Early post-metamorphic, Carboniferous blastoid reveals the evolution and development of the digestive system in echinoderms

Abstract

Inferring the development of the earliest echinoderms is critical to uncovering the evolutionary assembly of the phylum-level body plan but has long proven problematic because early ontogenetic stages are rarely preserved as fossils. Here, we use synchrotron tomography to describe a new early postmetamorphic blastoid echinoderm from the Carboniferous (approx. 323 Ma) of China. The resulting three-dimensional reconstruction reveals a U-shaped tubular structure in the fossil interior, which is interpreted as the digestive tract. Comparisons with the developing gut of modern crinoids demonstrate that crinoids are an imperfect analogue for many extinct groups. Furthermore, consideration of our findings in a phylogenetic context allows us to reconstruct the evolution and development of the digestive system in echinoderms more broadly; there was a transition from a straight to a simple curved gut early in the phylum's evolution, but additional loops and coils of the digestive tract (as seen in crinoids) were not acquired until much later.

Multiple paedomorphic lineages of soft-substrate burrowing invertebrates: parallels in the origin of Xenocratena and Xenoturbella

“…These sedentary features must be absent if such a form of early echinoderms directly descended from the freeliving worm-shaped enteropneusts, as commonly assessed currently. A majority of the confirmed early Cambrian echinoderm representatives are sedentary or semisedentary with Ushaped gut, like modern pterobranchs, but not enteropneusts [104][105][106]. Recent molecular analyses [107,108] clearly dismiss previous data [109] that the sedentary phylum Pterobranchia is a part of the clade of the free-living Enteropneusta.…”

<p class="HeadingRunIn"><strong>Reinterpretation of the enigmatic Ordovician genus <em>Bolboporites </em>(Echinodermata)</strong></p>

“…The theca was bearing all main body orifices (anus, hydropore, mouth) and, when present, various kinds of respiratory structures (e.g., epispires, diplopores, rhombs; Kesling 1968;Sprinkle 1973;David et al 2000). In life, the theca housed the main body cavity, which contained all internal organs (e.g., gut; Kesling 1968;Sprinkle 1973;Rahman et al 2015).…”

“…Microtomographic observations were performed by using a Bruker CT-scan (Skyscan 1174 model) at the Université de Bourgogne, Dijon (Morphoptics Service), to obtain virtual cross-sections through some specimens, and also to reconstruct a three-dimensional model of Bolboporites. In recent years, tomography has become a routine technique of imagery, so as to reveal internal structures in various fossils and, in particular, Palaeozoic echinoderms (Sutton et al 2005;Rahman & Clausen 2009;Rahman & Zamora 2009;Rahman et al 2010Rahman et al , 2015Briggs et al 2017). Data acquisition was obtained at 50 kV and 800 µA.…”

A new model of respiration in blastoid (Echinodermata) hydrospires based on computational fluid dynamic simulations of virtual 3D models

“…Although digital techniques, such as synchrotron imaging, are clearly preferable when available and can produce spectacular results (Rahman et al, 2015b), acetate peels provide important morphological data critical to studies of blastoid phylogeny that are currently unavailable using nondestructive imaging technology, particularly where there is low-density contrast within specimens. Even in these cases, the boundaries separating echinoderm plate material and hydrospires from spary and micritic thecal fill are clearly defined in acetate peels because echinoderm plates are constructed of microporous stereom.…”