‘ Snakes and lizards suffered DEVASTATING LOSSES , with 83 % of species BECOMING EXTINCT , and many KEY TAXA ELIMINATED .’
had a huge diversity of tooth sizes adapted for meat and plant matter ; primitive enantiornithine birds were also endowed with teeth . Only the advanced lineage of birds , the neornithines , had lost teeth and instead evolved a beak which most likely was adapted for consuming seeds . This represents a highly compelling hypothesis , for in the wake of the extinction , when most animal and plant matter was destroyed , seed ( which can withstand very harsh conditions ) would still have been available . Furthermore , seed-eating birds are recognised to be among the first species to recolonise disturbed sites . So the loss of teeth and the adoption of a granivorous ( seed-eating ) niche may have been the key for neornithine birds surviving the K-T Armageddon . This may also help explain the survival of the lineage that gave rise to us – the Mammalia .
Another compelling hypothesis for the differential success of neornithine avian and non-avian dinosaurs in surviving the K-T extinction relates to developmental rates . A study published just recently ( in 2017 ) by Erickson and colleagues analysed embryonic growth in dinosaurs based on tooth development , and revealed that dinosaurs had exceptionally long incubation periods - from three to six months ! Most birds today have an incubation period lasting about three weeks . Analysis of bone microstructure also suggests that dinosaurs and primitive birds had slow growth rates , whereas contemporaneous birds had supercharged growth rates , affording them resilience in the face of environmental upheavals . These factors would combine to make non-avian dinosaurs much more vulnerable to extinction .
Parental behaviour , almost ubiquitous among birds , may also have contributed . Although there is a lack of conclusive evidence to demonstrate
Left : Citipati was a genus of bird-like dinosaurs that lived during the Late Cretaceous Period in what is now Mongolia . Inage by Linda Bucklin . differences in parental behaviour among neornithines and the rest of the dinosaurs , if non-avian dinosaurs typically ‘ abandoned ’ their eggs in large mounds in the ground , as opposed to personally incubating and guarding the eggs as most birds today do , keeping them safe from the rigours of the environment , this may have aided in the survival of the neornithines , and the demise of other species . Even for those dinosaurs that did attend their eggs , their lengthy incubation would mean prolonged exposure ( for both eggs and parents ) to a destructive environment after the crash of the meterorite .
‘ Snakes and lizards suffered DEVASTATING LOSSES , with 83 % of species BECOMING EXTINCT , and many KEY TAXA ELIMINATED .’
Rather than being mutually exclusive , it is most likely a combination of these traits that set neornithines apart from their non-avian dinosaurian relatives ; enabling them to persist across the K-T extinction and go on to radiate into the highly-successful group of feathered , avian dinosaurs we see today .
What of other reptiles ?
So , some lineages of avian dinosaurs survived the K-T extinction , but what of the other reptiles ( crocodilians and lepidosaurs )? For we still have lizards , snakes and crocodiles , all of which had ancestors that were present during the reign of the dinosaurs , but survived the extinction event that completely wiped out their cousins .
The truth is that snakes and lizards also suffered devastating mass extinctions coinciding with the Chicxulub asteroid impact : 83 % of species became extinct , with many key lizard taxa being completely eliminated . Associated with this species-level loss was a loss in morphological diversity ; recovery was prolonged and lizard diversity did not approach that in the Cretaceous period for another 10 million years after the K-T extinction event . It appears that the ancestors of extant lizards were advantaged by their small body size , as only lizards and snakes under 500 grams persisted into the Paleogene . Their smaller size would have given them an edge by requiring less energy , and combined with ectotermy and an ability to brumate or aestivate , meant that they could survive long periods of food shortages , and hide in burrows or shelters where environmental conditions were less adverse . They also , like most lizards today , were adapted to a diet of insectivory or scavenging . With most vegetation being eliminated , this removed the food base for large herbivores , and thus also for larger predatory carnivores . However , the decaying organic matter would have still provided food for insects to feed and breed in , and thus provide a food source for small insectivores , as well as for scavengers . The small ancestral lizards also almost certainly had short incubation periods , and also short generation times , meaning that they could reproduce and evolve rapidly .
And what of the dinosaurs ’ closest relatives , the crocodlians ? At the time crocodiles were , like today , relatively large-bodied reptiles , and so their ability to persist across the K-T boundary cannot be due to any ‘ economy of scale ’! It is likely that , along with being ectothermic and able to withstand long periods without food , crocodiles also were able to avoid some of the extreme environmental adversities by being able to shelter in water . In addition , they had the opportunity to forage and scavenge on both land and water , and were higher order predators in detrital food chains . These were buffered from the crash in productivity after the meteorite collision and its effect on photosynthesis , which caused terrestrial food chains based on living plant-matter to crash .
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