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Vendozoa and Selective Forces on Animal Origin and Early Diversification: Reply to Rstb.Royalsocietypublishing.Org Dufour and Mcilroy (2017)

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Vendozoa and Selective Forces on Animal Origin and Early Diversification: Reply to Rstb.Royalsocietypublishing.Org Dufour and Mcilroy (2017) Vendozoa and selective forces on animal origin and early diversification: reply to rstb.royalsocietypublishing.org Dufour and McIlroy (2017) Thomas Cavalier-Smith Invited reply Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK Cite this article: Cavalier-Smith T. 2017 Vendozoa and selective forces on animal origin 1. Introduction and early diversification: reply to Dufour and The authors are right to emphasize that Vendozoa probably include both filter McIlroy (2017). Phil. Trans. R. Soc. B 373: feeders on plankton and those deriving nutrition directly from the underlying substratum on which they lie or move and to argue that hard and soft surfaces 20170336. would have provided habitats offering partially contrasting selective forces http://dx.doi.org/10.1098/rstb.2017.0336 favouring distinct body forms. However, my paper already argued that some Vendozoa were bifacial filter-feeding fronds and others likely to have been hori- Accepted: 26 September 2017 zontal dwellers on soft surfaces that may have fed phagocytically on substrate microrganisms by ventral non-choanocyte cells and dorsally on plankton by choanocytes. A better interpretation of Fractofusus than that of Dufour & McIlroy [1] might be that it was just such a dorsal collar-cell and ventral sub- strate feeder; if so it was a presponge, not a pre-placozoan. If Ediacaran organisms of that dual feeding mode existed, the dichotomy between plankton Author for correspondence: feeders and substrate feeders was less sharp than they imply. Their comment raises seven issues: (i) conceptually, how does the ‘pre- Thomas Cavalier-Smith placozoan grade of organization’ really differ from a presponge. (ii) Can we e-mail: [email protected] reliably infer from fossils the actual feeding mode and different cell types of Vendozoa? (iii) What are the phylogenetic relationships between Vendozoa, sponges and Placozoa; and the inferred phenotypes prior to each branch point? (iv) What is the relative timing of vendozoan, sponge and bilaterian ori- gins? (v) Does substrate feeding offer a transition to the first animal as plausible as the choanoflagellate to presponge path? (vi) What selective force was crucial for the origin of the nervous system? (vii) What is required for good explanations of major evolutionary transitions? 2. ‘Pre-placozoan grade’ is conceptually confused McIlroy and Dufour introduced the term ‘pre-placozoan grade’ to apply to the earliest rangeomorph fossils [2], but did not define or clearly explain it. They now say pre-placozoans ‘differ from presponges by their lack of basal pinaco- cytes and suspension feeding capabilities, and by a feeding mode that relies on establishing symbiosis with, or directly phagocytosing chemoautotrophic bacteria’ [1]. The hypothetical nature of bacterial prey is not helpful for defining a grade of organization—a morphological concept of organismal sub- structure. Thus, using it implies that an organism with identical structure (thus organizational grade) feeding not on chemotrophic but on heterotrophic or photosynthetic bacteria or on eukaryotes or osmostrophically would not be of ‘pre-placozoan grade’, which is unreasonable. Excluding prey type from this definition leaves only lack of basal pinacocytes, which does not adequately define a grade of animal organization. The authors also confusingly note that compared with presponges ‘The pre-placozoan model presents a similar body plan (simple epithelium surrounding mesohyl), but with only choanocyte-like cells, involved in feeding and water circulation’ [1]. A ‘similar body plan’ is effectively the same as having a similar grade of organization, yet if they had The accompanying comment can be viewed at only ‘choanocyte-like cells’ their grade of organization would be that of a http://dx.doi.org/10.1098/rstb.2017.0148; http://dx.doi.org/10.1098/rstb.2015.0476. & 2017 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited. multicellular choanoflagellate (if ‘choanocyte-like’ means animals) was a multicell with only one somatic cell type (pre- 2 having a microvillar collar), not an animal, and thus a simpler sumably choanocyte-like) that secreted extracellular material, rstb.royalsocietypublishing.org grade than presponges. If they had only cells with collars as yet had already evolved oogamy. It would have been effec- they imply, their dorsal cells should have been able to feed on tively a multicellular choanoflagellate with oogamy. They plankton exactly like a presponge, so asserting that they did do not suggest how this hypothetical animal ancestor fed not feed on plankton but only from their ventral surface or what selective force might have made it evolve oogamy seems contradictory, and would have lowered their feeding (extremely rare in protists; unknown in any members of king- effectiveness and