Translator Disclaimer
1 March 2009 The Burgess Shale Animal Oesia is not a Chaetognath: A Reply to Szaniawski ()
Author Affiliations +
Abstract

The Middle Cambrian Oesia disjuncta, a monospecific genus, is known only from the celebrated Burgess Shale of British Columbia. It has been re-interpreted by Szaniawski (Acta Palaeontologica Polonica 50:1–8; 2005) as a chaetognath, a distinctive phylum whose exact position in the protostomes is still controversial. Unequivocal chaetognaths, that have no similarity to Oesia, are already known to occur in the Chengjiang Lagerstätte (Lower Cambrian, S.W. China), and here I describe the first example of a chaetognath from the Burgess Shale itself. Comparisons between Oesia and chaetognaths fail to find any significant homologies. Whilst the phyletic position of Oesia is very uncertain, a place in the hemichordates may be worth exploring.

Introduction

Significant advances in metazoan phylogeny (e.g., Philippe et al. 2005; Dunn et al. 2008) continue to have wide-ranging implications for our understanding of evolution, not least in terms of the Cambrian “explosion”. New phylogenetic configurations have brought into evolutionary juxtaposition major groups which classical zoology had long regarded as only distantly related. These new trees are achieving a degree of stability, and necessarily beg the question as to what the common ancestors may have looked like, no easy task given their existing disparity. Important as these advances are, it is important to stress that this area by no means involves a one-way traffic, whereby relevant information is available only from molecular data. In principle, the fossil record can also contribute important, arguably unique, insights. In this context, Burgess Shale-type faunas are well known not only for their extraordinary fossil preservation but also serving as repositories of unfamiliar, even bizarre, animals whose phylogenetic status is a topic of active debate. Phylogenies of early metazoan evolution are drawing on fossil groups which until a few years ago would have simply been treated as “extinct phyla”, but are now realized to be at least potential stem-groups of known phyla and accordingly can throw key, and often unexpected, light on the assembly of bodyplans. Nevertheless, whilst there have been successes, or at least fertile hypotheses, a significant number of Burgess Shale-type taxa are still phylogenetically refractory and therefore a focus of renewed scrutiny.

Institutional abbreviation.

  • USNM, National Museum of Natural History, Washington, D.C., USA.

Is Oesia really a chaetognath?

Of the taxa from Burgess shale-type deposits that are phylogenetically controversial, one such example requiring investigation is Oesia disjuncta (hereafter referred to as simply Oesia, on account of its monospecificity). To date this animal has only been recorded from the Burgess Shale (Fig. 1A, B, D). The discoverer of this famous deposit, Charles Walcott, described it as a polychaete annelid (Walcott 1911; see also Tarlo 1960), but Lohmann (1922, 1933–1934) reassigned Oesia to the appendicularian tunicates. Since then, however, this animal has only received sporadic and passing mention (e.g., Whittington 1971: 1174; Conway Morris 1979: 336), while in The Fossils of the Burgess Shale (Briggs et al. 1994) it is not even illustrated (but see p. 221 where it is listed). Based on a re-examination of photographs, but not the original material located in the National Museum of Natural History (Washington, D.C), Szaniawski (2005) has argued that Oesia is best assigned as a chaetognath.

If it were correct, such an interpretation would be important for several reasons. Although the chaetognaths were for long allied to the deuterostomes, with the renaissance in the study of metazoan phylogeny and the major reassessments driven by molecular data, it was to be expected that notwithstanding their very characteristic and distinct bodyplan their place within the deuterostomes (or elsewhere) would have been rapidly resolved. This, however, has not proved to be the case (Bull and Miller 2006), and as Hausdorf et al. (2007: 2727) noted “the phylogenetic position of chaetognaths … remains elusive”. Thus whilst it is now clear that chaetognaths are protostomes (e.g., Papillon et al. 2004), there remain significant divergences in opinion and it is widely conceded that long branch attraction remains a serious impediment (e.g., Podsiadlowski et al. 2008; see also Halanych 1996). Earlier proposals for a relationship to the ecdysozoans (e.g., Halanych 1996; Zrzavy et al. 1998) continue to receive some support, with a possible relationship to the priapulids being mooted (Helmkampf et al. 2008). Others, however, identify a relationship to the lophotrochozoans as more likely (e.g., Matus et al. 2006; see also Haase et al. 2001), whilst yet others argue that the chaetognaths are more basal and possibly a sister group to all other protostomes (e.g., Marlétaz et al. 2006; Helfenbein et al. 2004; see also Halanych 2004). Continuing work appears to lean in favour of a lophotrochozoan relationship, but unfortunately, these recent studies still make it difficult to distinguish between the two latter alternatives (Matus et al. 2007).

