5 metres.

    2.Diagram showing the relationships between the various mammal-like reptiles. Mammal-like reptiles did not survive beyond the end of the Triassic period, but one group, the Cynodontia, gave rise to the first mammals at the end of the Triassic, about 200 million years ago.

    3.Three views of the skull of the Upper Permian dicynodont Emydops. It had only a pair of tusks in the upper jaw and a few small teeth in the palate and lower jaw, and upper and lower horney, tortoise-like beaks were used for slicing and crushing vegetation. The temporal opening behind the eye socket was very large in dicynodonts, and housed powerful jaw-closing muscles. the skull is approximately 8 cm long.

    4.The formidable gorgonopsian Rubidgea, with a body length of over 2 metres, stalks the dicynodont Diictodon. Rubidgea was among the last of the Karoo gorgonopsians, a group which did not survive beyond the end of the Permian period.

    5.Skull of Lystrosaurus, a dicynodont common in the lower Triassic of the Karoo. Fossils of Lystrosaurus, which had an average body length of 70 cm, are also known from Antarctica, India, china and Russia, showing that at one time these continents lay closer to Africa than they do today.

    6.Animals besides Lystrosaurus which lived in lower Triassic Karoo times were Chasmatosaurus (top), a 1,5-metre-long early archosaurian reptile, Capitosaurus (centre), a large amphibian, and the cynodont Thrinaxodon (bottom), a 25-cm-long mammal-like reptile.

  3. #3

    Post Re: Mammal-like reptiles

    the later mammal-like reptiles

    Together with Lystrosaurus lived several primitive members of an advanced group of mammal-like reptiles, the Cynodontia. These animals, very likely derived from Permian Therocephalia, show several advances over other mammal-like reptiles, and in many respects they foreshadowed the first true mammals.

    Thrinaxodon, a small Lystrosaurus Zone cynodont, shows many of the characters of the group. Especially striking are the post-canine teeth, each of which carries several well-developcd cusps.

    Upper and lower tooth rows did not meet during chewing and biting activity, however, and the cusps were not worn into facets as in mammals. Another important advance over more primitive mammal-like reptiles seen in Thrinaxodon, is the development of a bony secondary palate in the roof of the mouth. This structure, universally present in mammals, serves to shift the internal opening of the respiratory passage to the back of the mouth; as a result, the animal is able to chew food in the front of the mouth and breathe at the same time. Since a high metabolic rate requires uninterrupted respiration, it is safe to assume that in certain respects Thrinaxodon and other cynodonts were approaching the mammal grade.

    Other parts of the Thrinaxodon skull show equally important refinements. The side of the skull is more completely ossified than in earlier forms, and the brain more fully enclosed. In the lowerjaw the tooth-bearing dentary bone is, in mammalian fashion, considerably enlarged, while the other lower jaw bones are respondingly reduced in size.

    The primitive Thrinaxodon is well removed from the immediate ancestry of mammals, but later cynodonts show some of the changes that brought the group close to the mammal boundary. Several cynodont families were present in the warmer and drier environment of succeeding Cynognathus Zone times. Lystrosaurus was now totally absent, and dicynodonts were represented by the large Kannemeyeria.

    Cynognathus itself was a powerful flesh-eating cynodont, but numbers of plant- eating types such as Diademodon and Trirachodon were also present. In Trirachodon transversely elongated upper and lower post-canine teeth met during the chewing process, and food was actively broken down in the mouth prior to its being swallowed.

    The Molteno Beds, lying above the Cynognathus Zone strata, have yielded no significant reptile fossils and represent a break in the fossil record of the Karoo. However, several highly specialized descendants of cynodonts were present in the succeeding Red Beds and Cave Sandstone, the last of the fossil-bearing Karoo strata. These animals lived in a Karoo Basin which was becoming drier and warmer, and are generally of small size. Tritylodon, representative of one group, had a well-developed secondary palate, a very mammal-like lower jaw with the dentary greatly enlarged and the remaining jaw bones reduced to tiny vestiges, and complex upper and lower post-canine teeth which crushed and sliced food during feeding. Tritylodon was clearly close to the mammalian level of organization and was, in fact, classified as a mammal by early investigators.

