Decoupling of taxonomic diversity and morphological disparity during decline of the Cambrian trilobite family Pterocephaliidae

Divergent vertebral formulae shape the evolution of axial complexity in mammals

Complexity, defined as the number of parts and their degree of differentiation, is a poorly explored aspect of macroevolutionary dynamics. The maximum anatomical complexity of organisms has undoubtedly increased through evolutionary time. However, it is unclear whether this increase is a purely diffusive process or whether it is at least partly driven, occurring in parallel in most or many lineages and with increases in the minima as well as the means. Highly differentiated and serially repeated structures, such as vertebrae, are useful systems with which to investigate these patterns. We focus on the serial differentiation of the vertebral column in 1,136 extant mammal species, using two indices that quantify complexity as the numerical richness and proportional distribution of vertebrae across presacral regions and a third expressing the ratio between thoracic and lumbar vertebrae. We address three questions. First, we ask whether the distribution of complexity values in major mammal groups is similar or whether clades have specific signatures associated with their ecology. Second, we ask whether changes in complexity throughout the phylogeny are biased towards increases and whether there is evidence of driven trends. Third, we ask whether evolutionary shifts in complexity depart from a uniform Brownian motion model. Vertebral counts, but not complexity indices, differ significantly between major groups and exhibit greater within-group variation than recognized hitherto. We find strong evidence of a trend towards increasing complexity, where higher values propagate further increases in descendant lineages. Several increases are inferred to have coincided with major ecological or environmental shifts. We find support for multiple-rate models of evolution for all complexity metrics, suggesting that increases in complexity occurred in stepwise shifts, with evidence for widespread episodes of recent rapid divergence. Different subclades evolve more complex vertebral columns in different configurations and probably under different selective pressures and constraints, with widespread convergence on the same formulae. Further work should therefore focus on the ecological relevance of differences in complexity and a more detailed understanding of historical patterns.

Unexpected bite-force conservatism as a stable performance foundation across mesoeucrocodylian historical diversity

Effective interpretation of historical selective regimes requires comprehensive in vivo performance evaluations and well-constrained ecomorphological proxies. The feeding apparatus is a frequent target of such evolutionary studies due to a direct relationship between feeding and survivorship, and the durability of craniodental elements in the fossil record. Among vertebrates, behaviors such as bite force have been central to evaluation of clade dynamics; yet, in the absence of detailed performance studies, such evaluations can misidentify potential selective factors and their roles. Here, we combine the results of a total-clade performance study with fossil-inclusive, phylogenetically informed methods to assess bite-force proxies throughout mesoeucrocodylian evolution. Although bite-force shifts were previously thought to respond to changing rostrodental selective regimes, we find body-size dependent conservation of performance proxies throughout the history of the clade, indicating stabilizing selection for bite-force potential. Such stasis reveals that mesoeucrocodylians with dietary ecologies as disparate as herbivory and hypercarnivory maintain similar bite-force-to-body-size relationships, a pattern which contrasts the precept that vertebrate bite forces should vary most strongly by diet. Furthermore, it may signal that bite-force conservation supported mesoeucrocodylian craniodental disparity by providing a stable performance foundation for the exploration of novel ecomorphospace.

Giant Mesozoic coelacanths (Osteichthyes, Actinistia) reveal high body size disparity decoupled from taxic diversity

The positive correlation between speciation rates and morphological evolution expressed by body size is a macroevolutionary trait of vertebrates. Although taxic diversification and morphological evolution are slow in coelacanths, their fossil record indicates that large and small species coexisted, which calls into question the link between morphological and body size disparities. Here, we describe and reassess fossils of giant coelacanths. Two genera reached up to 5 m long, placing them among the ten largest bony fish that ever lived. The disparity in body size adjusted to taxic diversity is much greater in coelacanths than in ray-finned fishes. Previous studies have shown that rates of speciation and rates of morphological evolution are overall low in this group, and our results indicate that these parameters are decoupled from the disparity in body size in coelacanths. Genomic and physiological characteristics of the extant Latimeria may reflect how the extinct relatives grew to such a large size. These characteristics highlight new evolutionary traits specific to these "living fossils".

