Machaeridia

Plumulites bohemicus, from Withers (1926).

Belongs within: Annelida.

Yay, machaeridians!
Published 14 January 2008

The fossil record is replete with taxa whose nature is largely mysterious owing to incomplete remains. Most deposits only preserve the mineralised parts of an organism and, all too often, that just isn’t enough. And so, in 2008, we were all excited to learn that 150 years after their initial discovery, we finally had soft-body remains of a machaeridian.

Plumulites bengtsoni, from Vinther et al. (2008). A, Holotype YPM 221134, part. B, Camera lucida drawing of the part. Colours indicate the trunk (yellow), parapodia (red), chaetae (gray), attachment of shell plates (green), gut (purple) and dorsal linear structure (blue). Abbreviations: os, outer shell plate; is, inner shell plate; aos, anterior outer shell plate; ls, linear structure; cw, cuticular wrinkles; r, rami evidenced by divergent bundles of chaetae. Scale bar = 5 mm.

Machaeridians were small invertebrate animals found from the Ordovician to the Carboniferous. Like a number of other Palaeozoic shelled taxa, they possessed a body armour composed of multiple sclerites rather than a single solid shell. Despite often being implicitly dismissed as ultimately inferior or primitive with regard to the more familiar shelled animals that we see around us today, the scleritome body plan was actually very successful (the Ordovician to the Carboniferous was no small stretch of time) and survives to this day in groups such as the chitons. Vinther et al. (2008) state that machaeridian sclerites are near-ubiquitous in benthic marine assemblages of the appropriate time period. The problem is that because scleritome plates are not directly fused to each other, on death the scleritome generally becomes disarticulated and it can be exceedingly difficult if not impossible to reconstruct the appearance of the entire animal in life. As a result, scleritome animals have been swept into the too-hard basket in the past and not received the attention that they probably deserve. Even with the massive renaissance of interest in problematica that popularised the Cambrian explosion and the Ediacara fauna in the 1990s, the Machaeridia retained their internationally ignored status. Guesses as to their affinity ranged from molluscs to annelids to arthropods (specifically barnacles) to echinoderms.

Selected machaeridians, from Caron (2008). Top, a complete machaeridian scleritome from 425-million-year-old deposits in New York state. Length, 5.4 mm. Bottom, dorsal reconstructions of presumed complete scleritomes (not to scale) showing the diversity of scleritomes within the three families of machaeridians—running upper to lower, the Lepidocoleidae, Turrilepadidae and Plumulitidae. The fossil specimen (top) is a lepidocoleid; that described by Vinther et al. is a plumulitid. All scleritomes are shown with the presumed head to the right. (Top image courtesy of A. Högström; bottom images courtesy of J. Dzik.)

Hence my excitement at the publication of Vinther et al. (2008), which gives us our most complete picture of a machaeridian to date, in the form of an articulated specimen of Plumulites bengtsoni from the Lower Ordovician of Morocco preserving soft body parts. The specimen is not perfect—the preserved soft parts are a little smeared, and the head is missing—but what we have is very informative. Most significant are what seem to be bristle groups running down each side of the body, which the authors feel are probably parapodia. Parapodia are groups of chitinous bristles (chaetae) found in polychaete annelids, and their presence in machaeridians is about as clear as indication of annelid affinities as you can get. Polychaete parapodia are divided into upper and lower clusters of chaetae, and a few of the apparent parapodia in P. bengtsoni do appear to show branching compatible with such an arrangement.

Iphione ovata, from here.

Accepting that machaeridians are closer to annelids than other living animal phyla, it still remains difficult to establish their exact position relative to that group, whether as a stem outgroup or derived ingroup. The phylogeny of living annelids has long been problematic. Annelids possess relatively few phylogenetically useful morphological characters and molecular analyses have struggled to even recover their monophyly relative to other phyla (Rousset et al., 2007). A possible relationship to the annelid order Phyllodocida, some members of which possess dorsal plates as shown above, in Vinther et al.‘s phylogenetic analysis was dependent on coding the mineralised machaeridian sclerites as homologous with the chitinous phyllodocidan elytra. Leaving this character as uncertain caused the annelid interrelationships to collapse to a polytomy, with the possibility remaining that machaeridians sit on the annelid stem. As it has also been suggested that phyllodocidans represent a paraphyletic grade relative to other annelids, the two results are not necessarily incompatible.