    Another highly specialized group, known as the Ictidosauria, were very small animals where the dentary bone of the lower jaw was actually in contact with the skull and effectively formed the entire lower jaw. By this character the Ictidosauria could be classified as mammals, but other features suggest that they were approaching the mammalian grade independently of the true cynodont-mammal line.

    As yet, no specific cynodont can be confidently pointed to as being directly ancestral to the first mammal. However, among the many forms which show trends, some in parallel fashion, towards the mammal boundary, a line leading from an early Thrinaxodon-like animal through a succession of more specialized forms to culminate in the first true mammals in Late Triassic times can be discerned. Recently-discovered mammals from the Upper Triassic Red Beds of the Karoo include Erythrotherium and Megazostrodon, and these tiny specimens show that by the end of the Karoo period, 190 million years ago, a new class of animal had appeared and stood ready to initiate, much later, the great Age of Mammals.

    The final stages of mammal-like reptile evolution took place in a Karoo environment very different from that of earlier times. The climate during the Red Bed period became increasingly hot and dry, and the succeeding Cave Sandstone was laid down in semi-desert conditions, where wind-blown sand covered large areas of the Karoo. Finally, all life in the Karoo Basin was swept away by a series of great volcanic outbursts. Streams of lava flowed over the landscape and built up to a huge thickness capping the entire Karoo rock succession. Today these volcanic rocks, known as the Drakensberg basalts, form the towering Drakensberg Mountains in the central and eastern parts of South Africa. In the late Karoo, advanced mammal-like reptiles and their mammalian descendants were small animals which formed an insignificant part of the total reptile fauna. From their overwhelmingly dominant position in the early Karoo of Tapinocephalus Zone times, they were now, in turn, dominated by newcomers to the Karoo, the dinosaurs and their allies.

  4. #4

  5. #5

    Arrow Re: Mammal-like reptiles

    The Therapsids of the Permian — the Ancestors of the Mammals
    This is a story about ancestors — your ancestors. Two hundred and seventy million years ago — 50 million years before the very first dinosaur — a new group of animals, the therapsids, burst onto the scene.

    These therapsids were unlike any animals that have appeared on the earth — before or since. Elephantine plant-eaters with horns, or thick skulls, for head-butting and sharp tusks for self- defense. Saber-toothed carnivores who killed their prey with a single stab. Strange tusked and turtle-beaked plant-eaters as small as a rat or as big as a hippo. Even a lion-sized meat-eater with venomous fangs like a snake.

    And one small, seemingly insignificant, otter-like therapsid — Procynosuchus — who just
    happens to be the distant ancestor of you, me, and every other hot-blooded mammal.

  6. #6

    Post Re: Mammal-like reptiles

    The Cynodont Radiation

    The cynodonts, or 'dog teeth', were the most successful and one of the most diverse groups of therapsids. Their Late Permian and Triassic evolutionary radiation included such forms as the large, carnivorous cynognathids, equally large herbivorous traversodonts, and the small and extremely mammal-like tritylodontids and trithelodonts. The trithelodonts (ictidosaurs) are almost certainly close to the direct ancestry of the Mammalia. The extremely mammal-like structure of cynodonts has been known for nearly a century, but only within recent years have we learned enough about them and about the very early mammals to say with confidence that all mammals are indeed descended from a single group of cynodonts

    Even the earliest cynodonts, the Procynosuchidae of the Late Permian, show many advanced mammalian characteristics, such as a reduced number of bones in the lower jaw, a secondary bony palate and a complex pattern of the crowns of their cheek teeth. It is likely that Cynodonts were at least partially if not completely warm-blooded, covered with hair, which would have insulated them and helped to maintain a high body temperature.

