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ian therapsids this post is the third installment in a multi part series on the evolution of mammal like reptiles time has turned it to rock but despite the passage of so many millions of years the head of a gorgonopsid still retains an aura of unspeakable ferocity gorgonopsia is a suborder of predaceous therapsids mammal like reptiles that arose during the middle permian for much of their early evolutionary history gorgonopsians were rare modest sized carnivores eclipsed in numbers and ecological relevance by thick skulled meat eating dinocephalians the rather unexplained demise of the dinocephalians at the end of the middle permian threw open new predatory niches for these gorgon faced hunters to exploit the most striking feature of the gorgonopsid skull is the presence of two enlarged canines reminiscent of the dagger like fangs of sabre toothed cats from the far more recent pleistocene epoch that hang menacingly from the upper jaw the lower canines were also massive often only a third shorter than their upper counterparts to sufficiently clear the canines before delivering a lethal bite these animals must have been capable of drawing back their lower jaws far enough to produce gape angles in excess of 90 degrees kemp s 1969 functional analysis suggests that gorgonopsid jaws were capable of performing two kinds of bites 1 a killing bite in which the animal would use its fearsome canines as stabbing devices piercing through hide and flesh to disable its prey the front end of the lower jaw bears a large massy chin that may have served as the mechanical equivalent of a war club packing an extra kinetic punch when the jaws were drawn shut 2 a more surgical type of bite in which the incisors interlocked when the jaws were closed allowing the animal to slice out a jagged edged hunk of flesh from a carcass the transtion from biting mechanism 1 to 2 was made possible by the movement of the lower jaw to a more forwardly position so that the serrated incisors could intermesh the large size of the snout indicates that smell was key to the hunting behavior of these ancient predators arctops watsonii a south african gorgonopsid image credit богданов compared to pelycosaurs like dimetrodon gorgonopsids have rather gracile post cranial skeletons with limbs that are longer and more vertically oriented there was probably less lateral swaying of the trunk during locomotion and the back was kept straighter the tail was smaller the body was held well clear off the ground and some skeletal reconstructions of gorgonopsids even evoke a sense of wolf like nimbleness according to some workers the nature of the articulation between the femur and the acetabular socket of the pelvis suggests that the gorgonopsid hindlimb was capable of switching between a primitive splayed posture and a more erect mammalian stance similar to the high walk of a crocodile the precise character of limb suspension in gorgonopsids which probably lay somewhere in the gray zone between sprawling and fully erect is still debated the hind limbs which accounted for the bulk of the animal s speed and propulsive power were more upright than the fore limbs while body form did not vary considerably among different gorgonopsid genera there was certainly a range in size the smallest known gorgonopsids are comparable in body dimensions to modern day canids the largest gorgonopsids members of the genus inostrancevia for example measured around 3 5 meters from nose tip to tail end making them longer than any living land carnivore these late permian super predators were well adapted for hunting huge stocky slow moving plant eating dicynodonts and paraeiasaurs in some ways this mode of life was replicated in a much later era by saber toothed cats that preyed heavily on herbivorous megafauna large gorgonopsids hunted big game in this reconstruction inostrancevia kills a juvenile pareiasaur image credit dmitry bogdanov it is worth listing some of the other important features gorgonopsids bear in relation to the pelycosaur to mammal transition the advanced traits are italicised and the primitive pelycosaur ish traits are indicated in normal font 1 a secondary palate is absent 2 the dentary of the lower jaw is expanded 3 the stapes is reduced 4 cervical and lumbar ribs are present 5 they display the same advanced bone histology seen in dinocephalians 6 no definite statements on the presence or absence of body hair in gorgonopsids can be made there is however a limited amount of evidence indicating that their snouts may have borne vibrissae or whiskers 7 the relative size and shape of the brain had not changed significantly from the pelycosaur condition the gorgonopsids were wiped out by the end permian extinction event after their disappearance new non therapsid animals would come to take on the mantle of terrestrial super predator among them raiusuchians and theropod dinosaurs in the next post in this series we ll spend some time on two more therapsid groups