Selectivity and the effect of mass extinctions on disparity and functional ecology

Selectivity of mass extinctions is thought to play a major role in coupling or decoupling of taxonomic, morphological, and ecological diversity, yet these measures have never been jointly evaluated within a single clade over multiple mass extinctions. We investigate extinction selectivity and changes in taxonomic diversity, morphological disparity, and functional ecology over the ~160-million-year evolutionary history of diplobathrid crinoids (Echinodermata), which spans two mass extinctions. Whereas previous studies documented extinction selectivity for crinoids during background extinction, we find no evidence for selectivity during mass extinctions. Despite no evidence for extinction selectivity, disparity remains strongly correlated with richness over extinction events, contradicting expected patterns of disparity given nonselective extinction. Results indicate that (i) disparity and richness can remain coupled across extinctions even when selective extinction does not occur, (ii) simultaneous decreases in taxonomic diversity and disparity are insufficient evidence for extinction selectivity, and (iii) selectivity differs between background and mass extinction regimes.

Disparities in the analysis of morphological disparity

Analyses of morphological disparity have been used to characterize and investigate the evolution of variation in the anatomy, function and ecology of organisms since the 1980s. While a diversity of methods have been employed, it is unclear whether they provide equivalent insights. Here, we review the most commonly used approaches for characterizing and analysing morphological disparity, all of which have associated limitations that, if ignored, can lead to misinterpretation. We propose best practice guidelines for disparity analyses, while noting that there can be no 'one-size-fits-all' approach. The available tools should always be used in the context of a specific biological question that will determine data and method selection at every stage of the analysis.

Diversity and Disparity of Therocephalia: Macroevolutionary Patterns through Two Mass Extinctions

Mass extinctions have the potential to substantially alter the evolutionary trends in a clade. If new regions of ecospace are made available, the clade may radiate. If, on the other hand, the clade passes through an evolutionary “bottleneck” by substantially reducing its species richness, then subsequent radiations may be restricted in the disparity they attain. Here we compare the patterns of diversity and disparity in the Therocephalia, a diverse lineage of amniotes that survived two mass extinction events. We use time calibrated phylogeny and discrete character data to assess macroevolutionary patterns. The two are coupled through the early history of therocephalians, including a radiation following the late Guadalupian extinction. Diversity becomes decoupled from disparity across the end-Permian mass extinction. The number of species decreases throughout the Early Triassic and never recovers. However, while disparity briefly decreases across the extinction boundary, it recovers and remains high until the Middle Triassic.

Morphospace saturation in the stem-gnathostomes pteraspidiformes heterostracans: an early radiation of a 'bottom' heavy clade

Ostracoderms (fossil armoured jawless fishes) shed light on early vertebrate evolution by revealing the step-wise acquisition of jawed vertebrate characters, and were important constituents of Middle Palaeozoic vertebrate faunas. A wide variety of head shield shapes are observed within and between the ostracoderm groups, but the timing of these diversifications and the consistency between different measures of their morphospace are unclear. Here, we present the first disparity (explored morphospace) versus diversity (number of taxa) analysis of Pteraspidiformes heterostracans using continuous and discrete characters. Patterns of taxic diversity and morphological disparity are in accordance: they both show a rise to a peak in the Lochkovian followed by a gradual decline in the Middle-Late Devonian. Patterns are largely consistent for disparity measures using sum of ranges or total variance, and when using continuous or discrete characters. Pteraspidiformes heterostracans can be classified as a "bottom-heavy clade", i.e., a group where a high initial disparity decreasing over time is detected. In fact, the group explored morphospace early in its evolutionary history, with much of the subsequent variation in dermal armour occurring as variation in the proportions of already evolved anatomical features. This Early Devonian radiation is also in agreement with the paleobiogeographic distribution of the group, with a maximum of dispersal and explored morphospace during the Lochkovian and Pragian time bins.

