~ The Induan Stage ~
251.902 to 251.2 mya
Early Triassic
The Induan stage is the first stage of the Mesozoic and begins directly after the Permian-Triassic Extinction Event. The bottom (or beginning) of the stage is marked in the fossil record where the eel-like conodont species Hindeodus parvus first appears; others mark it at the end of the negative oxygen isotope anomaly caused by the Permian-Triassic extinction. The top (or end) of the Induan, where the next stage begins, is marked by the first appearance of the ammonite species Meekoceras gracillitatis. Ammonites play a key role in the 'epochs' of the Triassic; this is because they diversified like wildfire during the Mesozoic, so much so that ammonite researchers view the Mesozoic not as the Age of Dinosaurs but as the Age of the Ammonites. During the Induan stage, global biodiversity was low. Most of the world remained all but lifeless, deserted, hot and dry. Terrestrial ecosystems wouldn't wholly recover until the Carnian stage of the Upper Triassic; in the Induan both the seas and much of the freshwater bodies were anoxic (lacking in oxygen).
 |
| a pair of Lystrosaurus |
Ammonites, fishes, insects, and the vast majority of tetrapods (four-legged animals including reptiles and amphibians) were rare during the Induan. Still, there were hold-outs, such as the lizard-like Procolophon, the primitive amphibian Wetlugasaurus, and numerous therapsids, such as Galesaurus, Regisaurus, Scaloposaurus, and Thrinaxodon. The presence of these creatures in the Induan was spotty at best; the titles of dominant species go to the happy-go-lucky herbivorous lystrosaurids and the carnivorous proterosuchids. Lystrosaurs were diverse during the Upper Permian, but they were nearly wiped out in the Permian-Triassic extinction. Their dynasty continued thanks to a species of Lystrosaurus that, for some reason or another, was able to hold its own in the bare-naked world of the early Triassic. Lystrosaurus was a four-legged, herbivorous dicynodont therapsid. Its prevalence in the early Triassic is marked by the fact that this animal's remains comprise nearly ninety-five percent of the early Triassic fossil beds. It's theorized that a single species of Lystrosaurus was able to survive the extinction, and in a world absent significant competition and predators, it was able to thrive and blossom into a number of different species. Lystrosaurus became the most common group of terrestrial vertebrates during the early Triassic; this is the only time a single species or genus of land animal dominated the planet to such a degree.
Paleontologists scratch their heads at Lystrosaurus' success. Why did it fare so well when so many others died out? Several theories have been proposed, but none are air-tight. Perhaps Lystrosaurus survived because it was a burrower and able to cope with the 'stale air' of the early Triassic. The negative oxygen content and the bloom of carbon dioxide would've made most animals feel like they were suffocating, but Lystrosaurus may have been able to handle the change without too much pain. Its fossils reveal that its barrel-like chest could hold large lungs; short internal nostrils facilitated rapid breathing; and high neural spines could enable the muscles to expand and contract the chest much better than other animals. Opponents to this idea point out that Lystrosaurus' chest cavity wasn't over-proportioned when compared to other dicynodonts that did go extinct; the high neural spines were likely adapted to the animal's posture, locomation, and body size rather than lung capacity; and, besides all this, there were other Triassic burrowers - such as Procolophon and Thrinaxodon - that didn't fare too well. Burrowing mustn't be the key to Lystrosaurus' success. Others have suggested that Lystrosaurus had a semi-aquatic lifestyle that enabled it to keep trucking on; but early Triassic temnospondyls (basically primitive amphibians) had the same lifestyle, and those that survived were barely holding on. Still others theorize that the surviving Lystrosaurus line wasn't that specialized. Mass extinctions collapse food webs, and the more specialized species are hit the hardest since they can't adapt. Numerous lystrosaur species died out at the end of the Permian; perhaps these doomed species were specialized to feed on niche diets (such as on Dicroidium or Glossopteris trees); the surviving Lystrosaurus may have been more 'banal' in the sense that it lacked such narrow specialization and was able to make do on the food that remained. Its descendants would've branched out to become more specialized as the planet began to recover. All these theories aside, perhaps the best explanation is that Lystrosaurus just enjoyed damn good luck.
The herbivorous Lystrosaurus wasn't without predators. The proterosuchids dominated the carnivorous niche of the Induan. These were a type of early archosauriformes (which include diapsid reptiles; one branch of archosauriformes led to the proterosuichids, whereas another led to the 'true' archosaurs that include crocodiles, pterosaurs, dinosaurs, and birds). The proterosuchids were meat-eaters, and in the post-apocalyptic Induan they were able to prey on the numerous lystrosaurs. Just as the lystrosaurs were able to radiate and thrive because they held sway as herbivores, so the proterosuchids did in the carnivore chain. The earliest proterosuchids were slender, medium-sized (about five feet long), long-snouted, and superficially resembled crocodiles (though they lacked the armored scutes of crocodiles, and their skeletons were far more primitive). The proterosuchids had short limbs and a sprawled posture, like contemporary lizards but unlike most later archosaurs. Their most distinctive feature was a down-turning of the premaxilla (the front of the upper jaw, which overhung the lower jaw). The proterosuchids are seen in the Late Permian and Early Triassic, and it's believed they eventually gave rise to the erythrosuchids in the early Triassic. Two prominent examples from the Induan stage will suffice.
