Nowakia acuaria, from the Czech Geological Survey.

Belongs within: Eumetazoa.
Contains: Dacryoconarida, Tentaculitida, Hyolitha, Brachiopoda.

The Cricoconarida, tentaculitids and related taxa, are a group of small, conical shells of uncertain affinities known from the Lower Ordovician to the Upper Devonian. Cricoconarid shells are narrow and straight, and bear transverse rings, ringlets and striae. They may be divided between the Tentaculitida, in which the embryonic part of the shell is bluntly pointed and conical, and the Dacryoconarida with a demarcated teardrop-shaped embryonic chamber (Fisher 1962).

Tommotiids revealed!
Written 27 June 2008
Disarticulated mitral sclerites of Micrina xiaotanensis, from GeoScience World.

Earlier this year, I wrote several posts where I looked at a range of fossil organisms that were originally described from bits of disassociated external skeleton. Some of these were still of unknown live appearance, some had turned out once soft-body fossils were discovered to look very different from what anyone had imagined. One such group that I didn’t cover (though I did refer to them in passing) was tommotiids. Tommotiids are part of the Cambrian assemblage of scleritome animals that may or may not be related to each other, and may or may not include basal lophotrochozoans (the clade that includes brachiopods, annelids and molluscs). Tommotiids in particular have been suggested to be related to brachiopods, with which they share a similar shell ultrastructure (Holmer et al. 2002). Until recently, no articulated tommotiid specimens had been found, but comparisons of tommotiid sclerites with those of Halkieria and Wiwaxia had led to suggested reconstructions of tommotiids as bilateral armoured slug-like animals. A new paper by Holmer et al. (2008) suggests a quite different image.

An articulated tommotiid scleritome was recently described by Skovsted et al. (2008), though frustratingly I don’t have access to the paper. Far from the imagined bilateral slug, sclerites of the tommotiid Eccentrotheca were joined into an expanding tube-shaped structure. Skovsted et al. inferred that Eccentrotheca was a sessile, vermiform (worm-shaped) filter-feeder. Such an interpretation, they argued, fit well with the potential brachiozoan (brachiopod + phoronid) affinities of tommotiids, though Eccentrotheca may have been more similar in appearance to the worm-like phoronids rather than the brachiopods.

The reconstruction of Holmer et al. (2008) focuses on another tommotiid, Micrina, which is the most brachiopod-like of the tommotiids. Micrina possessed two types of sclerite, the smaller and flatter sellate and the larger, cap-shaped mitral. By comparison with Halkieria, Williams & Holmer (2002) suggested that the two sclerites could have been situated at either end of a slug-shaped animal. However, the revelation from Eccentrotheca that at least some tommotiids might be sessile suggested that this reconstruction should be re-examined.

Reconstruction of Micrina, from Holmer et al. (2008).

The new reconstruction of Micrina from Holmer et al. (2008) is shown above. Rather than being a slug-like animal, Micrina is reconstructed as a sessile filter-feeder like Eccentrotheca. However, Micrina differs from Eccentrotheca in being cup-shaped rather than vermiform. What it does bear a distinct resemblance to is a basal brachiopod similar to the diagram I used elsewhere, which are also sessile filter feeders attached to a substrate by a short pedicle. In contrast to brachiopods, the two valves of Micrina would not have been able to form a sealed chamber, but the shell ultrastructure of Micrina does suggest the presence of a fringe of setae that Holmer et al. suggest could have served a protective function. One potential issue with the reconstruction is that mitral valves are generally preserved in much greater numbers than sellate valves when the reconstruction suggests they should be equally abundant, but this may be a preservation artefact resulting from the smaller and lighter construction of the sellate valve.

A sessile reconstruction for tommotiids has interesting implications for the interpretation of other Cambrian scleritome animals. Halkieria was suggested as a stem-brachiopod by Conway Morris & Peel (1995), but this was debated by Vinther & Nielsen (2005) who interpreted Halkieria as closer to molluscs. While the sessile reconstruction of tommotiids does not entirely rule out a halkieriid ancestry for brachiozoans (one could still potentially argue that halkieriids were ancestral to a tommotiid + brachiozoan clade), it does make it significantly less likely. Conway Morris & Peel (1995) suggested that the two large subterminal sclerites at each end of Halkieria could have been brought into apposition to form the two valves of the brachiopod shell, but the sessile tommotiids suggest that the equal-sized valves of brachiopods could have been derived from an unequally-valved ancestor.

They are also interesting by way of analogy with the chancelloriids, those incredibly confusing Cambrian animals whose sclerite structure demands they be lophotrochozoans, but whose sessile habit and radial organisation screams non-bilaterian. While there is no reason to suggest an actual phylogenetic connection between tommotiids and chancelloriids, the presence of a sessile habit in the former, which are almost undeniably lophotrochozoans, suggests that the radial nature of the latter may not be so difficult to resolve with a lophotrochozoan ancestry after all.

More crunchy scleritome goodness
Written 25 March 2009
Tommotiids. On the left, the articulated Eccentrotheca. On the right, sclerites of Micrina placed to show their suggested life positions. Photo from here.