the dicynodonts and the therocephalians sources 1 kemp thomas stainforth on the functional morphology of the gorgonopsid skull philosophical transactions of the royal society of london series b biological sciences 1969 1 83 2 kemp thomas stainforth the origin and evolution of mammals oxford oxford university press 2005 3 kemp thomas stainforth and t s kemp mammal like reptiles and the origin of mammals london academic press 1982 4 chinsamy turan anusuya ed forerunners of mammals radiation histology biology indiana university press 2011 6 van valkenburgh b l a i r e and i jenkins evolutionary patterns in the history of permo triassic and cenozoic synapsid predators paleontological society papers 8 2002 267 288 30 comments therapsids part 2 terrible heads and food webs december 29 2012 9 26 pm filed under uncategorized generations of fossil hunters have visited the arid fields of the karoo basin in south africa in search of ancient life the rock layers of the karoo constitute an invaluable storehouse of information on the evolutionary history of the therapsids they have over the course of a century yielded up many strange and forgotten permian beasts mineralized relics from a time when the karoo was a far greener place the karoo basin in south africa has provided great insights into the story of therapsid evolution during the permian period what are dinocephalians and what do they have to do with therapsid evolution the dinocephalians represent the first significant wave of therapsid diversification they were a group of medium to large sized herbivores and carnivores that enjoyed global distribution during the middle permian paleontologists have identified a few features that seem to unite the group 1 the upper and lower incisors interlocked when the jaws were drawn shut the lower incisors had surfaces on the tongue ward side called heels that met with the upper incisors to form food crushing platforms 2 the nose projects ventrally in many forms giving the face a markedly downturned appearance 3 the jaw is shortened and the jaw joint is is more anteriorly situated 4 significant thickening of the skull bones pachytosis is a commonplace feature hence the name dinocephalia which means terrible heads 5 the temporal fenestra is longer along the dorso ventral axis than the antero posterior axis large herbivorous dinocephalians like tapinocephalus and moschops attained weights and lengths comparable to modern oxen and rhinoceri i seem to recall one author referring to moschops an awkward looking and ponderous creature judging from standard skeletal reconstructions as the fat albert of the permian the post cranial skeleton of moschops is heavy set with several features that assisted in weight bearing including a massive shoulder girdle arm bones with broad flattened ends and short stout vertebrae the forelimbs probably assumed something of a sprawling posture while the smaller hind limbs were more erect the frontal and parietal bones of the skull display an extraordinary degree of pachytosis with thicknesses of upto 11 5 cm the functional significance of the thickening of the skull roof in dinocephalians is unclear the most popular theory is that the male members of these species used their skulls as rams and butted heads in dominance rituals moschops a herbivorous dinocephalian image credit dmitry bogdanov were dinocephalians warm blooded the temperature physiology of these animals is a point of uncertainty dinocephalians were apparently capable of living at high latitudes with highly seasonal weather regimes their pelycosaur forebears were as far as we know restricted to equatorial climes and their bone tissues display patterns of vascularization and growth similar to what is seen in mammalian bones these are the arguments usually presented in favor of dinocephalian warm bloodedness however it must also be noted that the dinocephalians lacked a number of key skeletal features often associated with endothermy in mammals for example they did not possess a secondary palate an anatomical structure that separates the nasal cavity from the oral cavity in mammals the presence of this separation allows mammals to breathe continuously while feeding plant eating dinocephalians almost certainly found themselves from time to time in the predatory cross hairs of carnivorous dinocephalians the anteosaurs for example were a group of medium and large bodied meat eating dinocephalians with high narrow skulls thickened skull roofs and large robust canines they were heirs to a set of ecological niches that during the early permian would have been occupied by sphenacodontid pelycosaurs including dimetrodon the largest known anteosaurs had skulls measuring over a meter in length a russian anteosaur titanophoneus image credit mojcaj therapsids and food chains grasses carpet a hinterland plain the grasses are gnawed down by great itinerant masses of wildebeest the wildebeest are preyed upon by prides of lions food chains of this nature in which energy flows from land