    By Early Triassic times, cynodonts had diverged into large predaceous carnivores such as Cynognathus and moderate large omnivorous and herbivorous types such as Trirachodon and Diademodon. The Middle Triassic saw a major radiation of herbivorous forms included in the family Traversodontidae. From this family evolved the highly specialized and extremely mammal-like Tritylodontidae of the Late Triassic to Middle Jurassic, the "rodents" of the early Mesozoic and culmination of the herbivorous cynodont radiation. At the same time, the descendents of Cynognathus evolved into medium-sized to small carnivorous and insectivorous forms. It is interesting that as the archosaurian reptiles were becoming larger, the cynodonts became smaller, perhaps nocturnal. The hot arid Triassic conditions favored the ectothermic reptilian metabolism of the archosaurs over the warm-blooded mammalian organization of the cynodonts. (In his Dinosaur Heresies, Bob Bakker has clamed that even the early thecodont archosaurs like Erythosuchus were warm-blooded, and out-competed the cynodonts for this reason, but this position is almost never held nowadays.)

    In the end, the small advanced cynodonts and their mammalian descendents became nocturnal, depending on hearing and smell and leaving the day to the visual-orientated archosaurs. Cynodont and early mammalian brains were larger than sauropsid (reptilian) brains not because they were more intelligent, but because of the enlarged olfactory and auditory bulbs. The small cynodonts and Mesozoic mammals owned the cool night, or lived in trees, the large thecodonts and dinosaurs ruled the day and the ground. It was to be some 150 million years before a combination of environmental stress and cometary or asteroid impact brought about the end of the dinosaurs and the other great reptiles, and allowed the mammals to emerge and take control of the Earth.

    Masseter-Minds of the Mesozoic
    The cynodonts are a very well-known and intensely studied volume of phylospace. In particular, a series of interdependent changes in the cynodont head -- mostly soft tissue changes -- has been the focus of much science and not a little inspired speculation. The principle issues are (a) the development of the characteristic mammalian jaw musculature and jaw articulation (b) the enlargement of the brain (c) the evolution of the mammalian middle ear, and (d) the beginnings of the unique mammalian feeding style and molar dentition. This is far too much to swallow all at one bite, even for well-educated hominids with all of the aforesaid brain, jaw, ear and dental equipment. Palæos is an ambitious project, and we have not shied away from some reasonably technical matters. However, any attempt to integrate all of these issues at once would decisively cross the line between ambition and hubris. Of these four transformations, the change in jaw musculature may have been the most advanced at the end of the Permian. Accordingly, as a first iteration, we will outline the changes in jaw musculature and note how those developments may have related to some of the others on our list.

    As Rowe (1996) points out, the early cynodonts were the first synapsids in which the brain filled the endocranial cavity. This is, perhaps, not the most impressive of accomplishments because the cynodonts were, quite literally, narrow-minded. That is, the brain was a long, tube-like affair (notably priapiform, in fact) trapped within a very narrow skull. [1] However, precisely because space was limited, any further expansion of the brain required remodeling the skull -- a constraint which had profound consequences for both the structure of the ear and the location of the jaw muscles. All three of these structures -- brain, jaw muscles and ear -- compete for space at the back of the head. This is particularly true of the brain and jaw muscles since both are made up of a single basic type of cell and the power of both is, roughly speaking, a linear function of the number of cells.

    In more basal therapsids, the jaw was closed reptile-fashion, by the adductor mandibularis, which originated on the braincase. The bulk of the muscle mass was located in an adductor chamber between the braincase and the dermal skull bones. The adductor mandibularis inserted on the coronoid process and internal surface of the lower jaw. Fenestration of the temporal area of the skull allowed more room for muscle mass. However, even the earliest cynodonts had taken this trend just about as far as it could go.

    One crucial change seems to have occurred in the precynodont therapsid lineage. In order to make more room for the adductor, the bar under the temporal fenestra (jugal + squamosal) became more robust and moved laterally, bowing out from the side of the skull. By itself, this development might have left the braincase overly exposed, but the parietal grew down and covered the braincase laterally. Thus, the attachment area for the adductor was both changed and split. The point is extremely important for later developments, and -- at the risk of beating a dead horse -- worth doing one more time, with feeling, and in more rigorous detail. See, generally, Carroll (1988).

