How the Decline of the Dinosaurs Occurred is a Mystery
The cause of extinction for the non-avian dinosaurs remains a mystery. Yes, there are many theories about this, and palaeontologists have whittled them down to a few, e.g. an asteroid striking the Earth, huge volcanic eruptions, changes in sea level and climate change; yet how these possible occurrences actually caused the extinction of complete lineages of organisms, is still under discussion.
Were Dinosaurs Thriving Before the End of the Cretaceous Period?
One of the hot questions concerns whether dinosaurs were actually thriving up to the end of the Cretaceous, or whether there were signs before then that indicates some kind of decline of the dinosaurs. Evidence from the Hell Creek Formation in western North America, where paleontological finds are made from rock that is 66 million years old, suggests that dinosaur diversity – based on number of species – is less pronounced than evidence from 10 million years earlier in the same area suggests. Such evidence of decline of the dinosaurs, however, relies on the methods used to identify species before counting them, which in turn relies on the way in which we distinguish between different species of dinosaur (or other groups) that are only described from fossils.
For example, Triceratops and Torosaurus are known as two different genera, which means that two large ceratopsid genera existed towards the end of the Cretaceous in western North America. However, should we believe that the taxon we call Torosaurus actually represents the more mature form of Triceratops, based on skeletal evidence, then there is only half of the originally suggested diversity in ceratopsids. Considering that records of fossilized remains are never perfect, and even in the best conditions for preservation we are merely sampling a few bits and pieces, then equating species counts to actual diversity is complicated.
Did Variety in Form Contribute to the Decline of the Dinosaurs?
In a recent paper in the journal ‘Nature Communications’, four palaeontologists approach the matter differently. Instead of tracking number of species and genera, the authors – Stephen Brusatte, Richard Butler, Albert Prieto-Márquez and Mark Norell – investigated differences in morphology by studying variation across seven major clades from a regional and global perspective. The theory is that variety in form relates to different kinds of behavior and lifestyle, which allows an estimation of diversity at a particular time, without the need for taxonomic certainty. This allows researchers to gauge which dinosaur clades may have been thriving, and which may have been showing decline over time.
Data Analysis Helps to Explain the Decline of the Dinosaurs
The authors analysed trends in morphological disparity from the late Campanian to the Maastrichtian age (i.e. the 12 million years at the end of the Cretaceous), in the following seven dinosaur groups: ankylosaurs, ceratopsids, hadrosauroids, pachycephalosaurids, sauropods, tyrannosauroids and the non-avian coelurosaurs. They found that no one pattern fits all the groups; there was decline of the dinosaurs in some groups, but others were stable. The latter proved true for the ankylosaurs (a group with heavy armour), pachycephalosaurs (with their domed heads), coelurosaurs (that were small and feathery) and the infamous tyrannosaurs. These groups did not exhibit obvious changes in disparity, and in the sauropods (with their long necks and large size) there was only a slight increase in disparity. On both a local and a global level, there was no evidence that these groups were affected by the decline of the dinosaurs suspected prior to extinction.
Hadrosaurs and Ceratopsids Showed Signs of Being Affected by the Decline of the Dinosaurs
A different picture emerged for the hadrosaurs (with their shovel beaks) and the ceratopsids (with their horns). Ceratopsids exhibit a noticeable drop in disparity from the Campanian to the Maastrichtian, which is partially explained by the loss of an entire subgroup of ceratopsids. The Campanian age saw both centrosaurines (e.g. Centrosaurus) and chasmosaurines (e.g. Chasmosaurus) roaming North America, while only the chasmosaurines survived to see the Maastrichtian.
Globally the hadrosaurs only dipped slightly in terms of disparity, but different continents show different patterns for this group. Asian hadrosaurs exhibit a small increase in disparity, while North American hadrosaurs show a marked decline over the last 12 million years of the Cretaceous. However, the situation for North American dinosaurs cannot be extrapolated to a global level.
This Evidence Shows Not All Dinosaurs Were Equal Prior to Extinction
The authors point out that past studies concentrated on faunal abundance or richness of species, while their current study on levels of disparity gives a ‘more nuanced picture’ of the ultimate 12 million years of the history of the dinosaurs. It debunks the idea that a decline of the dinosaurs came about for all species or that all species were thriving. It rather shows the last 12 million years to be a period of flux for dinosaurs, particularly in North America. Here certain lineages remained in stable state, while others (the largest and most abundant herbivores) exhibited a decline in variability.
The increase in disparity showed by the sauropods at the end of the Cretaceous is of particular note. The popular image is one of sauropods being titans of the Jurassic that were replaced by groups with superlative plant-shearing ability (e.g. hadrosaurs and ceratopsids). New evidence seems to suggest that sauropods actually clung on and may even have expanded again, while the hadrosaurs and ceratopsids were considerably affected by the “decline of the dinosaurs.”
What Would the Decline of the Dinosaurs Meant if the Cretaceous Extinction Event Never Happened?
What would have happened if the Cretaceous extinction never took place? We will never know, but if the non-avian dinosaurs had survived, human kind may never have evolved to ask this very question.
One thing is clear: one should never consider the dinosaurs as a monolithic group. There is variability everywhere – in the pressures that lay behind their evolution, in the reasons they went extinct, between groups, and between geographic locations. The more we research them the more we realise that their history is complex, and that there is still so much more to learn. Our present understanding of the non-avian dinosaur extinction is based on evidence from sites in western North America. These sites are relatively accessible and give scientists a record that spans the final days of the dinosaurs to a time where mammals dominate the world. No matter how well we study these sites, they will always give only a small glimpse of the global dinosaur extinction; the situation in North America may not represent what happened across the globe. It is possible that these North American sites and fossil records reflect a local anomaly influenced by ‘extreme fluctuations of the inland Western Sea’, the formation of mountains, and a unique biogeography – all factors that could affect the local evolution of dinosaurs in ways that differ from other continents.
To truly understand dinosaur evolution and ultimate extinction, scientists will have to adopt a different approach – one that does not lump all dinosaurs in one group, that considers localised evidence and that operates at a more refined level. Despite years of study and speculation about the evolution and decline of the dinosaurs, we are only starting to put together an image of the last days of the Cretaceous.