Halkieriid, from Palaeos.com.

Machaeridians are but one of a number of groups of Palaeozoic scleritome animals that appear to occupy basal positions on the trochozoan family tree. -)ther examples include the sachitids, tommotiids, wiwaxiids and halkieriids*. Interestingly, there is no indication that these groups all form a clade. Halkieriids may be related to brachiopods (Holmer et al., 2002), to which tommotiids are also ultrastructurally similar (Vinther et al., 2008). Wiwaxiids have been interpreted by different authors as related to molluscs and/or annelids, while molluscs of course include chitons (one extinct order of which, the Multiplacophora, possessed a more complex scleritome than modern species—Vendrasco et al., 2004). It seems not unlikely that the scleritome animals as a whole represent the ancestral group of a brachiopod-annelid-mollusc clade, with the three modern lineages arising independently from scleritome ancestors**. The scleritome would have been reduced and lost in the annelids, while brachiopods and molluscs each independently evolved towards more consolidated armation (if halkieriids are brachiopod relatives, then the two larger terminal plates were expanded in the ancestral brachiopod to the expense of the other plates; the mode of evolution in molluscs is a bit more obscure due to more uncertain phylogeny). Of course, this scenario is still pretty provisional, and there are a number of basal shell-less taxa involved such as Odontogriphus, phoronids and aplacophorans that could still potentially cause it to collapse into a quivering heap. As I’ve so often said in relation to matters of phylogeny, watch this space.

*Many of these groups have been included in a taxon called Coeloscleritophora that was also suggested to include the strange Cambrian organisms called chancelloriids. Individual sclerites of chancelloriids do have a similar structure to those of other “coeloscleritophorans”, but articulated specimens indicate a radically different body plan for the entire animal with sclerites arranged radially over a vase-shaped form. It seems most likely that chancelloriids were sponge-grade animals unrelated to the other “coeloscleritophorans”, but the question is complicated by the complete absense of a living sponge group that is even remotely similar to chancelloriids beyond superficial appearance.

**Recent genetic analyses show that these three phyla fall within a clade called Spiralia (or Lophotrochozoa, a name which is used often but for various reasons gives me grief, so I try to avoid it) that also includes a number of other taxa such as nemerteans and platyzoans. Relationships within Spiralia have not been satisfactorily sorted out. It is possible that some other spiralian taxa may fall within the annelid-brachiopod-mollusc clade, which would even further complicate my scenario.

More crunchy scleritome goodness
Published 25 March 2009

Yep, it’s time for another installment on my favourite assemblage of polyphyletic problematica. Two significant new additions have been made to the repertoire of articulated scleritomes:

An assortment of articulated Lepidocoleus (each about an inch long). Take especial note of figure d! From Högström et al. (2009).

The initial discovery of machaeridian soft-tissue remains by Vinther et al. (2008) was exciting enough. But the following year, Högström et al. (2009) described a collection of articulated machaeridians from the Devonian Hunsrück Slate in Germany, including a second specimen with soft-tissue remains.

There are a few reasons why this was a very satisfying discovery. Firstly, the specimen supported the annelid affinities proposed for machaeridians by Vinther et al. (2008). Secondly, the Hunsrück specimens represented a different family (Lepidocoleidae) from the Fezouata specimen (Plumulitidae; I should point out that articulated lepidocoleid scleritomes had been found before, but not preserving any soft tissue). This confirmed that the machaeridians represent a monophyletic grouping, and are not unrelated taxa that have convergently developed similar sclerite morphologies (always a possibility with animals only known from disarticulated sclerites). Whereas plumulitids appear to have had rather loosely articulated sclerites, giving them an ornamental spined appearance, lepidocoleids had a much more tightly-woven, armour-plated scleritome.

And thirdly, just as a kind of cherry on the top, one of the other Hunsrück specimens, while it may not have had soft-tissue remains, had something else to commend it that some palaeontologists would probably find even more exciting. It’s sitting neatly positioned at the end of a well-preserved trail. Trace fossils are often the best evidence you can get for working out the behaviour of extinct animals, but it can be a frustrating exercise because often the best conditions for preserving traces are not very good for preserving the animals that made them, and vice versa. When I was on a palaeontology field trip as an undergrad, I was told that one of the lecturers had a standing offer of a crate of beer for anyone who found a body fossil in association with a trace fossil. Finding a machaeridian in association with a trace fossil, I feel, would have warranted at least two.