Prolacerta was a smaller proterosuchid, reaching only about twenty inches in length. It was probably an active, terrestrial carnivore or insectivore due to fang-like teeth of roughly the same size and shape. Though it was likely quadruped most of the time, because its hind limbs were larger and longer than the front, there's a possibility it could be bipedal during moments of high activity (such as fleeing a predator or chasing prey). Though Prolacerta was originally considered the ancestor of modern lizards, a 1975 study reclassified it as closer to archosaurs.
 |
| a rendition of Prolacerta in both the bipedal and quadrupedal stance |
A much larger proterosuchid was Proterosuchus. This creature reached five to seven feet in length and resembled a primitive crocodile with long jaws, powerful neck muscles, short legs, and a lengthy tail - but its hook-shaped mouth and long rows of simple, cone-shaped teeth fit it firmly as a proterosuchid. It was theorized to be an ambush predator, much like modern crocodiles; it would lie in wait just under the water's surface, and when a careless Lystrosaurus ambled close for a drink, it would spring its trap, lurching out of the water, clamping down on its prey, and using its jaw-hook as an anchor to pull the writhing prey into the water. Proterosuchus' stout legs, positioned elbows-out with a semi-erect gait, would've enabled it to move overland. The ability to move between land and water enabled Proterosuchus to control its body temperature: it could sunbathe to warm up and submerge in water to cool down. As an ambush predator, it would be able to stay in a single environment its whole life; this enabled it to conserve energy so that it could potentially survive months at a time without food - a great advantage in the harsh world of the Induan. Studies of Proterosuchus' scleral rings indicate that it may have been cathemeral, most active at dawn and dusk.
 |
| Proterosuchus with a freshly-caught Lystrosaurus |
~ The Olenekian Stage ~
251.2 to 247.2 mya
Early Triassic
The bottom of the Olenekian stage is at the lowest occurrence of the ammonite species Hedenstroemi or Meekoceras gracillitatis and of the conodont Neospathodus waageni. Its top is at the lowest occurrences of the genera Japonites, Paradunabites, and Paracrochordiceras; and of the conodont Chiosella timorensis. Some paleontologists further subdivide the Olenekian into the Smithian and Spathian substages. Highlights of the Olenekian include the emergence of the mostly aquatic tanystrophids, the genesis of the ichthyosaurs, and the appearance of the erythrosuchids from the proterosuchid line.
The emergence of the tanystropheidae, a family of mostly marine archosauromorph reptiles that lived through the Triassic Period, began during the Olenekian. The earliest representatives of this group were Augustaburiania and Exilisuchus; the latter was terrestrial, despite most tanystrophids being aquatic. The type tanystrophids would have long, stiff necks formed from elongated cervical vertebrae and long cervical ribs. A later species, Tanystropheus from the Middle Triassic, had a several-meters-long neck that was much longer than the rest of its body. The tanystrophids would diversify in the later stages of the Triassic and live along much of the coastline of the Tethys Sea; the recent discovery of fossils in western North America indicates that they were more ecologically diverse than once thought, extending into the Panthalassa Ocean and colonizing the coastal regions around Pangaea.
The proterosuchids continued to diversify in the Olenekian. Though Proterosuchus continued hanging around, it was outdone by the emergent erythrosuchids, such as the monster Erythrosuchus. Chasmatosuchus was an emergent proterosuchid: it was over six feet long and thought to have behaved like a modern crocodile. Its mouth had two distinct features: the top of its jaw hooked downwards to aid in holding prey (like its forerunner Proterosuchus), and the upper palate was lined with a row of teeth (a primitive feature lost in later archosaurs).
 |
| a Chasmatosuchus moves in for the kill on an early Triassic amphibian |
The erythrosuchids emerged from an off-shoot of the proterosuchid line, and they rose to dominate the Olenekian in the titanic Erythrosuchus. This was the largest erythrosuchid and the top predator of its day. It had a large head and short neck; it walked on all fours and had limbs positioned semi-vertically under its body; its head reached up to three feet in length, resembled that of a later dinosaur, and was filled with sharp, conical teeth. It had skeletal features indicating that it was on its way towards a digitigrade posture (i.e. it was close to walking on its toes rather than placing the entire foot on the ground, a trait we find in dinosaurs).