Whilst the exact position of the chaetognaths in the scheme of metazoan phylogeny may be difficult to pin down, a broadly basal position evidently has major implications for both the nature of ancestral triploblasts as well as their functional morphology and ecology. Thus chaetognaths might be informative as to key ancestral characters within the bilaterian metazoans, including embryology (e.g., Shimotori and Goto 2001), the nature of the mesoderm (e.g., Shinn 1994) and also the musculature (e.g., Casanova and Duvert 2002), as well as coelomic body cavities and the fate of the blastopore. In addition, the spinose protoconodonts which appear at the dawn of the Cambrian explosion, are reliably attributed to the chaetognaths (e.g., Szaniawski 2002). This is consistent with this group being amongst the earliest effective predators in the pelagic realm (e.g., Hu et al. 2007), and has important implications for the exploitation of higher trophic levels by basal lophotrochozoans, if not basal triploblasts.

Nevertheless, as with a few other phyla e.g., sipunculans, the chaetognaths have a very conservative bodyplan, and even the specialized denizens of the hydrothermal vent community show little modification (Casanova and Moreau 2005). Similarly the few innovations, notably the development of limb-like appendages, are evidently autapomorphic novelties and have no wider phylogenetic context (Casanova et al. 2003). Given this anatomical uniformity, then clearly any palaeontological data relevant to the origin and early history of chaetognaths would be of very considerable interest.

Here I suggest that the claim for Oesia being material to this argument (Szaniawski 2005) is difficult to substantiate. Whilst this assignment by Szaniawski has already been treated with considerable skepticism (Vannier et al. 2006), other authors have evidently either kept an open mind (Hu et al. 2007; in passing I might note that their claim that I have reinterpreted the Burgess Shale fossil Nectocaris as a chaetognath (Conway Morris 1998) is a misunderstanding) or more significantly have supported this proposal to the extent of annotating illustrations of Oesia with ostensible chaetognathic descriptors (Bull and Miller 2006). Accordingly, it is timely to assess the evidence for and against Oesia being any sort of chaetognath. While a full redescription of Oesia is still necessary, the thesis put forward by Szaniawski can be questioned on the basis of two lines of evidence. First, on the basis of my investigations I argue that Oesia has no meaningful similarity to any known chaetognath. Nor does there appear to be any compelling to identify this taxon as either a stem-group chaetognath or some other basal protostome that might be allied to this enigmatic phylum. In fairness this begs the question of what any such stem-group would actually look like given the morphological isolation of the chaetognath bodyplan, but as suggested below there is little a priori evidence from Oesia to support this view. Second, and more tellingly, unequivocal chaetognaths are known from Burgess Shale-type localities, and to date those described have no significant similarity to Oesia.

The basis of Szaniawski's (2005: 4) analysis is, of course, that there are “numerous close structural similarities” between Oesia and chaetognaths.

A key feature would be the diagnostic grasping spines, yet Szaniawski (2005: 4) concurs that these are “not […] wellpreserved”. My close examination of the available suite of Oesia leads me to conclude that no trace of grasping spines is evident (Fig. 1A3, B, D3), and their highly tentative identification in one specimen (Szaniawski 2005: figs. 1C, 2C; see also Tarlo 1960: fig. 3) cannot be substantiated. Szaniawski (2005) explains this difficulty by using a taphonomic explanation, specifically suggesting that the grasping spines might have been vulnerable to selective destruction in the sediments of the Burgess Shale. Such selectivity is, of course, common-place in taphonomy, but it is less plausible in the context given that the putative spines would be chitinous, and thus presumably similar to otherwise well-preserved chitinous bodies of the numerous arthropods. To be sure, Szaniawski's (2005) proposal echoes the earlier hypothesis of Butterfield (2003) who argued that the principal taphonomic filter in the Burgess Shale is destruction of non-extracellular structures. On this basis he argued that a chaetognath affinity for the worm Amiskwia was far more probably than hitherto thought (see Conway Morris 1977). There appears, however, to be no meaningful similarity between Amiskwia and Oesia, and so no compelling reason to accept Amiskwia (or indeed Oesia) as a chaetognath.

