So, a few days ago I promised you pterosaurs.
What I knew about pterosaurs before last week could largely be written on the back of the proverbial postage stamp or, at the most, one side of A4. I knew about Pteranodon (thanks to Robert Mash's excellent little book, How to keep dinosaurs, which incidentally opened my eyes to the possibility of pycnofibres - essentially fur-like filaments - covering the beast and to the wisdom of using Deinonychus - you remember Deinonychus from the last post - as police dogs to quell unruly crowds or as a fuck-off alternative to Dobermans or Rottweilers). I knew about Sordes Pilius (dirty fur) from a Nature article, when I was researching Bakker and the Dinosaur Renaissance, Pteradactylus antiquus, antique wingfinger and the archetypal pteradactyl, and finally Queztalocoatlus northropi*, because everyone has heard of Queztalocoatlus northropi, that giant, living flying machine; the size, at least, of a medium-sized microlite.
What I hadn't realised was how successful the pterosaurs had actually been; over one hundred 'discovered' and described species spread over the entire reign of the dinosaurs and fossils by the truckload! I suspect that we have only found a tithe of the actual number of different species. After the arthropods had initially conquered the air in the form of the flying insects during the Carboniferous period - 360-299 million years ago - (and some of them were huge, relatively speaking, with wingspans of up to eighteen inches - think about that the next time you swat a fly), a glorious opportunity arose for any animal that could exploit that rich food source denied to ground-dwelling therapsids or dinosaurs. One only has to look at the diversity of the other two 'clades' of sky-borne species (birds number approximately 10,000 species and bats, who make up about 20-25% of known mammalian species number about 1250) to realise what opportunities are there for the taking if only you can get airborne.
At first sight the pterosaurs seem ill-equipped to deal with life on land, bi-pedal postures have been postulated but don't really stand up to scrutiny; pterosaurs are just too 'top-heavy' to maintain a bi-pedal posture for very long. Birds have grasping feet which allow them to frequent the trees and wings which fold back neatly across their tails. Bats, which are not at all related but which share the pterosaur's elongated fingers to support the wing membrane, although only the fourth was elongated in the pterosaurs, have relatively strong feet which allow them to roost upside down when they are not flying; something early reconstructions of pterosaurs tended to mimic. However, there seems little doubt that pterosaurid feet do not seem to possess the bauplan or the musculature to 'hang on' in the manner of bats and were simply too big and/or heavy. And so we must assume that they hopped about on the ground, when they got down from the sky, and were prey to any carnivore that happened to fancy a pterosaur sandwich.
However, a variety of biomechanical analyses and the discovered 'trackways' of what appear to be pterosaurids 'walking', seem to show locomotion at least as good as a 'bunny-hop' and probably better. How much this is due to Bakker's (and by extension Ostrom's) postulate that dinosaurs were both 'endothermic' (having a mammal-like metabolism) and were highly active and dynamic and how much is down to real reasearch, one might never know; but certainly the whole concept of pterosaurid morphology and bioecology has gone through a sea-change over recent decades.
So what did it for the pterosauria. Was it truly an asteroid impact which wiped out 75% of all known species at the end of the Cretaceous, 65 million years ago? Me, I am inclined to a more relaxed view. If the fossil record can be believed and it is not just a collection of Lagerstaetten, which preserve fossils more readily that others, and that truly the paucity of species which we perceive from the fossil record actually represent 'reality' then we were already in a 'crisis' situation by the late Cretaceous. Most of the herbivorous fossils recovered from the late cretaceous were hadrosaurs, ankylosaurs and ceratopsians, most of the giant sauropods had already bit the dust and the diverse 'herbivore lineage' had all but died out; the predominant predator was T.Rex and its cousins; small to mid-sized predatory dinosaurs also do not appear in great numbers. The pterosaurs are themselves restricted to a few types; the Azhdarchidae and maybe the Nycosauridae in the main. The sea-going mososaurs and plesiosaurs, from the evidence of fossils, had begun to die off millions of years before.
It is difficult to determine what might have been going on during the last twenty-five million years of the Cretaceous, I am inclined to agree with those palaeontologists who conjecture that something was stressing the dinosaurs and related groups during that period which led to a decline in diversity. Species come and go regularly (about every 5 million years give or take) but usually another species evolves to fill the niche left by the extinct species; this, I feel, is not the case during the late Cretaceous. Of course, we, humans or penguins, probably would be very different if entire clades had not been wiped out and it would have been a likely descendent of a non-avian dinosaur which blasted off from Cape Gondwana and landed on the crater known, to them, as the Sea of Tethys!
So what did happen to wipe out an almost entire ecosystem; no one really knows. Most of the earlier extinction events (four out of 'the big five', the fifth was the end-Cretaceous, K-Pg {or the Kt-T}) were thought to be multiple catastrophes involving two or more causes; I deem the K-Pg boundary event to be no more a single-event phenomena than the others. Something, climate, oxygen level, toxic gases, vulcanism, greenhouse or reverse-greenhouse effects, was, in my opinion, screwing up the environment; that asteroid or comet (opinion is divided) was the final straw!
It is comforting to know that I, as you did, survived the asteroid collision, but I can trace my ancestry back to Tyrannosaurus Rex or Gigantoraptor and the best you can do is a miniscule shrew, a mere Laurasiatherian titchy life form; that is immensely gratifying!
I am enormously indebted to Mark Whitton, at the University of Portsmouth (hell, I know it sounds too much like the Polytechnic of Colwyn Bay), for giving me so much to think about. I am too old now, to think of a career in ptersosaur palaeontelogy, but you have made me, at least, wish I were younger.
Footnote. In case you're interested, the other four major extinctions were: the Ordovician-Silurian (O-S, 450-440 million years ago) which killed around 60-70% of known species; the Late Devonian (D-C, 375-360 million years ago), which appears to have come in 'pulses' over a period of about 15-20 million years, and which finally saw the last remaining trilobites (my favourites!) who had been decimated at the O-S boundary - that too saw about 70% of all known species go extinct including most of the first invaders of the land; the Permian-Triassic (P-Tr 252 million years ago), which is also known as the Great Dying, in which 90-95% of all known species became extinct and is thought to have also been a 'pulsed' event also with possible multiple causes; the Triassic-Jurassic (Tr-J, 201 million years ago) which saw again around 70-75% of all species become extinct (notice a pattern here?), although the Dinosauria and their close relatives managed to avoid that particular catastrophe. There have other mass extinction where perhaps 25-30% of species went extinct but they are too numerous to mention here and may just be a pattern of life down the ages. If you are wondering why the mass extinction events always occur at the end of one period, it's just that often major periods of geological time are split solely for that reason; they are that catastrophic!
Sadly, future palaeontologists may mark down the end of the Cenozoic (our current era) as the sixth extinction event if humans continue on their present course; a truly unique event. It will be the first time that the biota has itself caused such an extinction but don't worry; life has a habit of bouncing back.
* So named after, I believe, Jack Northrop and early pioneer of the 'flying wing' design for aircraft (and that was what Queztalocoatlus was, a flying wing) which ultimately led to the American, cold-war B2 'Spirit' bomber. And yes, I also know that the Horton brothers in Germany in the '30s got there first but they were trying to build efficient gliders and only later did they get sidetracked by the Nazis into putting engines on!
No comments:
Post a Comment