Discrete and continuous character-based disparity analyses converge to the same macroevolutionary signal: a case study from captorhinids

The relationship between diversity and disparity during the evolutionary history of a clade provides unique insights into evolutionary radiations and the biological response to bottlenecks and to extinctions. Here we present the first comprehensive comparison of diversity and disparity of captorhinids, a group of basal amniotes that is important for understanding the early evolution of high-fiber herbivory. A new fully resolved phylogeny is presented, obtained by the inclusion of 31 morphometric characters. The new dataset is used to calculate diversity and disparity through the evolutionary history of the clade, using both discrete and continuous characters. Captorhinids do not show a decoupling between diversity and disparity, and are characterized by a rather symmetric disparity distribution, with a peak in occupied morphospace at about the midpoint of the clade's evolutionary history (Kungurian). This peak represents a delayed adaptive radiation, identified by the first appearance of several high-fiber herbivores in the clade, along with numerous omnivorous taxa. The discrete characters and continuous morphometric characters indicate the same disparity trends. Therefore, we argue that in the absence of one of these two possible proxies, the disparity obtained from just one source can be considered robust and representative of a general disparity pattern.

Novelty and Innovation in the History of Life

The history of life as documented by the fossil record encompasses evolutionary diversifications at scales ranging from the Ediacaran-Cambrian explosion of animal life and the invasion of land by vascular plants, insects and vertebrates to the diversification of flowering plants over the past 100 million years and the radiation of horses. Morphological novelty and innovation has been a recurrent theme. The architects of the modern synthesis of evolutionary theory made three claims about evolutionary novelty and innovation: first, that all diversifications in the history of life represent adaptive radiations; second, that adaptive radiations are driven principally by ecological opportunity rather than by the supply of new morphological novelties, thus the primary questions about novelty and innovation focus on their ecological and evolutionary success; and third, that the rate of morphological divergence between taxa was more rapid early in the history of a clade but slowed over time as ecological opportunities declined. These claims have strongly influenced subsequent generations of evolutionary biologists, yet over the past two decades each has been challenged by data from the fossil record, by the results of comparative phylogenetic analyses and through insights from evolutionary developmental biology. Consequently a broader view of novelty and innovation is required. An outstanding issue for future work is identifying the circumstances associated with different styles of diversification and whether their frequency has changed through the history of life.

What limits the morphological disparity of clades?

The morphological disparity of species within major clades shows a variety of trajectory patterns through evolutionary time. However, there is a significant tendency for groups to reach their maximum disparity relatively early in their histories, even while their species richness or diversity is comparatively low. This pattern of early high-disparity suggests that there are internal constraints (e.g. developmental pleiotropy) or external restrictions (e.g. ecological competition) upon the variety of morphologies that can subsequently evolve. It has also been demonstrated that the rate of evolution of new character states decreases in most clades through time (character saturation), as does the rate of origination of novel bodyplans and higher taxa. Here, we tested whether there was a simple relationship between the level or rate of character state exhaustion and the shape of a clade's disparity profile: specifically, its centre of gravity (CG). In a sample of 93 extinct major clades, most showed some degree of exhaustion, but all continued to evolve new states up until their extinction. Projection of states/steps curves suggested that clades realized an average of 60% of their inferred maximum numbers of states. Despite a weak but significant correlation between overall levels of homoplasy and the CG of clade disparity profiles, there were no significant relationships between any of our indices of exhaustion curve shape and the clade disparity CG. Clades showing early high-disparity were no more likely to have early character saturation than those with maximum disparity late in their evolution.