Systematics of Machaeridia
<==Machaeridia [Turrilepadomorpha]VRB08
    |  i. s.: Deltacoleus crassus Withers 1926D94
    |--Zhijinitidae [Cambroclavida, Cambroclavitidae]WS93
    |    |--Zhijinites longistriatus Qian 1978WS93
    |    |--Cambroclavus fangxianensis Qian & Zhang 1983WS93
    |    `--Parazhijinites Qian & Yin 1984CA06
    |         `--*P. guizhouensis Qian & Yin 1984CA06
    `--TurrilepadidaWS93
         |--Plumulites Barrande 1872 [Plumulitidae]VRB08
         |    |--*P. bohemicus Barrande 1872VRB08, WS93
         |    |--P. bengtsoni Vinther, Van Roy & Briggs 2008VRB08
         |    |--P. gumunsoensis Choi & Kim 1989HE04
         |    |--P. kutscheriVRB08
         |    |--P. primus Kobayashi 1934HE04
         |    |--P. richorumVRB08
         |    `--P. tafennaensisVRB08
         `--CuniculepadidaPTV14
              |--TurrilepadidaeWS93
              |    |--Mojczalepas multilamellosa Dzik 1986WS93
              |    |--Clarkeolepis clarkei Elias 1958WS93
              |    `--TurrilepasPTV14
              |         |--T. whitersi Elias 1958WS93
              |         `--T. wrightianaPTV14
              `--LepidocoleidaeWS93
                   |--Plicacoleus robustus Dzik 1986WS93
                   |--Aulakolepos elongatum Dzik 1994WS93, D94
                   `--LepidocoleusPTV14
                        |--L. jamesi (Hall & Whitfield 1875)HE04
                        |--L. ketleyanusPTV14
                        |--L. sarleiPTV14
                        |--L. strictus Withers 1926HE04
                        |--L. suecicus Moberg 1914HE04
                        `--L. ulrichi Withers 1926HE04

*Type species of generic name indicated

References

Caron, J. B. 2008. Ancient worms in armour. Nature 451: 133–134.

[CA06] Clausen, S., & J. J. Álvaro. 2006. Skeletonized microfossils from the Lower–Middle Cambrian transition of the Cantabrian Mountains, northern Spain. Acta Palaeontologica Polonica 51 (2): 223–238.

[D94] Dzik, J. 1994. Evolution of ‘small shelly fossils’ assemblages of the early Paleozoic. Acta Palaeontologica Polonica 39 (3): 247–313.

[HE04] Hints, O., M. Eriksson, A. E. S. Högström, P. Kraft & O. Lehnert. 2004. Worms, wormlike and sclerite-bearing taxa. In: Webby, B. D., F. Paris, M. L. Droser & I. G. Percival (eds) The Great Ordovician Biodiversification Event pp. 223–230. Columbia University Press.

Högström, A. E. S., D. E. G. Briggs & C. Bartels. 2009. A pyritized lepidocoleid machaeridian (Annelida) from the Lower Devonian Hunsrück Slate, Germany. Proceedings of the Royal Society of London Series B—Biological Sciences 276: 1981–1986.

Holmer, L. E., C. B. Skovsted & A. Williams. 2002. A stem group brachiopod from the Lower Cambrian: support for a Micrina (halkieriid) ancestry. Palaeontology 45 (5): 875–882.

[PTV14] Parry, L., A. Tanner & J. Vinther. 2014. The origin of annelids. Palaeontology 57 (6): 1091–1103.

Rousset, V., F. Pleijel, G. W. Rouse, C. Erséus & M. E. Siddall. 2007. A molecular phylogeny of annelids. Cladistics 23: 41–63.

Vendrasco, M. J., T. E. Wood & B. N. Runnegar. 2004. Articulated Palaeozoic fossil with 17 plates greatly expands disparity of early chitons. Nature 429: 288–291.

[VRB08] Vinther, J., P. Van Roy & D. E. G. Briggs. 2008. Machaeridians are Palaeozoic armoured annelids. Nature 451 (7175): 185–188.

[WS93] Wills, M. A., & J. J. Sepkoski Jr. 1993. Problematica. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 543–554. Chapman & Hall: London.

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