plant producers eg grass to a class of abundant herbivorous primary consumers eg wildebeest and finally to a relatively small population of carnivorous secondary consumers lions are quite familiar to us however terrestrial ecosystems may not have always relied so heavily on a productive base of land bound photosynthesizers it has been suggested that prior to the appearance of the therapsids land ecosystems were generally more dependent on aquatic productivity this hypothesis is based on two observations 1 the unusually high ratio of carnivorous pelycosaurs to herbivorous pelycosaurs in early permian assemblages an exclusively land based food web would require a massive preponderance in numbers of herbivores over carnivores 2 the abundance of fish eating forms like ophiacodon that may have served as intermediate links in food chains that tied land to freshwater the was an increase in the relative number of land going vertebrate herbivores to carnivores after the rise of the therapsids the first large bodied terrestrial plant eaters were not dinocephalians both the caseids and the diadectids had played that part before during the early permian see my post on pelycosaurs and on amniotes but dinocephalian diversification and therapsid diversification in general may have played a role in the creation of food webs that were more plant based and less fish based the reign of the dinocephalians was short lived geologically speaking they perished shortly after reaching the pinnacle of their taxonomic diversity around 263 million years ago leaving no living descendants but the therapsid story was far from over in the next few posts we ll discuss some of the therapsid groups that ruled during the late permian starting with the gorgonospids and therocephalians sources 1 kemp thomas stainforth the origin and evolution of mammals oxford oxford university press 2005 2 kemp thomas stainforth and t s kemp mammal like reptiles and the origin of mammals london academic press 1982 3 chinsamy turan anusuya ed forerunners of mammals radiation histology biology indiana university press 2011 4 maclean paul d jan j roth and e carol roth the ecology and biology of mammal like reptiles washington dc smithsonian institution press 1986 8 comments therapsids part i december 26 2012 5 09 pm filed under uncategorized introduction to the therapsids 2 3 3 3 3 that s the number of bones in your thumb index finger middle finger ring finger and little finger respectively the same numbers apply to the phalangeal bones in your toes starting with your big toe this basic structural plan common to most land mammals has a very ancient pedigree the therocephalia a group of carnivorous mammal like reptiles that walked the earth over 250 million years ago during the permian period had hands and feet constructed in just this fashion four out of the five digits on each extremity contained 3 bones while the remaining digit corresponding to the thumb or big toe contained only 2 many of the most unique and widely conserved features of mammalian anatomical design and physiology including a bony secondary palate a three ossicle ear endothermy an erect limb posture and quite probably hair and lactation originated among ancestral groups of mammal like reptiles during the permian and early triassic periods much of this series will be concerned with bridging some of the evolutionary distance that separates the pelycosaur haptodus left by nobu tamura from the early mammal morganucodon right by michael b h so how did the evolutionary process derive a class of highly active fur clad milk producing animals from ancestral forms that were so fundamentally reptile like in appearance this is the question that i shall attempt to answer in a series of posts over the course of this winter we discussed the biology and classification of some of the earliest mammal like reptiles in my post on pelycosaurs in this post we shall begin to deal with their permian heirs the therapsids the first few installments of this series will deal with specific groups of mammal like reptiles that lived during the middle and late permian while the remaining parts will deal with some of the long term evolutionary trends we can glean from the therapsid fossil record what is a therapsid a very crude evolutionary tree the word clouds around the ends of each of the branches represent different terms and animals associated with the taxonomic grouping therapsida is a clade that owes its evolutionary origins to a pelycosaurian grade ancestor and includes all of the most advanced synapsids including modern mammals if that sentence is gibberish to you perhaps reading the first few paragraphs of my post on pelycosaurs will help demystify things to summarize birds mammals and modern reptiles are amniotes by descent the first amniotes were terrestrially adapted animals that laid water proof eggs on dry land synapsida is a subgrouping of amniotes that includes modern mammals and their various...
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