Morphological diversity in tenrecs (Afrosoricida, Tenrecidae): comparing tenrec skull diversity to their closest relatives

It is important to quantify patterns of morphological diversity to enhance our understanding of variation in ecological and evolutionary traits. Here, we present a quantitative analysis of morphological diversity in a family of small mammals, the tenrecs (Afrosoricida, Tenrecidae). Tenrecs are often cited as an example of an exceptionally morphologically diverse group. However, this assumption has not been tested quantitatively. We use geometric morphometric analyses of skull shape to test whether tenrecs are more morphologically diverse than their closest relatives, the golden moles (Afrosoricida, Chrysochloridae). Tenrecs occupy a wider range of ecological niches than golden moles so we predict that they will be more morphologically diverse. Contrary to our expectations, we find that tenrec skulls are only more morphologically diverse than golden moles when measured in lateral view. Furthermore, similarities among the species-rich Microgale tenrec genus appear to mask higher morphological diversity in the rest of the family. These results reveal new insights into the morphological diversity of tenrecs and highlight the importance of using quantitative methods to test qualitative assumptions about patterns of morphological diversity.

Origins, bottlenecks, and present-day diversity: patterns of morphospace occupation in marine bivalves

It has long been known that species should not be distributed randomly in morphospace (a multidimensional trait space), even under simple models of evolution. However, recent studies suggest that position in morphospace can affect aspects of evolution such as the durations of clades and the species richness of their constituent taxa. Here we investigate the dynamics of morphospace occupancy in living and fossil marine bivalves using shell size and aspect ratio, two functionally important traits. Multiple lines of evidence indicate that the center of a family's morphospace today represents a location where taxonomic diversity is maximized, apparently owing to lower extinction rates. Within individual bivalve families, species with narrow geographic ranges are distributed throughout the morphospace but widespread species, which are generally expected to be extinction resistant, tend to be concentrated near the center. The morphospace centers of most species-rich families today (defined as the median value for all species in the family) tend to be close to the positions of the family founders, further suggesting an association between position in morphospace and net diversification rates. However, trajectories of individual subclades (genera) are inconsistent with the center of morphospace being an evolutionary attractor.

Dynamic evolutionary change in post-Paleozoic echinoids and the importance of scale when interpreting changes in rates of evolution

How ecological and morphological diversity accrues over geological time has been much debated by paleobiologists. Evidence from the fossil record suggests that many clades reach maximal diversity early in their evolutionary history, followed by a decline in evolutionary rates as ecological space fills or due to internal constraints. Here, we apply recently developed methods for estimating rates of morphological evolution during the post-Paleozoic history of a major invertebrate clade, the Echinoidea. Contrary to expectation, rates of evolution were lowest during the initial phase of diversification following the Permo-Triassic mass extinction and increased over time. Furthermore, although several subclades show high initial rates and net decreases in rates of evolution, consistent with "early bursts" of morphological diversification, at more inclusive taxonomic levels, these bursts appear as episodic peaks. Peak rates coincided with major shifts in ecological morphology, primarily associated with innovations in feeding strategies. Despite having similar numbers of species in today's oceans, regular echinoids have accrued far less morphological diversity than irregular echinoids due to lower intrinsic rates of morphological evolution and less morphological innovation, the latter indicative of constrained or bounded evolution. These results indicate that rates of evolution are extremely heterogenous through time and their interpretation depends on the temporal and taxonomic scale of analysis.

Extinction space--a method for the quantification and classification of changes in morphospace across extinction boundaries

Three main modes of extinction are responsible for reductions in morphological disparity: (1) random (caused by a nonselective extinction event); (2) marginal (a symmetric, selective extinction event trimming the margin of morphospace); and (3) lateral (an asymmetric, selective extinction event eliminating one side of the morphospace). These three types of extinction event can be distinguished from one another by comparing changes in three measures of morphospace occupation: (1) the sum of range along the main axes; (2) the sum of variance; and (3) the position of the centroid. Computer simulations of various extinction events demonstrate that the pre-extinction distribution of taxa (random or normal) in the morphospace has little influence on the quantification of disparity changes, whereas the modes of the extinction events play the major role. Together, the three disparity metrics define an "extinction-space" in which different extinction events can be directly compared with one another. Application of this method to selected extinction events (Frasnian-Famennian, Devonian-Carboniferous, and Permian-Triassic) of the Ammonoidea demonstrate the similarity of the Devonian events (selective extinctions) but the striking difference from the end-Permian event (nonselective extinction). These events differ in their mode of extinction despite decreases in taxonomic diversity of similar magnitude.