Coelophysis Cove: April 2020, Part IIII (Expedition Memoirs)

The next day started late, with breakfast around 11 a.m. — but it was a holiday and so there was no rush to be anything or go anywhere.  Sometime towards the middle of the afternoon, we armed ourselves with insect repellant, bear repellant, and a pack full of beers, snacks, and water and hiked down to the stream we had seen running to the west of our campsite.  We had expected a longer hike, but instead found the stream lay some two hundred yards from the road, which was itself another two hundred yards or so from our encampment.

            We spent a few hours along the stream, called Shaws Fork, crossing from one shore to another on fallen trees which spanned the water like bridges.  One some you could see their evident use by animals, most likely racoon and opossum, based on the wear of branches and the bark running along the ventral side of the trunks. Sasha opted against the bridges, leaping with a notable lack of concern into the icy waters and leaping over and ducking under fallen trees like some long-familiar obstacle course.

            Here we found a surprisingly deep portion of the stream where a fallen tree had dammed a portion and thus created a deep pool.  Gnarly vines and the twisted roots of long-gone trees peered at us from beneath the water’s surface, the home of numerous minnows, freshwater shrimp, and possibly a catfish or two.  I made a note to myself to return with fishing gear next time.  The waters of the stream were very cold, both a result of the miles of rock insulation beneath us and, unknown to us at the time, the presence of a freshwater spring some two hundred more yards down the stream.

Dimetrodon by Brian Franczak, 1992.

            In addition to the small pool, the damming caused by the fallen tree had also created what would in the ocean have been called a sandbar, but which is more easily described as a tiny island bisecting the rushing waters of Shaws Fork to one side and the deep waters of the cove’s pool on the other.  I nicknamed the island Dimetrodon Island and the cove as a whole Coelophysis Cove, for their resemblances to scenes drawn on dinosaur trading cards that I had as a child.  The name, I thought, suited the location and our present endeavor, giving what might otherwise be a mere bend in the stream the alluring façade of grandeur becoming of great outdoor expeditions.

Coelophysis by Brian Franczak, 1992.

            The association with the prehistoric past is not, however, related to mere aesthetic similarities of the location to the imagination of my childhood; in point of fact, the Appalachians constitute the remnants of a mountain range which spanned the earth in the Permian, some 300 to 250 million years in the past.  In those days, all the continents of today were fused in a supercontinent known as Pangea, with its own lakes, seas, rivers, and mountain ranges foreign to our eyes.  Even the hazy silhouette of the Appalachians would have been unrecognizable peaks more reminiscent of the Himalayas and Rockies than of the Blue Ridge and Great Smokies.  By the end of the Cretaceous period 66 million years ago, the Appalachians had been eroded to the flat layers of sedimentary stone which today characterize these mountains.  Subsequent tectonic activity during the Paleogene (66-23 million years ago) up-thrusted this stone, turning parts of the stone on its side to form the strangely angled stone outcroppings one sees throughout Appalachia, and forming what we today called the Appalachian Mountains.

Nursing Dimetrodon by Mike Keesey, 2013.

            Other descendants of this prehistoric mountain range can today be found not only in eastern North America, from Georgia to New Brunswick, but also in the rock of the present-day Atlas Mountains of Morocco, the Highlands of Scotland, and the Scandinavian Mountains of Sweden, Norway, and Finland.  We today refer to their ancestral range as the Central Pangean Mountains, and some 250 million years ago, they were the boundary between two sides of the world.  To the south, in what is today Africa and Arabia, the humid winds coming off the Paleo-Tethys sea fed primeval forests of conifers and ferns, not unlike the Gulf Stream which today feeds the greenery of the southeastern United States and the Caribbean coasts of Central and South America.  Here a time-traveling explorer would have found a forest teaming with a variety of amniote tetrapods – the stem family of reptiliomorphs which contain the common ancestor of modern reptiles, birds, and mammals.  In the Permian period from 295 to 272 million years ago, synapsids like Dimetrodon would have been among the apex predators of the time, ranging from the size of a modern alligator to well above a Komodo dragon.  It may in fact be better to compare Dimetrodon to something approaching the size of a Siberian tiger, and indeed Dimetrodon, despite its rather reptilian countenance, is more closely related to us and other mammals than to the mascot of the University of Florida.  Perhaps Mike Keesey’s illustration of a Dimetrodon nursing its young through the use of mammary-like glands is not so imaginative as it might at first seem.

Pangea from ExtremeTech.com

            The opposite side of the mountain range would have been a rather dramatic shift in scenery.  Here in what today is North America there was then a vast desert, the Central Mountains acting as a rain shadow – a sort of shield – against the humid air and rains coming off the Paleo-Tethys.  The resulting climate is not dissimilar to the way the Rocky Mountains shield the Pacific coast from the Gulf Stream, or more dramatically, the way in which the Tibetan plateau today shields the Taklamakan basin against the moisture of the South China and Indian seas, forming one of the driest places on Earth.  Under such environmental stresses, life would have evolved to be both small and efficient, and thus we find that fossils from that region show a general trend towards lighter frames intended for faster movement.  As a result, or due in part to this change, many of these animals likewise evolved bipedal locomotion, a feature not seen in widespread numbers among the fauna of the latest Permian and earliest Triassic periods, demonstrating the insular manner in which animals on the northern side of the Central Mountains evolved, compared with their near-reptilian and near-mammalian relatives of Pangea.

ChronostratChart2020-01
Geologic timescale chart by the International Commission on Stratigraphy (ICS).

