Exhaustion of Morphologic Character States Among Fossil Taxa

Abstract

Abstract. Frequencies of new character state derivations are analyzed for 56 fossil taxa. The hypothesis that new character states are added continuously throughout clade history can be rejected for 48 of theses clades. Two alternative explanations are considered: finite states and ordered states. The former hypothesizes a limited number of states available to each character and is tested using rarefaction equations. The latter hypothesizes that there are limited possible descendant morphologies for any state, even if the character has infinite potential states. This is tested using power functions. The finite states hypothesis explains states : steps relationships significantly better than does the ordered states hypothesis in 14 cases; the converse is true for 14 other cases. Under either hypothesis, trilobite clades show appreciably more homoplasy after the same numbers of steps than do molluscs, echinoderms, or vertebrates.The prevalence of the exhaustion pattern among different taxonomic groups implies that worker biases are not to blame and instead implicates biological explanations such as intrinsic constraints or persistent selective trends. Regardless of the source of increased homoplasy, clades appear to exhaust their available character spaces. Nearly all examined taxa show significant increases in proportions of incompatible character pairs (i.e., those necessarily implying homoplasy) as progressively younger taxa are added to character matrices. Thus, a deterioration of hierarchical structure accompanies character state exhaustion. Exhaustion has several implications: (1) the basic premise of cladistic analyses (i.e., that maximum congruence reflects homology rather than homoplasy) becomes increasingly less sound as clades age; (2) sampling high proportions of taxa probably is needed for congruence to discern homoplasy from homology; (3) stratigraphic data might be necessary to discern congruent homoplasy from congruent homology; and (4) in many cases, character states appear to have evolved in ordered patterns.

Trait-based diversification shifts reflect differential extinction among fossil taxa

“…The vast majority of clades showing excess HSC shows more disparity among early species than expected rather than less disparity. High early disparity corroborates the idea that clades rapidly exhaust available character states (34,58). It also raises the possibly that evolvability is greatest early in clade history (25).…”

The evolution of morphogenetic fitness landscapes: conceptualising the interplay between the developmental and ecological drivers of morphological innovation

“…Most change is relatively minor, constituting the first appearance of new character states of existing characters, or, as clades become older, an increasing reshuffling of already-seen character states (Wagner 2000). Change of this magnitude is usually reflected in the recognition of new species or genera.…”

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

“…This sort of pattern could be the result of an "early burst" of morphological diversification as taxa diverge followed by a slow-down in rates as ecological space becomes filled (5,6). Internal constraint or long-term selective pressures could also limit overall disparity, leading to a slowdown in the rate of new trait acquisition over time (7,8). However, only a small proportion of fossil disparity studies have also assessed changes in rates of evolution within lineages (e.g., along phylogenetic branches) thereby providing a more nuanced understanding of how this disparity came about (e.g., refs.…”