The identification of other purported chaetognathan features in Oesia are also questionable. There is, for example, little evidence for lateral fins (Fig. 1A5), although one needs to note that in the definitive Cambrian chaetognaths (see below) the evidence for fins (most likely originally delicate and apparently lacking fin rays) is tenuous. A stronger argument might be made on behalf of the identification of the supposed tail fin. It is difficult to see, however, any close similarity to the equivalent area in chaetognaths. This is because in Oesia this posterior-most region appears to have had a three-dimensional arrangement composed of a series of plate-like structures (Fig. 1A2). Whilst one cannot dismiss such an arrangement typifying a stem-group chaetognath, at the least this configuration begs a radical re-organization of the posterior region. Finally, although putative “seminal vesicles” are identified in one specimen, and conceivably represent reproductive tissue, given the general lack of correspondence between Oesia and any chaetognath this comparison would seem to carry less weight. So too other similarities would appear to be generalized and lack specificity. This applies particularly to the transverse structures, whilst I regard the identification of a ventral ganglion and the possible location of the anus as, at best, equivocal.

Cambrian chaetognaths

It can be concluded that the similarities between Oesia and the chaetognaths certainly merit discussion, but in no case can a diagnostic comparison e.g., unequivocal cephalic spines, be arrived at that would serve to support the affinity as proposed by Szaniawski (2005). This conclusion is reinforced by the existence of unequivocal chaetognath material from Burgess Shale-type deposits. Szaniawski (2005) is dismissive of the Lower Cambrian taxon Eognathacantha ercainella from the Chengjiang Lagerstätte of Yunnan, SW China (Chen and Huang 2002). Whilst the illustrations in this short report are not entirely satisfactory, and combined with the fact that Chen and Huang (2002) are relatively cautious in their assessment, so Szaniawski's (2005) scepticism has some basis. However, better illustrations of the same specimen (Chen 2004: figs. 347–348) are again consistent with the chaetognath interpretation. Moreover, although not mentioned by Szaniawski (2005) there is an independent report of a Chengjiang chaetognath (Protosagitta spinosa) by Hu (in Chen et al. 2002: 166–167, text-fig. 8–1.3, pl. 17: 6). Here too the diagnostic grasping apparatus is visible, and subsequent research (Vannier et al. 2005, 2006) confirms the systematic position of this fossil. Both Eognathacantha and Protosagitta are described on the basis of unique specimens, and the relationships between these two taxa (including possible synonymy) remain to be established. Whilst Vannier et al. (2005) accept Szaniawski's (2005) placement of Oesia, they add no new information nor attempt to explain the manifest differences between this taxon and Protosagitta (and Eognathacantha).

Fig. 1.

A possible hemichordate Oesia disjuncta Walcott, 1911 (A, B, D) and an undescribed chaetognath (C), both from the Burgess Shale (Phyllopod bed), Middle Cambrian, British Columbia, Canada. A. USNM 57630 (part A1, A3, A4; counterpart A2, A5), showing entire specimen in high (A1) and low (A4) angle light, and details of posterior (A2), anterior (A3) and mid-sections (A5). B. USNM 57632, details of anterior end. C. USNM 199540, showing array of feeding spines, interlocking, in bilateral arrangement. D. USNM 57631, showing entire specimen in high (D1) and low (D2) angle light, and detail of anterior (D3). Scale bars 10 mm (A1–A4, D1, D2), 5 mm (A5, B, D3), and 1 mm (C).

f01_175.eps

In addition, there are additional records of soft-bodied chaetognaths from the slightly younger Burgess Shale of British Columbia. A number of specimens that are strikingly similar to the Chengjiang material were collected by the Royal Ontario Museum excavations (Desmond Collins, personal communication 2000) and they are presently under investigation by Jean-Bernard Caron and Derek E.G. Briggs. Independently, and many years ago, I noticed in the collections of the USNM a fossil that I interpret as a part of the anterior of a chaetognath. This specimen (USNM 199540; see also Conway Morris (1998: 115) is now illustrated here (Fig. 1C). The specimen was evidently collected by Charles Walcott, and clearly comes from the celebrated Phyllopod bed. I deliberately leave the specimen in open nomenclature, given that more complete material is in the process of description by others.