The ichthyosaurs also appeared during the Olenekian stage of the Triassic. These early ichthyosaurs are known as the ichthyopterygia ('fish flippers'), and there's debate regarding their classification. Were the ichthyopterygia true ichthyosaurs, or were they the ancestors of the 'true ichthyosaurs' seen later on in the Mesozoic? One of the earliest ichthyopterygians was Utatsusaurus; it was nearly nine feet long with a slender body. Unlike more advanced ichthyosaurs, it had no dorsal fin and a broad skull with a gently-tapering snout. It had small fins with five digits, and these digits had extra finger bones. The tail had a low fin, suggesting that Utatsusaurus' main mode of locomotion was undulation rather than using its paddles and tails. Some scientists consider this creature a 'transitional fossil' between terrestrial reptiles and the 'true' (or, some would say, 'truer') ichthyosaurs.
 |
| Utatsusaurus frolicking in the Olenekian Tethys Sea |
 |
| a pod of Chaohusaurus |
Another Olenekian ichthyosaur was Chaohusaurus; it looked more like an aquatic lizard than the dolphin-like ichthyosaurs of the later Mesozoic. Its elongated body was due to elongated vertebrae (rather than an allotment of extra vertebrae), and it had a short head that spanned about a third of the length of its trunk. It had a narrow pointed beak and large eye-sockets. The teeth of the upper jaws were pointed and blunt whereas those of the lower jaw were bulbous and convex, perhaps due to a diet wherein the back teeth pulverized shellfish. It had a relatively long neck and flippers rather than webbed feet. The tail fin was short and wide-based in the side view. It was one of the smallest ichthyopterygians, estimated to be around five and a half feet long and weighing just over twenty pounds. Fossil discoveries have revealed that it gave birth to live young like its 'true' ichthyosaur descendants. Another notable ichthyosaur of the Olenekian was Grippia, which could grow up to five feet in length and had a dolphin-like shape just like its descendants would.
 |
| a family of Trirachodon |
The therapsids - of which our post-apocalyptic Lystrosaurus was a part - diversified in the Olenekian. Trirachodon was a small cynodont (maxing out around nineteen inches) with a short, narrow snout and a wide orbital region. It was a burrower, and its colonial-style burrows have been discovered in Africa. In one complex, the fossilized remains of more than twenty individuals have been found. The burrow's entrance shafts slope down at shallow angles under vaulted roofs. The tunnels tend to slightly curve as they progress deeper, with chambers branching off at right angles to the main tunnel. The worn floors hint that the tunnels were frequently used by inhabitants passing each other when moving back and forth. Trirachodon's semi-erect stature is interpreted as an adaptation for better movement in the tunnels. This animal's bones were oddly thick, which may have helped strengthen them for furious digging. Trirachodon's colonial lifestyle suggests complex social behaviors previously thought unique to Cenozoic mammals, as this is one of the earliest signs of cohabitation in a burrow complex by tetrapods (though a burrow cast associated with Thrinaxodon in the Induan Stage of the Triassic may be an earlier example). These burrows would've been great places to escape predators, reproduce, and rear young (not to mention helping in thermoregulation). It's believed that in the African find, the tunnels and their occupants were 'buried alive' in a flash flood.
 |
| Kannemeyeria, an ox-sized dicynodont |
Two other Olenekian therapsids are worth mentioning. Kannemeyeria was a large dicynodont and one of the largest herbivores of the Triassic. It was nearly ten feet long and the size of an ox. It was well-adapted to an herbivorous lifestyle: it had a powerful beak and strong jaw muscles designed for shearing plant material. Though its head was large, the skull was lightweight due to the size of the eye sockets and nasal cavity. It had limb girdles that formed massive plates of bone to support its heavily-built body. A much smaller therapsid was Cynognathus, a large-bodied cynodont that reached about four feet in length and which would thrive in the Middle Triassic. It was heavily-built with a large head that reached a foot in length. It had wide jaws and sharp teeth; though its hind limbs were placed directly beneath the body, the forelimbs were sprawled out in a more reptilian fashion. This form of 'double gait' (erect/sprawling) is seen in some primitive mammals today. The presence of a secondary palate indicates that Cynognathus could breathe and swallow simultaneous. The possible lack of belly ribs in the stomach region suggests an efficient diaphragm, an important muscle for mammalian breathing. Pits and canals on the snout bone indicate concentration of nerves and blood vessels; in mammals, such structures allow whiskers to be used as sensory organs. It's no surprise, then, that some scientists believe Cynognathus may be one of the first 'true mammals,' not merely a transitional link between reptile and mammal but a mammal in its own right.
 |
| Cynaognathus, perhaps the first mammal - and our ancestor |
No comments:
Post a Comment