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1. Approaches to understanding echinoderm origins. Part 2: Questioning conceptual models

   Mooi, R; Lefebvre, B; Guensburg, T; Nohejlova, M; Dupichaud, C (December 2024, Cahiers de Biologie Marine)

   Davoult, D (Ed.)

   Abstract: The two main approaches in interpreting the significance of non-radial fossil echinoderms (carpoids) have consistently produced the same two distinct clusters of results for over a century of investigation. Studies based on conceptual models imbued with Haeckelian precepts consider bilaterally symmetric or asymmetric morphologies of carpoids plesiomorphic for the phylum. These approaches do not find a place for carpoids within an existing phylogenetic framework for the phylum because it is assumed that they primitively lack pentaradiality. Emphasis on differences between these taxa and pentaradial echinoderms forces them outside of and downward from more crownward clades. It is crucial to examine the data supporting these supposed differences. Is it worth considering non-pentaradial echinoderms as members of a well-known group of echinoderms, the blastozoans, which already includes many secondarily-derived, non-pentaradial members? Followers of an empirical model think so, presenting an integration of paleontology, embryology, body wall homology, and image analysis that derives echinoderms from a bilaterian, archimeric larva, not bilateral adults. Unprecedented modification of a single mesocoel (hydrocoel) initiated the pentaradial adult echinoderm, most parsimoniously with five primary lobes in stem forms of each major clade within the phylum. The unique water vascular system led to rearrangement of adult axes that literally have no parallel with those of any other invertebrate, representing an iconic synapomorphy for the Echinodermata. There are few, if any, developmental or stratigraphic data defending carpoids as 'bilateral precursors'. Their free appendage is now shown to be an ambulacrum, undermining any supposition of a 'head', 'tail', or 'gill slits'. Pentaradiality is plesiomorphic for the phylum, obviating the requirement for a triradial intermediate (helicoplacoids) between carpoids and pentaradial forms. Carpoids, a subset of blastozoans, exploited motility as a feeding mode, leading to extraordinary adaptations that belie their interpretation as ancestral echinoderms. 

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2. Approaches to understanding echinoderm origins. Part 1: Conceptual and empirical models

   Lefebrvre, B; Mooi, R; Guensburg, T; Dupichaud, C; Nohejlova, M (October 2024, Cahiers de Biologie Marine)

   Davoult, D (Ed.)

   Abstract: Echinoderms are so highly derived compared with other deuterostomes, including their sister group, hemichordates, that comparisons of body plans are sometimes accompanied by points of view enjoying varying levels of morphological, paleontological, and especially, embryological support. No echinoderm taxon has been the subject of more contentious debate than the carpoids, a disparate assemblage of non-pentaradial, flattened echinoderms that includes the Cincta, Ctenocystoidea, Soluta, and Stylophora. Because of their unusual morphologies, the phylogenetic position and significance of carpoids concerning the origins of the Echinodermata are still being evaluated. A detailed review of carpoid research over the past century and a half reveals that the debate largely results from methodological issues employing two basic, but very different models. Conceptual models, usually imbued with Haeckelian principles, consider the absence of a single character (pentaradial symmetry) as a recapitulation of the pre-metamorphic larval stage of echinoderms, forcing unusual taxa that also lack pentaradiality down the phylum's phylogenetic tree. Such scenarios assume that first echinoderms had a bilaterian-type anterior-posterior axis. Empirical models rely on comparison of non-pentaradial early forms with a wide array of data obtained from extant and fossil echinoderms. These data support a view in which larval morphologies of echinoderms are not represented in the fossil record of echinoderms, and that pentaradial symmetry was secondarily lost in carpoids, just as it was in many other coeval types of echinoderms. 

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3. Genome-wide identification and spatiotemporal expression analysis of cadherin superfamily members in echinoderms

   https://doi.org/10.1186/s13227-023-00219-7  

   Chess, Macie M; Douglas, William; Saunders, Josiah; Ettensohn, Charles A (December 2023, EvoDevo)

