Evolutionary significance of differential species longevity in
Osagean–Meramecian (Mississippian) crinoid clades
Authors: Kammer Thomas W. , Baumiller Tomasz K. , Ausich William I. 
Published: August 2022

The pattern of differential species longevities among five
Osagean–Meramecian crinoid clades is analyzed for its evolutionary
significance. Differences in mean species longevity between clades may have
resulted from species sorting based on eurytopy (niche breadth). In order to
test the relationship between longevity and eurytopy it was first necessary
to recognize generalists (eurytopes) vs. specialists (stenotopes)
objectively. Three different approaches were used: (1) the “Eurytopy Index”
(EI), which is a measure of mean number of facies per species; (2) analysis
of crinoid functional morphology; and (3) use of canonical discriminant
analysis to analyze species distributions between facies in order to
separate generalists from specialists. Mean species longevity for each clade
was evaluated by four different approaches: (1) rarefaction was used to
control for differences in sample size, including both species richness and
number of occurrences, between clades; (2) potential facies control of
species longevity was evaluated by a bootstrap that compared the observed
data to a null model where species longevity was limited only by the actual
occurrences of each species known facies through time; (3) uniformity of
clade species richness through time was evaluated by the “Index of
Uniformity for Species Richness” based on the standard deviation of clade
species richness across the time intervals; and (4) potential species range
truncations were evaluated by a biostratigraphic gap analysis based on the
binomial distribution.

The general order of increasing longevity and eurytopy is (from least to
most): flexibles, advanced cladids, camerates, disparids, and primitive
cladids. In general the pinnulate crinoids (advanced cladids and camerates)
were specialists with lower mean species longevity, and the non-pinnulate
crinoids (disparids and primitive cladids) were generalists with higher mean
species longevity. Pinnulate crinoids were specialized for feeding in
high-energy currents and, thus, were limited in their facies distribution
and presumably more extinction-prone. The non-pinnulates could feed in both
low- and high-energy currents and, thus, were less limited in their facies
distribution and presumably less extinction-prone. The flexibles were the
exception in that they were non-pinnulate but had the lowest mean species
longevity, apparently because they were specialized for deeper-water clastic
environments.

On average, generalist clades have mean species longevities that at a
minimum are up to 45% (≈1.0 ± 0.7 m.y.) longer than specialist clades.
However, greater mean species longevity did not necessarily confer long-term
advantages to a clade. The specialist advanced cladids became the dominant
crinoid clade of the late Paleozoic and gave rise to the articulate crinoids
of the post-Paleozoic. This may have resulted from the more rapid species
turnover of stenotopes creating adaptive evolutionary novelties for their
clade. Alternatively, it could simply be the result of stochastic
processes.

The finer subdivision of niche space by specialists has led previous
workers to predict that specialist clades should have higher species
richness than generalist clades. The present study supports this
prediction.