The specimen (Fig. 1C) displays the following features. The most striking component is the two sets of grasping spines that overlap. Those of the left-hand side are relatively expanded in configuration, and about 12 spines are identifiable. On the right-hand side the arrangement is more crowded with extensive overlapping, but at least 16 spines can be counted. So far as can be discerned the spines of either side originated in a single row. The individual spines are all similar, of about the same size, have a recurved shape, and are relatively slender, albeit expanding towards the points of insertion. There are also some traces of soft tissue to the posterior, but the nodule-like structures are foreign to the specimen and presumably diagenetic.

The specimen is most likely somewhat decayed, but it is similar to the grasping apparatus of extant chaetognaths. This would explain the juxtaposition of right and left sides, as well as the absence of softer tissue. Moreover, their overall morphology and arrangement is directly comparable to the equivalent spines in the chaetognaths from the Chengjiang Lagerstätte (see, in particular, Vannier et al. 2007: fig. 1d, e). So too the shape of the individual spines is strongly reminiscent of the protoconodont elements which are plausibly identified as derived from chaetognaths (e.g., Doguzhaeva et al. 2002; Szaniawski 2002). This specimen, however, has no similarity to Oesia, and is further evidence against assigning this animal to the chaetognaths. The well-preserved grasping spines seen in this specimen also directly contradict Butterfield's (2003) taphonomic hypothesis, and provide no support for Amiskwia being a chaetognath (see Conway Morris 1977).

Study of the Cambrian “explosion” and especially Burgess Shale-type faunas has been shaken up by various attempts to assign supposedly “bizarre” fossils to stem-groups, even though they have bodyplans (e.g., halkieriids, vetulicolians, vetulicystids, yunnanozoans) radically at odds with popular assumptions as to the supposed, albeit hypothetical, appearance of ancestors of familiar phyla. In the case of the chaetognaths it needs to be acknowledged that their conservative bodyplan, combined with an enigmatic phylogenetic position, makes it sensible to re-assess critically the fossil record in the hope of finding forms that might potentially elucidate the wider relationships and deeper origins of this intriguing group. To a limited extent this has already been achieved with the soft-part record of Lower Cambrian chaetognaths from the Chengjiang Lagerstätte, notably the evidence (albeit tentative) for the more or less continuous lateral fin-fold and the apparent absence of fin-rays (Chen 2004). To include Oesia in this schema is not only hypothetical, but demands a set of arbitrary transformations. Similar remarks apply with equal force to Amiskwia.

If Oesia is excluded from the chaetognaths, it will be important to resolve its wider relationships. Its overall morphology is vaguely reminiscent of a balanoglossid hemichordate, with the anterior and swollen region conceivably comparable to the diagnostic proboscis. So too the sometimes prominent transverse structures that are generally regarded as segmental divisions and/or musculature conceivably housed gill openings. New material collected by the Royal Ontario Museum (Jean-Bernard Caron, personal communication 2007) may help to resolve some of these issues.

Acknowledgements.

I thank Sandra Last and Vivien Brown (University of Cambridge, Cambridge, UK) for preparing various iterations of the manuscript, whilst Dudley Simons (University of Cambridge) assisted with photography. Constructive reviews provided by John Peel (Uppsala University, Uppsala, Sweden) and an anonymous referee greatly improved the paper. Loan of the specimen from the USNM was facilitated by Jann Thompson and Doug Erwin, to whom I also render my appreciation. This is Cambridge Earth Sciences Publication 9419.