   Abstract BackgroundCadherins are calcium-dependent transmembrane cell–cell adhesion proteins that are essential for metazoan development. They consist of three subfamilies: classical cadherins, which bind catenin, protocadherins, which contain 6–7 calcium-binding repeat domains, and atypical cadherins. Their functions include forming adherens junctions, establishing planar cell polarity (PCP), and regulating cell shape, proliferation, and migration. Because they are basal deuterostomes, echinoderms provide important insights into bilaterian evolution, but their only well-characterized cadherin is G-cadherin, a classical cadherin that is expressed by many embryonic epithelia. We aimed to better characterize echinoderm cadherins by conducting phylogenetic analyses and examining the spatiotemporal expression patterns of cadherin-encoding genes duringStrongylocentrotus purpuratusdevelopment. ResultsOur phylogenetic analyses conducted on two echinoid, three asteroid, and one crinoid species identified ten echinoderm cadherins, including one deuterostome-specific ortholog, cadherin-23, and an echinoderm-specific atypical cadherin that possibly arose in an echinoid-asteroid ancestor. Catenin-binding domains in dachsous-2 orthologs were found to be a deuterostome-specific innovation that was selectively lost in mouse, while those in Fat4 orthologs appeared to be Ambulacraria-specific and were selectively lost in non-crinoid echinoderms. The identified suite of echinoderm cadherins lacks vertebrate-specific innovations but contains two proteins that are present in protostomes and absent from mouse. The spatiotemporal expression patterns of four embryonically expressed cadherins (fat atypical cadherins 1 and 4, dachsous-2, and protocadherin-9) were dynamic and mirrored the expression pattern of Frizzled 5/8, a non-canonical Wnt PCP pathway receptor protein essential for archenteron morphogenesis. ConclusionsThe echinoderm cadherin toolkit is more similar to that of an ancient bilaterian predating protostomes and deuterostomes than it is to the suite of cadherins found in extant vertebrates. However, it also appears that deuterostomes underwent several cadherin-related innovations. Based on their similar spatiotemporal expression patterns and orthologous relationships to PCP-related and tumor-suppressing proteins, we hypothesize that sea urchin cadherins may play a role in regulating the shape and growth of embryonic epithelia and organs. Future experiments will examine cadherin expression in non-echinoid echinoderms and explore the functions of cadherins during echinoderm development. 

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4. Three-dimensional visualization as a tool for interpreting locomotion strategies in ophiuroids from the Devonian Hunsrück Slate

   https://doi.org/10.1098/rsos.201380  

   Clark, E. G.; Hutchinson, J. R.; Briggs, D. E. (December 2020, Royal Society Open Science)

   null (Ed.)

   Living brittle stars (Echinodermata: Ophiuroidea) employ a very different locomotion strategy to that of any other metazoan: five or more arms coordinate powerful strides for rapid movement across the ocean floor. This mode of locomotion is reliant on the unique morphology and arrangement of multifaceted skeletal elements and associated muscles and other soft tissues. The skeleton of many Palaeozoic ophiuroids differs markedly from that in living forms, making it difficult to infer their mode of locomotion and, therefore, to resolve the evolutionary history of locomotion in the group. Here, we present three-dimensional digital renderings of specimens of six ophiuroid taxa from the Lower Devonian Hunsrück Slate: four displaying the arm structure typical of Palaeozoic taxa ( Encrinaster roemeri, Euzonosoma tischbeinianum, Loriolaster mirabilis, Cheiropteraster giganteus ) and two ( Furcaster palaeozoicus , Ophiurina lymani ) with morphologies more similar to those in living forms. The use of three-dimensional digital visualization allows the structure of the arms of specimens of these taxa to be visualized in situ in the round, to our knowledge for the first time. The lack of joint interfaces necessary for musculoskeletally-driven locomotion supports the interpretation that taxa with offset ambulacrals would not be able to conduct this form of locomotion, and probably used podial walking. This approach promises new insights into the phylogeny, functional morphology and ecological role of Palaeozoic brittle stars. 

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5. The concept of ‘heteromorph ammonoids’

   https://doi.org/10.1111/let.12443  

   Landman, Neil H.; Machalski, Marcin; Whalen, Christopher D. (August 2021, Lethaia)

   ‘Heteromorph ammonoids’ encompass all ammonoid species whose shapes do not conform to a closely coiled planispiral shell. The term is useful as a broad description for such ammonoids. However, as a concept, ‘heteromorph ammonoids’ no longer has any scientific value or explanatory power. Although such ammonoids have traditionally been considered aberrant forms, they represent instead an integral part of the evolutionary history of the Ammonoidea. ‘Heteromorph ammonoids’, as a whole, are a poly- phyletic group, consisting of a heterogeneous mixture of taxa without any phylogenetic, morphological or ecological coherence. Their treatment as a single entity risks conflating convergences and phylogenetic affinities. It also vastly oversimplifies the stunning array of morphologies and ecological niches occupied by these animals. Investigation into the uncoiling (and recoiling) of ammonoids is a legitimate and worthwhile enterprise, especially in view of the realization that this phenomenon occurred several times in the history of the Ammonoidea. However, few insights can be gained by treating ‘heteromorph ammonoids’ as a single entity. Studies of such ammonoids should focus on monophyletic groups within a well‐constrained phylogenetic and stratigraphical framework to yield meaningful results. 

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