Study Shows Dinosaurs Diverged Long Prior to the End of the Cretaceous
There is a popularist view that the dinosaurs were at their most diverse and at the peak of these evolution in terms of the number of new species evolving; at the end of the Cretaceous. The Chicxulub impact then wiped out the great dinosaur dynasty leaving the entire world for the mammals to exploit. The Chicxulub impact identifies the asteroid impact event that resulted in the demise of the dinosaurs sixty-five million years ago. Fossil evidence doesn’t support this idea, studies in the Hell Creek Formation (Maastrichtian faunal stage), of the western United States indicate that the number of species of dinosaur was declining in this the main world towards the end of the Cretaceous. Approximately ten different genera are known from the youngest Cretaceous sediments, whilst older strata out of this area show proof many more different dinosaur types.
Hell Creek Formation Data
Certainly some of the greatest known dinosaurs date from the end of the Mesozoic. Animals wandering the Hell Creek area by the end of the Cretaceous include Triceratops, what dinosaur has 500 teeth Ankylosaurus and needless to say Tyrannosaurus rex. Previously, these gigantic representatives of these dinosaur families, (Triceratops, Ankylosaurus and T. rex are only about the biggest type of dinosaur from these three families), were considered to indicate that dinosaurs just got too big and lumbering to survive and this is the reason they went extinct. Scientists now realize that the reason why for the end Cretaceous mass extinction event, the extinction not merely of the dinosaurs but in addition the Ammonites, Plesiosaurs, Mosasaurs, Pterosaurs and a complete host of other plants and animals, were complex and probably involved a number of factors.
A Family Tree for the Dinosauria
Given the limitations of the present dinosaur fossil record it is difficult to piece together a “dinosaur family tree” but a task to map dinosaur evolution and to highlight the key evolutionary shifts in Dinosauria has just been completed. The outcome of the study, led by a group of researchers from the University of Bristol has just been published in the British Journal Proceedings of the Royal Society B.
This study indicates that the dinosaurs as an organization diversified rapidly in the Late Triassic (225 – 200 million years ago) and then underwent a second evolutionary surge in the Mid Jurassic (170 -160 million years ago). The scientists studied a big percentage of the described dinosaur species and pieced together an evolutionary “family tree of dinosaurs” ;.The team estimate that their study covered something similar to 70 percent of all known and described dinosaur species.
Bursts of Evolution
This new study contradicts earlier research that shows the dinosaurs diversifying during the Cretaceous. The established view is that although dinosaurs as an organization diversified during their entire existence, using periods, the evolution of new forms was speeded up. One particular period was early to mid Cretaceous which saw the emergence of a better variety of Ornithischian dinosaurs – the rise of the Hadrosaurs, Ceratopsians and the Pachycephalosaurs, for example. These types of new dinosaur were evolving during a period when many life forms on Earth were diversifying. Dating from about 125 to 80 million years back, there appears to have been an enormous surge of increased terrestrial biodiversity. Now period is called the Cretaceous Terrestrial Revolution, life on Earth over this period changed dramatically. The Angiosperms (flowering plants), social insects, modern lizards, Mosasaurs and many types of mammals all evolved. It had been believed that the rapidly diversifying dinosaurs were part of the move towards greater biodiversity, the paper published by the Bristol team demotes dinosaur evolution in this period to a far more peripheral role. This new study indicates that by the time of the Cretaceous Terrestrial Revolution, all the key dinosaur types that have been to survive before the end of the Cretaceous were already established.
New Research Challenges Earlier Theories
This new work certainly contrasts with much of the accepted thinking regarding dinosaur diversity. Most palaeontologists believe that during early to middle Jurassic there were only four main sets of dinosaurs, whilst during the Cretaceous this expanded to nine, namely:
Megalosaurs/Allosaurs, Tyrannosaurs, Sauropods, Hysilophodontids, Hadrosaurs, Pachycephalosaurs, Ceratopsians, Ankylosaurs and Stegosaurs.
The fossil record for the terrestrial vertebrate life of the Mesozoic is very incomplete so it is difficult to trace evolutionary links between different types of animals. The work of the Bristol University team is certainly helping open up the debate, but lacking reviewed the actual paper we cannot really comment any further. It will be interesting to learn how the evolution of non-avian dinosaurs, the birds has been assessed in this study. Very little is famous about the evolution of birds, however they do seem to have diversified and developed new species rapidly during the mid to late Cretaceous, a growth in speciation that has been largely unchecked by the Cretaceous mass extinction event.
Late Triassic Diversification
Certainly, it is not surprising that the dinosaurs diversified during the Late Triassic, the entire world was just recovering from the Permian mass extinction (an event that saw an estimated 57% of all marine families and 70% of all terrestrial vertebrate genera becoming extinct). Life on Earth slowly began to recover and those types of organisms left begun to diversify to fill those environmental niches that have been empty and those soon to be left empty by the “dead clades walking” such as the last of the Lystrosaurs. It absolutely was after the Permian mass extinction event that a number of sets of vertebrates got a chance to diversify, including our personal mammalian ancestors.