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Before Passeriformes perched on the branches of our trees and sang their sweet, inspirational tunes, there was Enantiornithes, the "Opposite Birds". So far, we have discovered at least 80 species from five different families. (However, some of those names come from just one bone, so the nomenclature may not be valid.)

The really odd thing is that despite their diversity, they had become extinct during the Mesozoic-Cenozoic extinction event 66 million years ago, even though their rivals, the forefliers of today's waterfowl, shorebirds, tube-noses, rails and ratites, did not.

The question is: How come? Was there any paleobiology saying that this particular order of birds was too specialized to withstand an impact winter?

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    $\begingroup$ This is a fine question, but I don't think you are going to find answers. Simply, we don't know enough about the physiology or life history of these birds to know. All we have are skeletons (and some feather impressions). Even if we did know all about their biology, we still wouldn't be able to say why they went extinct. The same question could be asked of any speciose, seemingly successful group that went extinct. $\endgroup$ – kmm Oct 12 '16 at 1:44
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The short answer is that we will never know.

Luck can sometimes be a factor of who lives and who dies. But such an answer doesn't feel satisfactory given such a diverse group such as dinosaurs and other groups like Enantiornithes.

Together with hatchling specimens of the Mongolian Gobipteryx[35] and Gobipipus,[36] these finds demonstrate that enantiornithean hatchlings had the skeletal ossification, well-developed wing feathers, and large brain which correlate with precocial or superprecocial patterns of development in birds of today. In other words, enantiornitheans probably hatched from the egg already well developed and ready to run, forage, and possibly even fly at just a few days old.[34]

A 2006 study of Concornis bones showed a growth pattern different from modern birds; although growth was rapid for a few weeks after hatching, probably until fledging, this small species did not reach adult size for a long time, probably several years.[37] Other studies have all supported the view that growth to adult size was slow, as it is in living precocial birds (as opposed to altricial birds, which are known to reach adult size quickly).[26] Studies of the rate of bone growth in a variety of enantiornitheans has shown that smaller species tended to grow faster than larger ones, the opposite of the pattern seen in more primitive species like Jeholornis and in non-avialan dinosaurs.[38] Some analyses have interpreted the bone histology to indicate that enantiornitheans may not have had fully avian endothermy, instead having an intermediate metabolic rate.[39] https://en.wikipedia.org/wiki/Enantiornithes

So we have this. Enantiornithes weren't completely bird like. They may have invested less energy in parental care. Perhaps taken longer to reach adulthood, longer to reach reproductive age. May not be as warm blooded as birds but not as cold blooded as a crocodile. Maybe that is why they went extinct.

The Enantiornithes did not care much for their young, since they were born pretty much hardwired to be independent. This hard wiring probably worked find in a stable environment, allowing adult to spend more time and energy making more independent chicks. But in a hostile, changing world of post-impact, that was simply too much for a hard wired, semi-warm blooded chick. Especially one that took longer to reach reproductive age than most birds did. And birds by and large do invest in parental care of their offspring.

The answer is perhaps obvious if we could go back in time and observe the entire genera of Enantiornithes. But in the absence of a time machine and only fossils and estimated growth rates... we can only speculate.

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All kinds of things effect survival of an impact, you don't just have a winter, the impact sets off every fault and volcano causing earthquakes, eruptions, tsunami, acid rain, and widespread forest fires. It's not really about who is killed by these events but which groups are best at recovering from it. And too much of that can be determined by things that don't leave evidence in fossils. Slowly reproducing and specialized organisms tend to go extinct but they are by no means the only things that go.

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