References

  1. D.E. Briggs , D.H. Erwin , and F.J. Collier 1994. The Fossils of the Burgess Shale. xvii + 238 pp. Smithsonian Institution Press, Washington, DC. Google Scholar
  2. E.E. Bull and D.J. Miller 2006. Phylogeny: The continuing classificatory conundrum of chaetognaths. Current Biology 16: R593–R596. Google Scholar
  3. N.J. Butterfield 2003. Exceptional fossil preservation and the Cambrian explosion. Integrative and Comparative Biology 43: 166–177. Google Scholar
  4. J.-P. Casanova and M. Duvert 2002. Comparative studies and evolution of muscles in chaetognaths. Marine Biology 141: 925–938. Google Scholar
  5. J.-P. Casanova , M. Duvert , and T. Goto 2003. Ultrastructural study and ontogenesis of the appendages and related musculature of Paraspadella (Chaetognatha). Tissue and Cell 35: 339–351. Google Scholar
  6. J.P. Casanova and X. Moreau 2005. Calispadella alata n.gen., n.sp., the first chaetognath recorded from a hydrothermal vent site (Mid-Atlantic Ridge). Journal of Plankton Research 27: 221–225. Google Scholar
  7. J-Y. Chen 2004. The Dawn of Animal World [in Chinese]. xii + 366 pp. Jiangsu Science and Technical Press, Nanjing. Google Scholar
  8. J-Y. Chen and D-Y. Huang 2002. A possible Lower Cambrian chaetognath (arrow worm). Science 298: 187. Google Scholar
  9. L-Z. Chen , H-L. Luo , S-X. Hu , J-Y. Yin , Z-W. Jiang , Z-L. Wu , F. Li , and A-L. Chen 2002. Early Cambrian Chengjiang Fauna in Eastern Yunnan, China [in Chinese]. 199 pp. Yunnan Science and Technology Press, Kunming. Google Scholar
  10. S. Conway Morris 1977. A redescription of the Middle Cambrian worm Amiskwia sagittiformis Walcott from the Burgess Shale of British Columbia. Paläontologische Zeitschrift 51: 271–287. Google Scholar
  11. S. Conway Morris 1979. The Burgess Shale (Middle Cambrian) fauna. Annual Review of Ecology and Systematics 10: 327–349. Google Scholar
  12. S. Conway Morris 1998. The Crucible of Creation: The Burgess Shale and the Rise of Animals. xxiii + 242 pp. Oxford University Press, Oxford. Google Scholar
  13. L.A. Doguzhaeva , H. Mutvei , and R.H. Mapes 2002. Chaetognath grasping spines from the Upper Mississippian of Arkansas (USA). Acta Palaeontologica Polonica 47: 421–430. Google Scholar
  14. C.W. Dunn , A. Hejnol , D.Q. Matus , K. Pang , W.E. Browne , S.A. Smith , E. Seaver , G.W. Rouse , M. Obst , G.D. Edgecombe , M.V. Sorensen , S.H.D. Haddock , A. Schmidt-Rhaesa , A. Okusu , R.M. Kristensen , W.C. Wheeler , M.Q. Martindale , and G. Giribet 2008. Broad phylogenomic sampling improves resolution of the animal tree of life. Nature 452 745–749. Google Scholar
  15. A. Haase , M. Stern , K. Wächtler , and G. Bicker 2001. A tissue-specific marker of Ecdysozoa. Development, Genes and Evolution 211:428–433. Google Scholar
  16. K.M. Halanych 1996. Testing hypotheses of chaetognath origins: Long branches revealed by 18S ribosomal DNA. Systematic Biology 45: 223–246. Google Scholar
  17. K.M. Halanych 2004. The new view of animal phylogeny. Annual Review of Ecology, Evolution and Systematics 35: 229–256. Google Scholar
  18. B. Hausdorf , M. Helmkampf , A. Meyer , A. Witek , H. Herlyn , I. Bruchhaus , T. Hankeln , T.H. Struck , and B. Lieb 2007. Spiralian phylogenomics supports the resurrection of Bryozoa comprising Ectoprocta and Entoprocta. Molecular Biology and Evolution 24: 2723–2729. Google Scholar
  19. K.G. Helfenbein , H.M. Fourcade , R.G. Vanjani , and J.L. Boore 2004. The mitochondrial genome of Paraspadella gotoi is highly reduced and reveals that chaetognaths are a sister group to the protostomes. Proceedings of the National Academy of Sciences, USA 101: 10639–10643. Google Scholar
  20. M. Helmkampf , I. Bruchhaus , and B. Hausdorf 2008. Multigene analysis of lophophorate and chaetognath phylogenetic relationships. Molecular Phylogenetics and Evolution 46: 206–214. Google Scholar
  21. S.-X. Hu , M. Steiner , M.-Y. Zhu , B.-D. Erdtmann , H.-L. Luo , L.-Z. Chen , and B. Weber 2007. Diverse pelagic predators from the Chengjiang Lagerstätte and the establishment of modem-style pelagic ecosystems in the early Cambrian. Palaeogeography, Palaeoclimatology, Palaeoecology 254: 307–316. Google Scholar
