New comatulid crinoids from the Early Cretaceous Glen Rose Formation (Texas, USA): paleobiology and evolutionary relationships of an endemic, ephemeral giant

Abstract The free-living, stalkless comatulids make up the bulk of living crinoid diversity and are the only crinoids remaining in shallow water, but compared to the stalked crinoids their fossil record is fragmentary and understudied, especially outside Europe. We present new Albian fossil comatulids from the Glen Rose Formation, central Texas, and study them using computed tomography, scanning electron microscopy, morphometry, and cladistic analysis of discrete and continuous characters. New material comprises the previously described Decameros wertheimi and Semiometra klari ? and the new taxa Semiometra alveoradiata n. sp. and an extremely unusual new form Castaneametra hodgesi n. gen. n. sp. In addition to being exceptionally variable, this species is the largest known comatulid, has the most cirri of any known comatulid, and exhibits unique architectural features and voluminous, through-going coelomic cavities. We reconstruct its paleobiology, infer its phylogenetic affinities, argue for its origin from small Semiometra -like ancestors in a brief Albian North American radiation, and suggest new interpretations of early comatulid phylogeny based on our findings. Adaptive allometry related to respiratory demands, along with an origin by peramorphosis, may explain some features of this odd, short-lived giant.