            It was these and multiple other environmental stressors that forged the unusually efficient form of the dinosauromorphs and their descendants, the dinosaurs—including birds!  We see these first ancestral forms appear in the Middle Triassic, some 247 to 237 million years ago, in animals that might to modern eyes look like leathery birds – though some of these earliest forms almost certainly possessed feathers or feather-like integument, like quills.  The efficiency of these forms can perhaps best be seen in the Late Triassic dinosaur, Coelophysis, an ancestral form of later carnivorous dinosaurs like the more famous T. rex and the ubiquitous common pigeon.  If one looks at the ancestral lines leading to Coelophysis – examples of which include Eoraptor and Herrerasaurus – one will see a general slimming and increasing size in these earliest dinosaurs as they came to fill more of the ecological niches. Indeed, were it not for the waning extinction of the archosauromorphs and therapsids, holdovers of the prior era which inherited Pangea following the Great Dying at the Permian-Triassic (Pa-T) boundary, approximately 252 million years ago.

            So that we are clear, all life during the Triassic comprised the descendants of the greatest mass extinction event in known time, 252 million years ago, in which approximately 83% of all known animal genera became extinct.  What life which managed to cling on did so under great environmental stressors, and so it was to those forms of life most well adapted to inhospitable conditions which began to thrive, namely the desert plains of northern Pangea. Although initially slow to predominate, their successes were given a boost by nature when a second extinction event 201.3 million years ago. Although not nearly as dramatic as the end-Permian extinction, the Triassic-Jurassic (T-J) extinction event was still significant in its own right, wiping out about 42% of terrestrial vertebrates and 23-34% of marine life. As such, it can comfortably be compared with the most famous extinction event of them all, the Cretaceous-Paleogene (K-Pa, formerly K-T) mass extinction of 66 million years ago which wiped out all non-avian dinosaurs.  If one can for a moment imagine the vast differences in the world between one side of that last extinction and the other, then one can get a sense of the vast differences which divided the world of the earliest dinosaurs in the latter Triassic and the explosion of Dinosauria in the subsequent Jurassic and Cretaceous periods.  This demonstratable survivability is the reason dinosaurs continue to thrive to this day in the winged members of our own animal kingdom.

            Coelophysis again serves as an important example of the overall efficiency of the dinosaurian form.  Evolved from smaller, near-identical ancestors of the Middle Triassic, Coelophysis began in the desert to the north of the Central Mountains, in what is today Arizona and New Mexico in the southwestern United States. From here Coelophysis — and indeed all dinosaurs — spread from their initial range to the north of the Central Mountains to the rest of Pangea, with fossil evidence for Coelophysis being found in stone dated to the Early to Late Jurassic in modern South Africa and Zimbabwe. This sort of vast distribution testifies not only to the resiliency of the dinosaurian form, but its efficiency against the more water-dependent therapsids and archosauromorphs; indeed, both habits which Coelophysis lived in were deserts consisting of fluvial plains which might flood seasonally.

Late Jurassic Asian dilophosaurids Guanlong hunt ceratopsian Yinglong, ABelov2014, 2015.

This was not some mere accident of adaptation on the part of dinosaurs — indeed, were it not for events outside anyone’s control, dinosaurs might not have come to dominate the remnants of Pangea into the Jurassic and Cretaceous. In addition to demonstrating for us the survivability of the dinosaurian form, the Trassic-Jurassic extinction event had the added benefit of removing competition for the major ecological niches available in the Mesozoic. Dinosaurs thus evolved in both their range and size, adapting to the changing environment around them. Consider the success of the coelophysoid form in particular, which spans the Triassic-Jurassic extinction event for the particular genus Coelophysis. But this is only part of the story, for more derived forms continued to adapt to the changing world of the Jurassic, leading to larger predators like the ornamented dilophosaurids and basal members of Averostra like Ceratosaurus and the unusual Antarctic theropod Cryolophosaurus. Rather than hunting reptilian half-cousins, these new and far larger predators hunted their kin dinosaurs, the sauropodomorphs and the ornithischian, among the other prehistoric fauna which replaced the animals of the Triassic.

            With these thoughts at the forefront of my mind just the day prior, we had ourselves located some fossils of such vaunted prehistoric life.  Though not nearly as spectacular a find as a dinosaur bone, we did discover the fossilized remains of tree rings in the sedimentary stone, as well as the fossilized impression of a small, most-likely freshwater mollusk.  Although the stone’s guise as mountain dates to the period just before the dawn of the dinosaurs, the stone on which they were formed is itself far older and dates to the Carboniferous period, 358.9 to 298.9 million years ago—before Dimetrodon and before the Permian.  These fossils testified to the swamp jungles of the time where arthropods the size of nightmares and dog-sized amphibians hunted one another amid the brackish waters and mud.  These stones would have been the same which members of Dinosauria would have trod upon in the Triassic and Jurassic in their expansion across the broken remains of Pangea.  Perhaps a small relative of Coelophysis akin to Staurikosaurus or Compsognathus once hunted among these stones, unknowingly passing the fossilized remains of its primitive ancestors.

            We returned to camp satisfied with our mini-expedition and constructed a great bonfire to celebrate the day, around which we stood how primitive man might have stood around those first sources of heat and cooking.  Our own dinner consisted of diced green peppers, jalapeños, and kielbasa grilled on a skillet over the fire, and accompanied by roasted corn on the cob and plenty of beer.  We reflected on the complex evolution of life, and on the impermanence of all things.

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