EGU Blogs

Extinction

Why I think the Jurassic/Cretaceous boundary is super important

This was originally posted here.

Mass extinctions are insanely catastrophic, but important, events that punctuate the history of life on Earth. The Jurassic/Cretaceous boundary, around 145 million years ago, was originally thought of to represent a mass extinction, but has subsequently been ‘down-graded’ to a minor extinction event based on new discoveries.

However, compared to other important stratigraphic boundaries, like the end-Triassic or the end-Cretaceous, both time periods representing mass extinction events, the Jurassic/Cretaceous (J/K) boundary actually remains really poorly understood. This is both in terms of what was going on with different animal groups at the time, and what environmental changes were occurring alongside this.

Well, I have a new research paper out now that synthesises more than 600 research articles, bringing them together to try and build a single picture of what was going on around this time! It’s free to read here, and is essentially the literature review from my thesis, or as I like to think of it, the justification for my existence as a researcher!

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The early evolution of birds – more complicated than trying to untangle your headphones..

Birds are a phenomenal story of evolutionary success. As modern-day dinosaur descendants, they occupy almost all environments and ecosystems around the globe, and are truly animals that capture our imaginations. However, how did they become so diverse, both in number and form? This is something only the fossil record can divine for us.

Birds first appear in the Middle to Late Jurassic of China and latest Jurassic of Europe (hello, Archaeopteryx), around 160-150 million years ago. Their first radiation, in terms of increasing species numbers, appears to have occurred in the Early Cretaceous of China, based on the fossil graveyards of the 125 million year old Jehol Biota. However, it has been argued that the timing of this radiation is strongly influenced by ‘the Lagersttäten effect’ – that is, periods of exceptional preservation in the fossil record. For the early evolution of birds, this is complicated by the fact that the earliest Cretaceous (around 145 to 125 million years ago) fossil record of birds is known from only rare and fragmentary material.

One of the earliest known birds, Archaeopteryx, from the infamous Solnhofen beds of Bavaria, Germany. (source)

One of the earliest known birds, Archaeopteryx, from the infamous Solnhofen beds of Bavaria, Germany. (source)

At some time around then, however, it is though that birds underwent a phase of rapid diversification of body forms, particularly geared towards increasingly small body sizes. This has important implications for the evolution of flight, but that’s another story.

A possible trigger for this diversification might have been with an extinction event at the end of the Jurassic, 145 million years ago, which saw the decimation of smaller-sized pterosaurs (their non-dinosaurian, winged cousins), particularly those known as rhamphorhynchids. Ecologically speaking, this would have opened up ‘ecospace’, for other animals to radiate into and occupy. Animals, such as birds.

What you would expect to see if this is the case, is increasing diversity of birds, which we do see, as well as increasing diversity of their body forms, their morphology, and ecological variants.

Were pterosaurs duking it out against birds for millions of years? (source)

Were pterosaurs duking it out against birds for millions of years? (source)

However, is this what we the fossils record for us? A recent study has shown that, despite the high diversity represented by the Jehol Biota, we actually see constrained levels of morphological diversity – known as disparity.

Much of the Jehol bird fauna seems to have been comprised of ground foragers, which is curious as they would all have been flight capable. Interestingly, this seems to be at odds with the local pterosaur fauna. These chaps were still owning the skies, and diversifying into an increasingly bizarre suite of forms.

Could it be, perhaps, that there was a sort of ‘fight for the skies’ happening at this time? Perhaps while both pterosaurs and birds radiating, they were competing to constrain the extent to which the others could evolve, and restricted them to particular ecologies. This hypothesis is certainly appealing, and tells of a sort of ‘fight for the skies’ in the early origin of birds.

Understanding why this was happening, and which roles birds were most successful in, is important for understanding their survival through the end-Cretaceous mass extinction and the demise of non-avian dinosaurs, as well as their rise to fame in modern times.

Reference

Mitchell, J. S. and Makovicky, P. J. (2014) Low ecological disparity in Early Cretaceous birds, Proceedings of the Royal Society, Series B: Biological Sciences, 281, 20140608. (link)

IPC4 Day 1 – Using the past to inform the present

Welcome to the fourth International Palaeontology Congress! 900 palaeontologists have piled into the land of steak, sun, and malbec in Mendoza, Argentina, for the biggest palaeontology conference that draws from all parts of the field.

What I want to do with these posts is just provide snapshot summaries of the talks I’ve been at to provide a window into the conference and the amazing diversity of research being conducted by a global team of awesome researchers. It’s not all just dinosaurs you know! Results will not be discussed in any detail for obvious reasons.

The first symposium I attended was on the “Coevolution of the Earth and life: the role of the physical environment in species’ evolution.”

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Last dinosaur of its kind found in the land that time forgot

In terms of iconic dinosaurs, the gargantuan sauropods are certainly up there. Along with the mostly meat eating-theropods, and herbivorous and often armoured ornithischians, they form one of the three major groups, or clades, of dinosaurs, and were the biggest animals to ever walk this Earth.

The end of the Jurassic period, some 145 million years ago, was a pretty important time for sauropods. Their diversity was already in decline through some of the latter part of the Jurassic, but it seems that they were hit pretty badly at the Jurassic/Cretaceous (J/K) boundary, in an extinction event that may have been quite severe among land and marine-dwelling animals.

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