Álvarez-Armada N et al. (2022), Heterochrony and parallel evolution of echinode...

ECB-ART-50339

Proc Biol Sci 2022 May 11;2891974:20220258. doi: 10.1098/rspb.2022.0258.

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Heterochrony and parallel evolution of echinoderm, hemichordate and cephalochordate internal bars.

Álvarez-Armada N , Cameron CB , Bauer JE , Rahman IA .

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Deuterostomes comprise three phyla with radically different body plans. Phylogenetic bracketing of the living deuterostome clades suggests the latest common ancestor of echinoderms, hemichordates and chordates was a bilaterally symmetrical worm with pharyngeal openings, with these characters lost in echinoderms. Early fossil echinoderms with pharyngeal openings have been described, but their interpretation is highly controversial. Here, we critically evaluate the evidence for pharyngeal structures (gill bars) in the extinct stylophoran echinoderms Lagynocystis pyramidalis and Jaekelocarpus oklahomensis using virtual models based on high-resolution X-ray tomography scans of three-dimensionally preserved fossil specimens. Multivariate analyses of the size, spacing and arrangement of the internal bars in these fossils indicate they are substantially more similar to gill bars in modern enteropneust hemichordates and cephalochordates than to other internal bar-like structures in fossil blastozoan echinoderms. The close similarity between the internal bars of the stylophorans L. pyramidalis and J. oklahomensis and the gill bars of extant chordates and hemichordates is strong evidence for their homology. Differences between these internal bars and bar-like elements of the respiratory systems in blastozoans suggest these structures might have arisen through parallel evolution across deuterostomes, perhaps underpinned by a common developmental genetic mechanism.

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	Figure 1. . Virtual reconstructions of internal bars and bar-like structures in modern and fossil deuterostomes. (a) Lagynocystis pyramidalis (NHMUK E29043), external view showing the position of the internal bars. (b) Close-up of the internal bars in Lagynocystis pyramidalis (NHMUK E16107, left; NHMUK E29453, right). (c) Generalized diagram visualizing the distribution and morphological characteristics of the internal bars in Lagynocystis pyramidalis. (d) Cryptoschisma sp. (MGM-3383D), external view (left) and close-up of a single hydrospire group (right). (e) Generalized diagram visualizing the distribution and morphological characteristics of the hydrospires folds (bar-like structures) in Hyperoblastus reimanni and Crysptoschisma sp.; however, the latter lacks slits in the C-D interray. (f) Strobilocystites polleyi (CMC IP 36209), external view showing the position of the rhomb-pores (top) and close-up of a single rhomb-pore bearing complex (bottom). (g) Generalized diagram visualizing the distribution and morphological characteristics of the pores (bar-like structures) in Strobilocystites polleyi. (h) Schizocardium sp., lateral views showing external and internal morphological features. (i) Branchiostoma floridae, lateral views showing external and internal morphological features. (j) Generalized diagram visualizing the distribution and morphological characteristics of the gill bars in hemichordates and cephalochordates. Scale bars = 1 mm.
	Figure 2. . Linear discriminant analysis (LDA) (a,c) and principal component analysis (PCA) (b,d) of the dimensions (length, width, depth and spacing) of internal bars and bar-like structures in modern and fossil deuterostomes. (a) LDA plot of the resultant morphospace. (b) PCA plot of the resultant morphospace. (c,d) Biplot vectors showing the contribution of each dimension of the internal bars and bar-like structures to the variation in the dataset for LDA (c) and PCA with vectors colour coded by the square cosines contribution to the variation of the data (d). Abbreviations: B.f, Branchiostoma floridae; B.sp, Balanoglossus sp.; C.sp, Cryptoschisma sp.; H.r, Hyperoblastus reimanni; J.o, Jaekelocarpus oklahomensis; L.p1, Lagynocystis pyramidalis (NHMUK E29453); L.p2, Lagynocystis pyramidalis (NHMUK E16107); S.p, Strobilocystites polleyi.; S.sp, Schizocardium sp. (Online version in colour.)
	Figure 3. . A generalized phylogenetic tree of the deuterostomes showing the evolution of key characters linked to the pharyngeal openings.

References [+] :

Álvarez-Armada, Heterochrony and parallel evolution of echinoderm, hemichordate and cephalochordate internal bars. 2022, Pubmed, Echinobase