A Diploblastic Radiate Animal at the Dawn of Cambrian Diversification with a Simple Body Plan: Distinct from Cnidaria?

Abstract

BackgroundMicrofossils of the genus Punctatus include developmental stages such as blastula, gastrula, and hatchlings, and represent the most complete developmental sequence of animals available from the earliest Cambrian. Despite the extremely well-preserved specimens, the evolutionary position of Punctatus has relied only on their conical remains and they have been tentatively assigned to cnidarians. We present a new interpretation of the Punctatus body plan based on the developmental reconstruction aided by recent advances in developmental biology.Results Punctatus developed from a rather large egg, gastrulated in a mode of invagination from a coeloblastura, and then formed a mouth directly from the blastopore. Spiny benthic hatchlings were distinguishable from swimming or crawling ciliate larvae found in cnidarians and sponges. A mouth appeared at the perihatching embryonic stage and was renewed periodically during growth, and old mouths transformed into the body wall, thus elongating the body. Growing animals retained a small blind gut in a large body cavity without partitioning by septa and did not form tentacles, pedal discs or holdfasts externally. A growth center at the oral pole was sufficient for body patterning throughout life, and the body patterning did not show any bias from radial symmetry.ConclusionsContrary to proposed cnidarian affinity, the Punctatus body plan has basic differences from that of cnidarians, especially concerning a spacious body cavity separating ectoderm from endoderm. The lack of many basic cnidarian characters in the body patterning of Punctatus leads us to consider its own taxonomic group, potentially outside of Cnidaria.

Developmental biology of the early Cambrian cnidarian Olivooides

“…This interpretation of the aperture as the blastopore and future mouth was followed by Yasui et al . (). Steiner et al .…”

“…Whether or not the pentaradial aperture develops from the blastopore, as has been argued explicitly or implicitly (Yasui et al . ) is a moot issue that cannot be determined on the basis of the available evidence; the key point is that it is not the blastopore.…”

“…Yasui et al . () followed Yao et al . () in identifying the stellate embryonic stage as a gastrula, and recorded the development of the mouth directly from the blastopore.…”

“…Tomographic analyses of the post‐embryonic thecate stage by Yasui et al . () revealed a vast open lumen occupied merely by a very short extension of the aperture, which they interpret as a gut. The limited extent of the gut and the apparent absence of tentacles led the authors to conclude that Olivooides had endosymbionts.…”

A new scyphozoan from the Cambrian Fortunian Stage of South China

“…Qinscyphus might have had a developmental mode and growth pattern similar to those of Olivooides (Yasui et al . ) and Quadrapyrgites (Liu et al . ).…”