LAUSR

Recently we documented a bilaterally symmetrical, solitary organism, Yanjiahella biscarpa from the early Cambrian (Fortunian) of China1. We interpreted that Y. biscarpa possessed an echinoderm-like plated theca, a muscular stalk similar to hemichordates and a pair of long, feeding appendages. Our interpretation and our phylogenetic analysis suggest that Y. biscarpa is a stem-echinoderm, which would confirm that echinoderms acquired plates before pentaradial symmetry and that their history is firmly rooted in bilateral forms. Zamora et al.2 however, have criticized our interpretation, arguing against an echinoderm affinity, instead suggesting that the phylogenetic placement of Y. biscarpa is dubious and its significance for understanding deuterostome evolution is uncertain. This criticism2 seems to stem from our interpretation of particular morphological features in Y. biscarpa1 and the perceived lack of echinoderm synapomorphies. Echinoderms possess a calcitic skeleton with a distinctive three-dimensional mesh-like microstructure called stereom, that is considered a major synapomorphy of the Echinodermata3. Zamora et al.2 highlighted the absence of stereom in Y. biscarpa, additionally stating that we had omitted appropriate methods, specifically latex casting, that may confirm the presence of stereom in our specimens. We concede that initially we did not latex cast any specimens of Y. biscarpa, predominantly due to the fragile nature and the associated risk of damaging the specimens in question. In lieu of latex casting we employed Scanning Electron Microscopy (SEM) to investigate the surface and the details of Y. biscarpa specimens. SEM has been extensively used in the past to study stereom microstructure4–6 and if such a microstructure was preserved in Y. biscarpa it would have been detected using this technique. In response to Zamora et al.2 we have cast a specimen in latex to investigate whether stereom can be detected in Y. biscarpa. No pitting, resembling stereom is evident on the plates (Fig. 1) and the latex casts do not appear to offer any further information that was not detectable using light photography and SEM, as predicted. The statement from Zamora et al.2 that latex casting could have resolved many of the uncertainties surrounding the composition, shape and arrangement of the plates and the morphology of the stalk is an overstatement and ambitiously delivered without direct examination of the fossil specimens. The stereom microstructure of Cambrian echinoderms is poorly known5, as the majority of specimens are preserved as moulds, or recrystallized skeletons and the primary three-dimensional morphology of their stereom has been obscured4–6. Generally, only superficial pitting is preserved and it is this pitting that is interpreted as stereom7, typically in specimens that are already recognized or interpreted as echinoderm taxa. Although stereom is a synapomorphy of the Echinodermata, its presence is often presumed and a number of echinoderm taxa have been documented without stereom clearly preserved8. We openly state that we cannot observe stereom in the original manuscript and we maintain our stance that this likely is due to taphonomic factors. Zamora et al.2 raise additional questions over the interpretation of particular morphological features of Y. biscarpa. Zamora et al.2 highlight that the configuration of the two feeding appendages in Y. biscarpa differ from previously reported echinoderms, in particular Ubaghsicystis, Dibrachicystis and Pleurocystites. However, these genera are considered to be derived taxa1,7 and are not particularly useful in deciphering early echinoderm character traits. As stated in Topper et al.1, the precise nature (muscular, collagenous or hydrovascular) of the feeding appendages in Y. biscarpa is unclear (although their flexibility indicates that they are soft) and consequently without more details we are unable to comment further. Zamora et al.2 additionally note that the stalk of Y. biscarpa is unlike anything seen in the Echinodermata. We appreciated this agreeable statement, as the lack of an analogous stalk in the Echinodermata forms the crux of our argument1 morphologically linking Y. biscarpa with the hemichordates. Furthermore, https://doi.org/10.1038/s41467-020-14922-9 OPEN