thus have been selected against. Further- dom Protozoa) or multicellularity and is thus explanatorily more, it is unlikely that their ventral surface would retain empty with respect to the origin of animals. If it fed on plank- collars, in which case saying they had only choanocyte-like ton using a collar, as it must have done if choanoflagellate cells seems wrong. Conversely, if they are not postulated to and sponge collar cells are homologues, it is effectively a pre- have had collars, it was wrong to call them ‘choanocyte- sponge in structure and mode of feeding, so the last precursor Phil. Trans. R. Soc. B like’. Thus, properties suggested for ‘pre-placozoa’ are par- of all animals would have been functionally a presponge, tially contradictory and/or too loosely specified. making it incorrect and misleading to say that ‘pre-placozoa’ Earlier, the authors said the vendozoan Dickinsonia ‘was offer an alternative route to the origin of animals. At best, their of placozoan grade’ with ‘mucociliary sole’ [2], but neither pre-placozoan would be relevant to the origin of the sister then nor now specify how a ‘pre-placozoan grade’ differs from clade to sponges, but no good case is made for that. a ‘placozoan grade’. As I use ‘presponge’ [3], it embraces a The scheme also does not specify how the pre-placozoan, 373 spectrum of increasingly complex grades of organization which supposedly evolved later than this original collar-cell : 20170336 with two to many different cell types, ranging from the feeding animal, differs from it in structure or feeding mode, simplest with two layers (true diploblasts; epithelium only) and is thus equally explanatorily empty with respect to the to others with mesenchyme also and thus three tissue origin of this subclade. It also has nothing to say about the layers (simple triploblasts). Thus, saying it ‘consisted of two origin of Placozoa, coelenterates or bilateria. Worse still, cell types’ [1] oversimplifies and misrepresents my paper. making the animal ancestor effectively a very simple organ- I would regard the most complex presponges as having a ism with only one somatic cell type totally fails to account similar grade of organization to Placozoa (albeit one of for the sharing by sponges and their sister clade of a their cell types being choanocytes). common system of pattern formation, e.g. the Wnt antero- posterior gradient system, Notch signalling, homeobox and other spatially controlled switch genes, or the shared PIWI 3. Are any Vendozoa ‘pre-placozoa’? germline maintenance system [3]. It is implausible to argue either that such complex systems could have evolved at a One cannot see cells or cell types in fossil Vendozoa, so we are choanoflagellate grade of organization with only one somatic unsure whether they had ciliated cells or choanocytes, but can cell type and no germline soma distinction or that they make an informed guess. Their overall size and complexity is evolved separately in sponges and other animals; therefore, such that it is unlikely that they were not triploblastic with the their fig. 1 idea of such a simple last common ancestor is connective tissue secreted by mesenchyme cells sandwiched almost certainly wrong. Furthermore, sponges and cni- between two epithelia. Placozoa are also, in this sense, triplo- daria/bilateria share an internal body cavity and mouth/ blasts, and the blanket use of diploblast for any pre-bilaterian osculum and an adult grade of organization immensely animal [2] is thoroughly confusing and ought to cease. If they more complex than the pre-placozoan; these complexities have neither gut nor aquiferous system, they can reasonably be are arguably homologous morphologically and in their pat- regarded as of comparable organizational grade to Placozoa. tern-forming gradient system and germ–soma distinction. But, it is not reasonable to regard any of them large enough Moreover, sponges and cnidaria share a common life history to be found as fossils as of ‘pre-placozoan’ grade if that with a well-differentiated ciliated larva that uses aboral/ means organizationally simpler than Placozoa. In my view, aboscular secretory cells that mediate larval settling in similar all must have had substantially more complex connective ways and in sponges express numerous post-synaptic protein tissue organization than Trichoplax, so the authors’ idea that homologues. Despite these fundamental homologies in body they were organizationally simpler than Placozoa is implausi- plan, pattern formation and morphogenesis of both adults ble; that Vendozoa might have had only choanocyte-like cells and larvae of sponges and cnidaria, as well as in the likely is incredible. If Fractofusus had the tissue structure postulated transition mechanisms between them, Dufour & McIlroy earlier, it would have been a complex triploblast not a diplo- ([1], fig. 1) assume that these complexities evolved indepen- blast as stated [2]. Calling it a pre-placozoan (without dently twice in animal evolution. That is contrary to the defining that term at all) allowed the reader to think they sup- comparative evidence and evolutionarily incredible.
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