  22. H. Lohmann 1922. Oesia disjuncta Walcott, eine Appendicularie aus dem Kambrium. Mitteilungen aus dem Zoologischen Staatsinstitut in Hamburg 38: 69–75. Google Scholar
  23. H. Lohmann 1933–1934. Appendiculariae. In : W. Kükenthal and T. Krumbach (eds.), Handbuch der Zoologie: Tunicata , 15–202. Walter de Gruyter, Berlin. Google Scholar
  24. F. Marlétaz , E. Martin , Y. Perez , D. Papillon , X. Caubit , C.J. Lowe , B. Freeman , L. Fasano , C. Dossat , P. Wincker , J. Weissenbach , and Y. Le Parco 2006. Chaetognath phylogenetics: a protostome with deuterostome-like development. Current Biology 16: R577–R578. Google Scholar
  25. D.Q. Matus , R.R. Copley , C.W. Dunn , A. Hejnol , H. Eccleston , K.M. Halanych , M.Q. Martindale , and M.J. Telford 2006. Broad taxon and gene sampling indicate that chaetognaths are protostomes. Current Biology 16: R575–R576. Google Scholar
  26. D.Q. Matus , K.M. Halanych , and M.Q. Martindale 2007. The Hox gene complement of a pelagic chaetognath Flaccisagitta enflata. Integrative and Comparative Biology: 47: 854–864. Google Scholar
  27. D. Papillon , Y Perez, X. Caubit , and Y. Le Parco 2004. Identification of chaetognaths as protostomes is supported by the analysis of their mitochondrial genome. Molecular Biology and Evolution 21: 2122–2129. Google Scholar
  28. H. Philippe , N. Lartillot , and H. Brinkman 2005. Multigene analyses of bilaterian animals corroborate the monophyly of Ecdysozoa, Lophotrochozoa, and Protostomia. Molecular Biology and Evolution 22: 1246–1253. Google Scholar
  29. L. Podsiadlowski , A. Braband , and G. Mayer 2008. The complete mitochondrial genome of the onychophoran Epiperipatus biolleyi reveals a unique transfer RNA set and provides further support for the Ecdysozoa hypothesis. Molecular Biology and Evolution 25: 42–51. Google Scholar
  30. T. Shimotori and T. Goto 2001. Developmental fates of the first four blastomeres of the chaetognath Paraspadella gotoi: Relationship to protostomes. Development, Growth and Differentiation 43: 371–382. Google Scholar
  31. G.L. Shinn 1994. Epithelial origin of mesodermal structures in arrow worms (Phylum Chaetognatha). American Zoologist 34: 523–532. Google Scholar
  32. H. Szaniawski 2002. New evidence for the protoconodont origin of chaetognaths. Acta Palaeontologica Polonica 47: 405–419. Google Scholar
  33. H. Szaniawski 2005. Cambrian chaetognaths recognized in Burgess Shale fossils. Acta Palaeontologica Polonica 50: 1–8. Google Scholar
  34. L.B. Tarlo 1960. The invertebrate origins of the vertebrates. Report of the International Geological Congress, XXI Session, Norden, 1960. 22: 113–123. Google Scholar
  35. J. Vannier , M. Steiner , E. Renvoisé , S.-X. Hu , and J.-P. Casanova 2005. Arrow worms: small marine predators from “deep time”. Acta Micropalaeontologica Sinica 22 (Supplement): 189–190. Google Scholar
  36. J. Vannier , M. Steiner , E. Renvoisé , S.-X. Hu , and J.-P. Casanova 2006. Early Cambrian origin of modem food webs: evidence from predator arrow worms. Proceedings of the Royal Society of London B 274: 627–633. Google Scholar
  37. C.D. Walcott 1911. Middle Cambrian annelids. Smithsonian Miscellaneous Collections 57: 109–144. Google Scholar
  38. H.B. Whittington 1971. The Burgess Shale: History of research and preservation of fossils. In : E.S. Richardson (ed.), Symposium of the North American Paleontological Convention, Part I (Extraordinary fossils) , 1170–1201. Allen Press, Lawrence. Google Scholar
  39. J. Zrzavy , S. Mihulka , P. Kepka , S. Bezdek , and D. Tietz 1998. Phylogeny of the Metazoa based on morphological and 18S ribosomal DNA evidence. Cladistics 14: 249–285. Google Scholar
and Simon Conway Morris "The Burgess Shale Animal Oesia is not a Chaetognath: A Reply to Szaniawski (2005)," Acta Palaeontologica Polonica 54(1), (1 March 2009). https://doi.org/10.4202/app.2009.0120
JOURNAL ARTICLE
5 PAGES


SHARE
ARTICLE IMPACT
Back to Top