    FIRST In the old adductor chamber, the adductor attached to the inner surface of the parietal (which roofed the chamber) and to the braincase. In Thrinaxodon, this is geometrically impossible. Instead, the adductor attaches to the outside surface of the parietal, which covers the braincase. Thus the braincase no longer need be engineered to support muscle. There is also more room to expand. In this configuration, expansion of the braincase will actually increase the surface area available for muscle attachment, if some of the muscle body can be moved out of the way.

    SECOND Expansion of the old subtemporal bar into a laterally flared zygomatic arch does exactly that, and more. Not only does it open up the space for muscle mass, but it creates a whole new surface for muscle attachment. The old adductor mandibulae therefore splits. The portion attaching to the external surface of the parietal is now referred to as the temporalis. The portion attaching to the inside surface of the zygomatic arch is referred to as the masseter. The temporalis attaches generally to the coronoid process and medial jaw, as before. But the masseter is not only posterodorsal to the jaw, it is also lateral. It attaches to the outside or lateral surface of the coronoid process. This creates an entirely new level of jaw control. Since we now have adductor muscles on both sides of the lower jaw, it becomes possible to develop controlled, lateral grinding movements.

    The final step, taken by the late cynodonts, is illustrated in the figure of Probainognathus. Here, the masseter itself has split, with part moving anteriorly to originate on the anterior portion of the zygomatic arch, under the orbit and attach on the posterior end of the jaw. Now, we also have opposing adductors anterior and posterior, allowing front-to-back as well as side-to-side grinding. With this complete set of muscles, it becomes possible to make controlled grinding movements of the jaw in any direction. Since these are all adductor muscles, this can all be done while the jaw is closed. Given this level of fine control, it became possible, and advantageous, to evolve teeth with fixed and definite patterns of cusps. That is to say, all of this muscle reorganization set the stage for the waves of dental specialization which characterized the mammal lineage, as well as the expansion of the brain.

    As Crompton & Parker (1978) point out, this suite of muscles also permits the animal to generate impressive jaw forces on the teeth without generating much pressure at all on the jaw joint. In fact, given the hypertrophied muscles developed by the cynodonts, the jaw joint becomes almost irrelevant. These jaws were not designed to slice or cut using the momentum of a rapidly closing hinge joint. Rather, the cynodont jaw was optimized to shear, tear and grind by lateral movement after the mouth was already closed. The jaw, as such, needed only to be fast enough to work around an appropriate food item. Depending on diet, this might not require a hinge joint at all. Accordingly, the reconstruction of the cynodont jaw muscles also freed up the post-dentary bones to be exapted for other uses, such as hearing, and permitted a gradual shift of the joint to a more mechanically advantageous articulation between the dentary and squamosal. Conversely, with opposing sets of muscles actin on the jaw at various points, it would be disadvantageous to divide the structure of the mandible among several bones. This could easily result in the jaw ripping itself apart. Thus the development of the dentary as the only bone in the lower jaw, to the exclusion of all other mandibular bones, can also be understood in terms of the structure of the jaw muscles.

    But, to return to our starting point, one should not take this sort of simplistic analysis too seriously. Had we started with an analysis of brain expansion or dentition, we might have been able to make a similar case, creating the impression that these innovations drove all of the others. We can understand the various systemic changes in terms of their relationships with jaw musculature, but we should not confuse this understanding with actual causation. It is not even clear that "causation" is a meaningful concept when applied to co-evolving and interrelated systems of this sort. Rather, in focusing on jaw muscles, we have simply chosen a convenient point of view from which to observe certain points of a smooth series of state changes. ATW030214.

    Notes: [1] Kemp (1982) suggests that the brain may have been much deeper, hence larger, than is usually supposed

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