Hangenberg event defines tetrapods

In the latter half of the Devonian period there were as many as 10 different extinction events — points of time where the extinction rates observed in the fossil record were considerably higher than the background rate.  Two of them were extreme — the Kellwasser event (or perhaps two events) recently estimated at about 376 million years ago; and the Hangenberg event at or before the boundary of the Devonian and Carboniferous periods (about 359 million years ago).  The late Devonian dying is one of the 5 great mass extinction events in the history of life on Earth. The others are:

1. The end Permian Extinction (the “Great Dying”), about 250 million years ago, where 85% of all marine genera and 70% of land genera died off.

2. The event at the Ordovician–Silurian boundary,  440-450 million years ago, eliminating over half the world’s genera.

3. The K-T extinction event (now known as the K-Pg event) at the end of the Cretaceous, where the dinosaurs, pterosaurs, and large sea reptile were killed off by an asteroid 65 million years ago.

4. The end Triassic event about 205 million years ago which eliminated large amphibians, most therapsids (the ancestors to mammals) and devastated diapsids in such a way that crocodile ancestors and dinosaurs would dominate the land and non-archosaur reptiles the seas.

The late Devonian extinctions rank right after the first two above.

It has been generally thought that the Kellwasser event was the major event, responsible for the extinction of almost all agnatha (jawless) fish.  The Hangenberg was thought to be a final pulse to the events.

A news release by the National Science Foundation today announces the publication in the upcoming issue of the Proceedings of the National Academy of Sciences of a study of vertebrate fossils that shows the Hangenberg event was the more significant and the one that created the bottleneck from which tetrapods emerged:

Scientists have long theorized that the Late Devonian Kellwasser event–considered to be one of the “Big Five” extinctions in Earth’s history–was responsible for a marine invertebrate species shake-up.

But an analysis of the vertebrate fossil record by [Lauren] Sallan and [Michael] Coates [both of the University of Chicago]pinpointed a critical shift in diversity to the Hangenberg extinction event 15 million years later.

Prior to the extinction, lobe-finned forms such as Tiktaalik and the earliest limbed tetrapods such as Ichthyostega had made the first tentative “steps” toward a land-dwelling existence.

But after the extinction, a long stretch of the fossil record known as “Romer’s Gap,” is almost barren of tetrapods, a puzzle that had confused paleontologists for many years.

Sallan and Coates’ data suggest that the 15-million-year gap was the hangover after the traumatic Hangenberg event.

“Something that’s seen after an extinction event is a gap in the records of survivors,” Sallan said. “You have a very low diversity fauna, because most things have been killed off.”

When tetrapods finally recovered, those survivors were likely the great-great-grandfathers to the vast majority of land vertebrates present today.

Modern vertebrate traits–such as the motif of five-digit limbs that is shared by all mammals, birds and reptiles in utero–may have been set by this early common ancestor, the authors propose.

The conclusions are said to be based on a fuller fossil record and modern species sampling techniques.  The paper apparently will not discuss causes of the extinction.  The Devonian period went through an extensive reduction in atmospheric carbon dioxide owing to the rapid terrestrial radiation of vascular seed-bearing plants, which appears ot have caused extensive global cooling.  There was also widespread anoxia at the ocean floors.

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