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ce between the maternal uterine wall and the developing fetus it anchors the fetus to the uterus supplies the growing embryo with oxygen and nutrients and eliminates the metabolic waste produced by it it also acts as an important endocrine organ placenta bearing mammals probably emerged around the middle of the mesozoic and are represented today by over 5000 species from elephants to humans to bats these animals display a long gestation period and give birth to well developed live young early placental mammals in the fossil record are recognized on the basis of certain shared features of the teeth jaws leg bones foot bones and ankle joints naturally the presence or absence of a soft organ like the placenta cannot be used as a diagnostic tool when working with fossils the marsupial clade also arose in the mesozoic and managed to persist into the modern world though its contribution to the present day range of mammalian diversity is meager compared to that of placental mammals there are a total of only 343 known marsupial species mostly distributed in australia and south america marsupials are popularly thought of as pouched animals in fact the very name comes from the latin word for pouch marsupium but only 50 of living marsupial species actually possess a permanent pouch marsupials can more properly be distinguished from their placental counterparts on the basis of their reproductive cycle marsupials possess only a rudimentary placenta with limited nutrient and oxygen exchanging capabilities they have short gestation periods and give birth to tiny incompletely developed young the younglings are nursed on breast milk for an extended period of time the lactation period far exceeds the gestation period subtle features of the upper and lower molars in addition to the total number of molars in each jaw also distinguish marsupials from placental mammals marsupials generally have lower metabolic rates slower rates of postnatal growth and smaller brain dimensions than placentals of comparable size adding to the mammalian diversity of the late cretaceous was a group of primitive essentially rodent like mammals called the multituberculates the clinical sounding name refers to the fact that each cheek tooth in the jaws of these animals bore multiple rows of tiny cusps bumps or tubercules that operated against similar counter rows in the opposite jaw like modern rodents they bore a pair of enlarged shearing incisors at the front of each jaw in terms of geological longevity it could be argued that they were the most successful mammalian order of all time lasting for a span of over 120 million years marsupials and placental mammals are much more closely related to one another than either is to the multituberculates judging from the structure of the pelvis it seems very likely that multituberculates gave birth to immature live young rather than laying eggs like their reptillian forebears unlike the marsupials and placental mammals of the mesozoic the multituberculates left behind no living descendants it is worth mentioning here that a group of primitive egg laying mammals the monotremes also made it into the paleocene they are represented today by just one species of platypus and four species of echidna the sudden disappearance of the dinosaurs opened up a plethora of new niches for the mammals to radiate into the world was warmer and wetter in the paleocene than it is today with rainforests ranging over most of the continents the continents themselves while not entirely alien in shape and extent occupied markedly different longitudinal and latitudinal positions in the paleocene than they do today a map of the world at that time is included below a paleocene world map so we ve set the stage what sorts of mammals could we paint into a paleocene landscape the marsupials appear to have undergone a significant reduction in diversity at the cretaceous paleocene boundary only one genus peradectes is known to have made it across successfully the placentals and multituberculates sustained fewer casualties by comparison the explosive diversification of mammalian morphotypes to fill ecosystems effectively emptied of large vertebrates did not begin immediately after the fall of the dinosaurs for example it was only towards the end of the paleocene epoch that the first truly large bodied mammal herbivores and carnivores began to arrive on the scene agusti and anton 2002 go so far as to describe much of the paleocene as being an impoverished extension of the late cretaceous world any overview description of the mammals of this period is destined to devolve into a tiresome catalogue of strange names and anatomical characters i have tried my level best to supplement my writings with pictures to help you visualize the animals i describe below the multituberculates hereafter shortened to multis reached the peak of their evolutionary fortunes during the paleocene the ptilodonts can be regarded as typical multis they had