Dinosaurs and the Cretaceous Terrestrial Revolution

Abstract

The observed diversity of dinosaurs reached its highest peak during the mid-and Late Cretaceous, the 50 Myr that preceded their extinction, and yet this explosion of dinosaur diversity may be explained largely by sampling bias. It has long been debated whether dinosaurs were part of the Cretaceous Terrestrial Revolution (KTR), from 125-80 Myr ago, when flowering plants, herbivorous and social insects, squamates, birds and mammals all underwent a rapid expansion. Although an apparent explosion of dinosaur diversity occurred in the mid-Cretaceous, coinciding with the emergence of new groups (e.g. neoceratopsians, ankylosaurid ankylosaurs, hadrosaurids and pachycephalosaurs), results from the first quantitative study of diversification applied to a new supertree of dinosaurs show that this apparent burst in dinosaurian diversity in the last 18 Myr of the Cretaceous is a sampling artefact. Indeed, major diversification shifts occurred largely in the first one-third of the group's history. Despite the appearance of new clades of medium to large herbivores and carnivores later in dinosaur history, these new originations do not correspond to significant diversification shifts. Instead, the overall geometry of the Cretaceous part of the dinosaur tree does not depart from the null hypothesis of an equal rates model of lineage branching. Furthermore, we conclude that dinosaurs did not experience a progressive decline at the end of the Cretaceous, nor was their evolution driven directly by the KTR.

The origin and early radiation of dinosaurs

“…; and (2) are significant diversification shifts concentrated in any specific interval of time? By comparing their cladogram to one expected under the null "birthdeath" model, Lloyd et al (2008) identified several nodes that exhibit significant diversification shifts. These are essentially nodes that are significantly more speciose than their sister taxon, which is a violation of the null model that assumes random splitting over time (see Moore, 2002, 2005;Jones et al, 2005).…”

“…Diversity measurements continually change as new fossils are discovered and specimens are reinterpreted (for instance, compare the dinosaur diversity measurements of Dodson (1990) and Wang and Dodson (2006)). A profile of dinosaur diversity over time was recently provided by Lloyd et al (2008), who were also the first authors to provide a phylogenetic correction to diversity measures across all Dinosauria (based on a supertree of dinosaur phylogeny, which as a "summary tree" is a broad and inexact proxy for a correction) and examine the potential sampling biases implicit in the dinosaur fossil record. Their diversity curves, based both on observed fossils ("taxic estimate") and observed counts corrected for ghost lineages ("phylogenetic estimate"), indicate a steady increase in diversity from the Carnian through the Early Jurassic (Table 4).…”

“…(See also the recent phylogenetically-corrected diversity analysis presented by Barrett et al (2009)). Lloyd et al (2008) also used their dinosaur supertree to ask two important questions: (1) which specific nodes (branching events) represent significant diversification shifts? ; and (2) are significant diversification shifts concentrated in any specific interval of time?…”

Dinosaur morphological diversity and the end-Cretaceous extinction

“…Despite 30 years of intensive research, a fundamental question remains: were dinosaurs undergoing a long-term decline before intensive volcanism and the Chicxulub bolide impact in the latest Cretaceous, or did these contingencies of Earth history strike down dinosaurs during or near their prime (at a time when their global biodiversity was stable or even increasing) [3][4][5][6][7][8][9][10][11][12][13][14][15] ?…”

“…Diversity counts, however, are problematic because of the emerging realizations that observed taxonomic richness is heavily biased by uneven sampling of the fossil record 16,17 and that many supposed Late Cretaceous dinosaur species are likely juveniles or sexual morphs of other known taxa [18][19][20] . Attempts to 'correct' terminal Cretaceous diversity counts for such biases have produced conflicting results [10][11][12][13][14][15] . Similarly, abundance measures are also potentially distorted by taphonomic and palaeoenvironmental heterogeneity over time 21 .…”