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h clear signs of deformation by hypervelocity shock are also found at the boundary the products of a violent collision these and other pieces of evidence allow us to say with some certitude that the non avian dinosaurs were done in by a space rock does the evidence stack up in strong favor of a meteorite impact at the permian triassic boundary in a word no the evidence for a late permian impact event is limited and hotly contested for example cage shaped carbon molecules called fullerenes containing noble gases argon and helium with isotopic ratios that reflect an extraterrestrial origin were discovered at the permian triassic boundary by a group of university of washington researchers in 2001 but their results were not successfully replicated when another team from caltech analysed pt boundary material from the same test areas published evidence of unusually high iridium levels and shocked quartz from an impact event at the pt boundary has been met with considerable skepticism from many in the scientific community the most popular extinction model points to an episode of intense volcanism 250 million years ago in what is now siberia as the likely triggering mechanism the siberian traps a large expanse of volcanic rock in asian russia stretching across 2 million square kilometers were formed by this eruptive event the eruptive style is worth comment instead of viscous lava pouring out of classically conical volcanoes one must imagine highly fluid lava issuing from huge fractures in the earths crust and flowing out over thousands of kilometers successive lava flows built up an immense layered plateau of basalt basalt is essentially solidified lava the threat that this event posed to life did not lie so much in the incessant outpouring of hot fast moving lava which surely annihilated life within a regional radius as in the outgassing of prodigious quantities of carbon dioxide and sulphur dioxide there is good evidence to show that global temperatures rose considerably at the end of the permian and elevated atmospheric levels of heat trapping greenhouse gases like carbon dioxide and methane most likely accounts for this change the release of carbon dioxide by the siberian traps event could have driven global temperatures up enough for another serious greenhouse gas source to come into play gas hydrates gas hydrates are crystalline solids in which gases are locked up in cages made of hydrogen bonded water molecules carbon dioxide and methane containing gas hydrates may be found at great depths in sediments around the rims of continents and in permafrost it is thought that gas hydrates today contain trillions of tonnes of stored carbon much more carbon than can be found in the combined fossil fuel reserves of all of the nations on earth the mass melting of frozen gas hydrates would have injected tremendous amounts of methane and carbon dioxide into the atmosphere the high temperatures would have also decreased the solubility of oxygen in water leaving the world s oceans relatively oxygen depleted anoxia or oxygen deficiency might have been the axe by which the permian extinction cut down so many diverse families of marine life particularly those reliant on the construction of calcium carbonate shells several lines of evidence point to widespread anoxia in the seas of that time the presence of black mudstone for example in the layers that follow the pt boundary is a telltale sign of low oxygen concentrations anoxic conditions permitted the rapid proliferation of oxygen hating sulfate reducing bacteria that in the presence of light produce hydrogen sulfide a gas that is toxic to most life forms and harmful to the ozone layer to boot carbon dioxide is heavier than air and even above water can drive down oxygen levels by a process of simple displacement carbon dioxide is poisonous in high enough concentrations sulfur dioxide another major gaseous product of volcanic activity is a precursor to acid rain acid rain would have wreaked havoc on terrestrial plant and animal life and introduced cocktails of acids into enclosed bodies of water the sulfate aerosols that result from the interaction of sulphur dioxide with the earth s atmosphere are actually capable of reflecting and scattering light from the sun they may have induced a brief initial period of global cooling before the pronounced warming trend set in hydrogen sulfide emissions the spread of toxic ash global warming wild fires acid rain planet wide anoxia and a weakening ozone layer are all phenomena that have been implicated with good reason in the great dying of the end permian there can be no doubt that this was an apocalypse but it was not a simple apocalypse it involved a complicated chain of events and employed multiple agents of death sources 1 kemp thomas stainforth the origin and evolution of mammals oxford oxford university press 2005 2 donovan stephen k and s k donovan eds mass extinctions processes and evidence london belhaven press 