large rodent like incisors perched at the front of each jaw separated from the cheek teeth by a toothless space they had elongated blade like lower premolars designed for cracking nuts and hard seeds like most multis the ptilodonts appear to have been largely herbivorous perhaps supplementing their diet with the occasional invertebrate ptilodonts had grasping claws a prehensile tail large toes and feet with a wide range of motion these traits indicate a heavily arboreal lifestyle in summary the ptilodonts were squirrel like animals both ecologically and morphologically left the rodent like skull of a ptilodont by nobi tamura right the beaver sized taenolabis by bruce horsfall the largest known multi approximated the size of a beaver the short snouted heavily built taenolabis it had large grinding molars and was clearly a ground dwelling herbivore the multis ultimately bought the farm about 30 million years ago succumbing perhaps to stiff competition from true rodents primates and herbivorous ungulates the transition into the paleocene was turbulent for the marsupials and they never truly recovered their former level of diversity in the northern hemisphere the southern continents however were a different story marsupials formed a sizeable chunk of the mammalian fauna in paleocene south america up to 50 a number of these marsupials can be characterized as belonging to the same taxonomical order as modern opossums some of these were adapted for burrowing others for scaling trees interestingly marsupial equivalents of rodents and carnivorans also evolved in south america during the later phases of the paleocene the weasel sized arboreally proficient marsupial mayulestes probably sought out frogs and small mammals as prey it was similar in many ways to the marten mayulestes a marsupial weasel placental mammals piddling contributors to the range of mammal diversity for most of the mesozoic managed to outshine both the multituberculates and the marsupials during the paleocene we shall consider several unique and interesting placental animal groups that lived in the paleocene in the following passages the lepictids it is tempting to observe the tiny sizes small brain cases pointed snouts and insectivorous diet of hedgehogs shrews golden moles elephant shrews treeshrews tenrecs and moles and conclude that they all belong to a single taxonomic category of mammals this is not really the case and molecular analyses as well as studies in comparative anatomy demonstrate that these 7 animals represent up to five different mammalian orders erinaceomorpha hedgehogs soricomorpha shrews and moles macroscelidea elephant shrews scandentia treeshrews and afrosoricida tenrecs and golden moles the tree tops and underbrush of the paleocene were home to a great many such small to medium sized insectivorous creatures representing different genera families orders superorders infraclasses and subclasses a number of these in south america at least were marsupials others were multituberculates and some were early members of the currently existing placental orders listed in the previous paragraph still others belonged to placental families and orders that kicked the bucket by the end of the paleocene the long legged lepictids the shrew like paleoryctids or the semi aquatic fish eating otter like pantolestids for example one of the most charismatic placental mammals recovered from the paleocene is lepictidum a sort of strange cross between an elephant shrew and a kangaroo it was about 60 to 90 centimeters long had lengthy hind limbs shortened fore limbs and a slender snout that sported a short trunk like many other mammalian insectivores the skull was quite unspecialized it is unclear whether this animal ran on all fours or hopped like a wallaby and yes as fossil genera go lepictidum is unbelievably cute the video below represents one animator s impressive attempt at bringing this animal back to life plesiadapiforms we can tease out the beginnings of our own order primata from amidst this paleocene profusion of tree climbing and insect munching forms modern primates share a number of features among them a short muzzle forwardly directed eyes with stereoscopic vision hands with nails rather than claws and cheek teeth with rounded cusps most plesiadapiformes display none of these characters possessing a long snout strong curved claws and side facing orbits for the eyes they can however be related to primitive tarsier like primates mostly on the basis of shared features of the teeth and the auditory bulla a bony structure that encloses the bones of the middle ear it is likely that they were close relatives of true primates if not directly ancestral to them they were extremely abundant in the paleocene and are interpreted as being lemur like in appearance they had long digits and flexible limbs for maneuvering through the forest canopy over 25 genera and 75 species of plesiadapiformes have been discovered from this period leaping about the tree cover as far north of the tropics as