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.
Highlights
Microfossils from the earliest sediments of the Cambrian, in the Kuanchuanpu Formation, China, collectively called Small Shelly Fossils (SSFs), have provided exceptional fossils comprising reliable developmental series for some species [1,2,3,4]
Hatchling fossils were identified as being completely covered with stellate spines, excluding the oral region that was characterized by a striated surface [2,3,4,5]
Among many spiny globular fossils, candidates for the initial stage of oral formation were judged to be specimens completely covered with spines and radial furrows dividing the blastoporal surface into five sectors (Figure S1C), as hatchlings displayed five ridges radiating from the abblastoporal apex (Figure 1A)
Summary
Microfossils from the earliest sediments of the Cambrian, in the Kuanchuanpu Formation, China, collectively called Small Shelly Fossils (SSFs), have provided exceptional fossils comprising reliable developmental series for some species [1,2,3,4]. Despite the richness in number and developmental stages, the phylogenetic positioning of Punctatus is highly ambiguous because of the lack of reliable characters comparable to extant animals. Previous reconstructions of Punctatus interpreted conical forms as test or thecal remains, they were suggested to be flexible [1,5]. Tubular and pyramidal skeletal fossils coeval with Punctatus have been assigned to conulariids because their transverse ribbings and longitudinal sulci between flat sides (faces) were regarded as homologous, though the pyramidal forms were six-sided [1]. Microfossils 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
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