1989 3 ward peter d under a green sky global warming the mass extinctions of the past and what they can tell us about our future harpercollins 2007 4 benton michael j and m j benton when life nearly died the greatest mass extinction of all time vol 5 london thames hudson 2003 8 comments reigning reptiles the archosaurs june 25 2012 5 58 am filed under uncategorized many great dynasties of reptiles rose and fell during the early phases of the mesozoic era a hefty block of geological time sometimes called the age of reptiles that lasted from 250 to 65 million years ago the beaked rynchosaurs the agile erect walking predatory rauisuchians the crocodile like phytosaurs and armored aetosaurs all had their heydays within the first 50 million years of the mesozoic the names of these lineages successful though they were are probably only familiar to professionals and die hard enthusiasts the same cannot be said of the ever popular dinosauria whose first relatively unassuming representatives also appeared on the scene during this early mesozoic time interval around 230 million years ago the dinosaur clade has attained tremendous public notoriety for its evolutionary experiments in large body size defensive weaponry and bipedal carnivory as well as for its sudden fall from planetary dominance about 65 million years ago dinosaurs have penetrated the public perception of prehistoric life so completely that the casual museum goer is prone to indiscriminately identify any large extinct reptile on exhibit as a dinosaur the term dinosaur refers to the members of a specific branch on the greater tree of reptile evolution it is not a blanket descriptive term for anything extinct gigantic and ostensibly reptilian none of the early mesozoic reptiles i mentioned in the opening paragraph are dinosaurs hugely simplified archosaur family tree but what distinguished the earliest dinosaurs from their non dinosaurian reptile contemporaries are modern day crocodiles close relatives of dinosaurs answering these questions is not trivial and will require a fair amount of phylogenetic tree climbing step 1 dinosaurs as diapsids dinosaurs have two openings on each side of the skull behind the eye sockets these holes called temporal fenestrae have margins that provide secure anchorage points for muscles involved in jaw function the presence of these paired holes in the skull roof places dinosaurs within the group diapsida all living reptiles with the possible exception of turtles and tortoises are diapsids by ancestry the skull of a primitive diapsid two holes called temporal fenestrae are situated behind each eye orbit the diapsida may be sectioned up into two distinct groups of reptiles a group of reptiles that includes lizards and snakes in addition to a number of now vanished creatures such as the spectacular marine mosasaurs this group is formally called the lepidosauromorpha and is of no further concern to us in this account of dinosaur origins a group that includes modern crocodillians avian and non avian dinosaurs flying reptiles called pterosaurs and a bewildering assortment of lesser known extinct reptiles with varied body plans diets native habitats and temporal ranges these are the archosauromorpha step 2 dinosaurs as archosaurs archosaurs display a number of skeletal features that distinguish them from other reptiles an opening in front of each eye orbit called the anorbital fenestra an opening on each side of the jaw called the mandibular fenestra teeth that are laterally compressed that is flattened from side to side and serrated teeth set in sockets a large protrusion on the femur called the fourth trochanter that serves as an attachment point for a set of thigh retracting muscles a high narrow skull various anatomical characteristics that indicate an upright or semi upright limb posture as opposed to the sprawling limb posture observed in modern lizards an primitive archosaur skull one of the main thrusts of my post is this the earth was already inhabited by a wide variety of impressive large terrestrial reptiles most of them archosaurs or close relatives of archosaurs when the first dinosaurs made their appearance on the evolutionary stage left a marauding phytosaur by dmitri bogdanov and right an armored aetosaur by arthurweasley phytosaurs were semi aquatic crocodile like archosaurs from the early mesozoic they had elongate snouts and jaws rimmed with conical serrated teeth unlike modern day crocodilians they had nasal openings positioned high up on the skull close to the eye orbits in many cases rather than at the tip of the snout one can envision them snagging unsuspecting prey on the edges of rivers and lakes aetosaurs were herbivorous archosaurs armored in heavy interlocking dermal plates it is tempting to think of them as prehistoric armadillos many were armed with fearsome spikes and stout claws rynchosaurs were strange plant eating relatives of the archosaurs that had barrel shaped bodies stocky limbs and strong beaks that could cut through plant