northern wyoming which was warmer and less arid during the paleocene condylarths primitive ungulates called condylarths also existed in the paleocene many of them bear little semblance to modern hoofed animals like horses or deer for example arctocyon was a wolf sized condylarth with the limb proportions of a bear and a diet that included meat it had a pair of impressive lower canines and a long robust skull its cheek teeth indicate that it was primarily a plant eater it is identified as a primitive ungulate by subtle features of its limb joints and dentition dissacus was another example of a condylarth that consumed meat it had digits that terminated in hoof like structures many different groups of condylarths have been recognized in the fossil record primitive ungulates left arctocyon by dmitry bogdanov right phenacodon by charles m knight some of the condylarths were browsers like ectoconus or phenacodus and possessed clear signs of tapir like hooves some of these animals were well adaptated for running the condylarths are generally regarded as being the basal ancestral stock from which the odd and even toed ungulates arose titanoides a late paleocene herbivore by smokeybjb by the end of the paleocene various large bulky herbivores appeared on the scene some of them condylarthian in origin and others not titanoides was a strange non condylarthian herbivore that approached the size of a rhinoceros and had giant saber like canines and long forelimbs the earliest civet like ancestors of dogs cats hyenas and bears also evolved in the late paleocene though their story will be told in another section of finstofeet it is astonishing how many wildly different lineages ptilodonts marsupials and plesiadapiformes among others evolved chisel like incisors and rodent like skulls during this epoch true rodents arrived onto the scene at the end of it all competing with and ultimately eclipsing the various pseudo rodents of the paleocene were there any large non mammalian predators during the paleocene one of the most unusual aspects of the paleocene was the presence of large predatory birds and reptiles that occupied the topmost rungs of the food chain in terrestrial ecosystems across the world the k t extinction event did not put the dinosaurs out of business entirely they were survived in the succeeding age by birds some aspects of the dinosaur to bird transition have already been dealt with in the taking wing series fossil evidence suggests that birds weathered the cretaceous paleocene transition poorly with only a few taxa escaping extinction these surviving forms eventually gave rise to all the different families of birds that we currently observe there is molecular data that contradicts this picture pointing to a pre k t event divergence for many of today s major bird lineages gastornis the terror bird in any case by the late paleocene one group of birds had lost the ability to fly and assumed the form in likeness of some of their extinct theropod relatives of large land bound predaceous bipeds the gastornithidae these terror birds were up to 2 metres tall and small non functional wings they had large powerful beaks for capturing and tearing prey apart it is often depicted as feeding on carrion and mammals they are related to modern waterfowls update correction january 2013 some recent evidence seems to suggest that the gastornithidae were actually herbivores pristichampsus a cursorial crocodile illustration by robert bakker crocodillians and crocodile like champsosaurs flourished during the paleocene pristichampsus was a land walking 3 meter long crocodile from this period that had hoof like toes and long legs it was heavily armored and capable of galloping after fleeing prey archaic marine crocodiles roamed the seas while a variety of crocodiles alligators and now extinct champsosaurs inhabited swamps and marshes the world over relicts of a by gone age of reptiles just recently specimens of a monstrous 40 to 50 foot snake appropriately named titanoboa were uncovered from this period it was by far the longest and heaviest snake known to science i ll leave you with this rather entertaining teaser for the smithsonian channel s special on titanoboa note title card art by nobi tamura sources 1 agustí jordi and mauricio antón mammoths sabertooths and hominids 65 million years of mammalian evolution in europe columbia university press 2010 2 prothero donald r after the dinosaurs the age of mammals indiana university press 2006 3 http www paleocene mammals de accessed 23 may 2013 13 comments sauropods whale lizards may 13 2012 2 26 am filed under uncategorized the force of gravity together with certain physiological and ecological constraints holds in check the evolution of ever larger body sizes among mammals on land by becoming secondarily adapted to life in water however whales have been able to circumvent at least some of these size restrictions left a blue whale the largest living animal known...
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