An Inane Sentence About Dinosaur Extinction

This from the Daily Mail about the discovery of a sudden shift in Early Cretaceous climate and its potential impact on dinosaurs:

The temperature drop during the Cretaceous period would almost certainly have wiped out an 'abundance' of the world's dinosaurs.

The scientists behind the ground-breaking study claim this would have been the first major step dinosaurs took on their eventual road to extinction.

Hmmm...I had heard stories about lemmings committing mass suicide but didn't know that the dinosaurs too came to an agreement in the early Cretaceous to take decisive major steps towards eventual extinction.

I know..I know.. the writer didn't mean it that way...but the sentence does come out reading that odd.

More seriously Dr Gregory Price of Plymouth University, one of the scientists involved in the study suggests that climatic shifts possibly caused by a dysfunction to the nascent Gulf Stream which brings warm waters to the north Atlantic resulted in a temperature drop about several degrees beginning around 137 mya and similar events recurring throughout the Cretaceous may have caused a gradual dying out of the dinosaurs.

If that is true it should be reflected in the patterns of dinosaur diversity through the Cretaceous period. Dinosaur diversity should show a systematic decline beginning around 137 mya.

What does the paleontological record show?

I dug around for references and came across several studies that have measured and estimated dinosaur diversity through time. They show that:

1) There is no trend of a systematic gradual decline in dinosaur diversity beginning around 137 mya (Ref).

2) There seems to be an expansion of dinosaur diversity early in the history of the group in the late Triassic - mid Jurassic. After that there were periods of minor expansion and diversification - as recognized by origin of new dinosaur groups - in the later Jurassic and in the mid - late Cretaceous (Ref).

3) The raw data i.e. the observed diversity from the collected fossil record shows a major expansion of diversity in the mid -late Cretaceous. That however may be an artifact of sampling. This can occur in a number of ways. Perhaps the mid-late Cretaceous has been sampled more intensively due to an interest in the late Cretaceous mass extinction. Or more Cretaceous strata is exposed and is available for study than earlier times (Ref) .

Corrected for these biases the increase in diversity in mid-late Cretaceous is more muted.

Figure below shows the patterns of dinosaur diversity through the Mesozoic.

Source: Dinosaurs and the Cretaceous Terrestrial Revolution

Fig a is an estimate of the frequency of new lineages originating through time and the pattern suggests that the majority of new groups seem to have evolved in the earlier 1/3rd history of dinosaurs,

Fig b, solid line shows observed diversity with peaks in the early history of the group with a Jurassic maxima and a later increase in the mid-late Cretaceous. Dashed line shows diversity corrected for ghost ranges, which are estimates of the total geological range of the fossil. Many times the first appearance of a  group in the fossil record may not coincide with the origin of that group. Therefore the earlier existence of that group is estimated based on evolutionary relationships with related groups. So for example the appearance of several new groups in the mid-late Cretaceous may not mean those groups originated then. If one applies estimates of when that group actually originated i.e. includes their basal ghost range, then the diversification peaks for mid-late Cretaceous are moderated.

Dotted lines show diversity corrected for sampling bias. The result suggests that diversification rates are heavily influenced by sampling. In other words the data corrected for sampling bias indicates neither an increase nor a decrease in dinosaur diversity throughout the Cretaceous.

Fig c shows the likelihood of shifts in diversity through time. The trend line suggests an overall decrease in diversity shifts (lineage branching events) after an early (late Triassic - early Jurassic) peak. The likelihood of lineage branching events (increases in diversity) fluctuated moderately throughout the Cretaceous (Ref).

4) Some studies show that dinosaurs did decline in diversity a few million years (10 million or so) before the K-Pg mass extinction event. Others suggest that no such pattern in discernible once you take into account not just observed diversity but estimated diversity as well. Estimated diversity is a measure of how much of the actual diversity remains to be discovered.

In summary.. the data does not seem to support Dr. Price's contention that there was a gradual decline in dinosaurs initiated by climatic changes around 137 mya over a time period of some 70 million years before the K-Pg mass extinction and certainly does not even come close to supporting the crackpot headline in the Daily Mail that Dinosaurs 'killed off by a sudden drop in temperature and NOT by a comet'

Dinosaurs were a group with a world wide distribution. There would have been throughout their history local extinction events brought about by environmental perturbations. A possible cooling of the earth about 137 mya may have harmed dinosaurs in the very northern latitudes but did not set the stage (in a non-teleological sense of the phrase) for a long term globally widespread decline of the dinosaurs through the rest of the Cretaceous. In fact, there may have been a moderate expansion of dinosaur diversity in the mid-late Cretaceous. Globally the dinosaurs seemed to be doing quite well until their rapid demise 65 mya.