matter before the ecological ascent of the theropoda the dinosaur group that includes tyrannosaurus rex velociraptor and practically every other famous meat eating dinosaur the biggest land predators on the planet were rauisuchians they are generally understood to have been active toothy largely quadrupedal archosaurs related to modern crocodiles they were erect walking animals with limbs oriented vertically below the body rather than jutting out sideways one of several traits they share in common with dinosaurs the head of the femur fits into a downward facing socket in the hip girdle called the acetabulum the largest raiusuchians hit a length of over 7 meters far bigger than the carnivorous dinosaurs of their time and hunted large game postosuchus kirkpatricki a 4 5 meter long rauisuchian the first true crocodiles also appeared during the early mesozoic modern day crocodiles and alligators are built for a low energy semi aquatic lifestyle crocodiles walk for the most part with a sprawling lizard like gait and spend hours basking on river banks in a state of absolute intertia many early crocodillians were by contrast slender legged and fleet footed terrestrial creatures some of them were even herbivores crocodile evolution is a fascinating story in its own right and will be dealt with more fully in a future post step 3 dinosaurs as erect walking animals crocodillians raiusuchians aetosaurs and phytosaurs can be readily distinguished from dinosaurs on the basis of their ankle morphology in the former groups the functional ankle joint is situated between two foot bones of similar size the astragalus an bone that forms part of the ankle in these archosaurs it is sutured to the two bones of the lower leg the fibula and tibia and the calcaneum heel bone this configuration allows for considerable rotation of the foot during locomotion the ankle morphologies of dinosaurs and other archosaurs the dotted line represents the position of the ankle joint in dinosaurs however the articulation between the ankle bones is modified to produce a simple hinge joint the functional ankle joint is between the upper bones of the ankle the calcaneum and astragalus and the bones immediately distal to it refer to diagram the calcaneum was very small compared to the astragalus the important implication of this anatomical design is that the foot was restricted to motion in a vertical plane in other words it could move back and forth but could not be twisted this is a clear adaptation for upright speedy running a similarly structured ankle is also found in pterosaurs pterosaurs are aerially adapted reptiles and close relatives of dinosaurs both the raiusuchians and the dinosaurs independently evolved an erect gait where the limbs were positioned directly below the body rather than splayed outwards but their anatomical approaches to this condition differed three bones make up the pelvis the pubis the ischium and the illium at the junction of these three bones there is a socket into which the head of the femur fits called the acetabulum in dinosaurs the head of the femur is bent that is offset at an angle to the main body of the bone the head fits into the acetabulum the dinosaur acetabulum rather than being a solid pocket of bone actually has an opening in it an open acetabulum is one of the defining features of the dinosaur clade the dinosaur femoral head is rather more barrel shaped that the bulbous head of the human femur limiting thigh motion to a plane parallel to the body comparison of the primitive reptile dinosaur and rauisuchian hip configurations adapted from skepticwiki s article on dinosaurs the oldest known dinosaurs from the fossil record are lithe bipedal carnivores like staurikosaurus and herrerasaurus one of the interesting implications of this ancient bipedal body plan is that quadrupedal dinosaurs from later ages horned triceratops and mighty brachiosaurus among them belong to lineages that had to revert to walking on four legs from a state of two leged ness at some point in their evolution herrerasaurus one of the earliest dinosaurs art by nobu tamura on the road to upright walking bipedalism the dinosaurs evolved a host of anatomical features that taken in summation set them apart from the rest of reptiledom these include a distinct neck a backward pointing shoulder joint a hole in the center of the acetabulum longer hind limbs than forelimbs and long lower foot bones and digits dinosaurs did not comprise a significant portion of terrestrial animal diversity for much of the early mesozoic but they eventually supplanted the raisuchians aetosaurs rynchosaurs and other reptile groups as the dominant vertebrates on land their spectacular success would last them through the middle and later mesozoic and beyond given that birds are modern day descendants of dinosaurs but what made the ascent of the dinosaurs to ecological preeminence possible some researchers have suggested that the upright walking dinos...
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