In the section above, I reviewed a new reconstruction of the tommotiid Micrina presented by Holmer et al. (2008). This reconstruction, with two valves on either side of an attached stalk, was intriguing in its resemblance to a basal brachiopod. However, it was in fairly stark contrast to the previously described articulated tommotiid Eccentrotheca, which has its sclerites stacked one above another to form a tubular structure (Skovsted et al. 2008). New articulated tommotiids described by Skovsted et al. (2009) may just go some way towards bridging the divide.

Paterimicra. On the left, apical (above) and lateral (below) views of the large sclerite S1. On the right, S1 in suggested life position with an S2 sclerite within the triangular notch. Scale bars for this and the next figure = 200 μm. Figures from Skovsted et al. (2009).

Skovsted et al. (2009) have described articulated scleritomes of the tommotiid Paterimitra. Like Eccentrotheca, Paterimitra had more sclerites in its scleritome than the two suggested for Micrina. However, unlike the tubular Eccentrotheca, Paterimitra had the scleritome dominated by a single basal S1 sclerite, shaped a bit like a wonky four-sided pyramid with one side extended out further than the other. On each of these two opposing sides was a deep notch or sinus, with the notch on the steeper side much deeper and with an outwards-pointing flange at the bottom. Inside this deeper notch would sit the smaller, triangular S2 sclerite, which also had an outwards-pointing flange at its bottom end that lined up with the flange of the S1 to form a loose protective tube. There were also a number of smaller, twisted-plate-shaped L sclerites. I have to confess, I’m still trying to work those out to some extent, but as far as I can tell they stacked on one side on the top of the S1 to form some degree of protective covering for the opening of the pyramid.

Lateral view of a partially-articulated Paterimicra specimen with L sclerites fused to the top of the S1 sclerite. It is noteworthy that the available articulated Paterimicra specimens with fused sclerites (this one, which is the only one to retain the L sclerites in place, particularly) show signs of injury or pathology at some point in development. This suggests that sclerite fusion was a pathological response in these individuals, not a normal part of scleritome development, which may partially explain why articulated specimens are so rare. Figure from Skovsted et al. (2009).

Skovsted et al. suggest a sessile life position for Paterimitra with the S1+S2 pyramid standing point-downwards, attached to the substrate by an organic stalk (like the pedicle of modern sessile brachiopods) passing between the flanges of the sclerites. They suggest Paterimitra may have been derived from an Eccentrotheca-like ancestor by the enlargement of the basal sclerites. Another Eccentrotheca-type lineage may have lost the sclerites entirely to give rise to the modern worm-like phoronids (though note that a few recent authors have suggested, based on soft-body characters shared with linguloids, that phoronids may be derived from within brachiopods). Micrina (in its suggested form) could be derived from a Paterimitra-type animal essentially by the loss of the L-sclerites. After that, it’s simply a matter of extending the two remaining sclerites so that the shell is able to fully close (both Micrina and Paterimitra would have been permanently open to some degree, though Holmer et al. (2008) suggested a protective guard of long setae for Micrina), and what you’ve got is a quite passable basal brachiopod!

More giant larvae
Published 1 July 2009

I just thought that I’d show you something that I alluded to briefly elsewhere in my post on Planctosphaera. This is the giant phoronid larva described by Temereva et al. (2006), as illustrated in a figure from that paper:

For comparison, the animals to its right are more normal phoronid larvae (Actinotrocha is a form genus for such larvae, as it is not generally possible to identify a particular larva with its mature adult form). Phoronids are not the only marine animals for which such giant larvae have been found. If you’ve read the other post, you may recall that Planctosphaera was such an example. There’s also the famed giant leptocephalus larvae, similar to the ten-centimetre (at most) leptocephalus larvae of eels or tarpons but reaching lengths of over six feet. Findings of giant larvae have lead to speculations about the existence of truly gigantic adults (particularly, it hardly needs saying, in the case of the leptocephali), but these adults remain as yet undiscovered. Many researchers suspect that giant larvae are not spawned from giant adults, but instead are pathological larvae of more normal-sized species that have failed to mature in the proper manner.

Even if the majority of giant larvae are merely abortive freaks, they are not without interesting implications for our understanding of evolution. Temereva et al.‘s giant phoronid larva differed from other phoronid larvae in more than mere size. It also possessed a more fully developed circulatory system, as well as rudimentary gonads (which normally don’t appear in phoronids until maturity). It takes little imagination to see the next step leading to a phoronid larva attaining full maturity while maintaining its larval form. It would not be the first known case—in 1928, Heath described Graffizoon lobatum, an animal very similar to the larva of a polyclad flatworm except for its possession of fully-developed gonads (as a reminder of our lack of familiarity with marine life, Graffizoon does not seem to have been recorded since).

For comparison, this is what adult phoronids look like:

Phoronopsis viridis, from UCMP.

How difficult would it be to recognise the relationship between animals potentially only separated by a single generation?

Systematics of Brachiozoa
<==Brachiozoa [Conichonchia, Mitrosagophora, Phoronozoa, Phorozoa, Tommotiida]
    |  |    |--Bercutia cristataSB09, WS93
    |  |    |--GeresiaSB09
    |  |    |--Quilicanella Rusconi 1952F62
    |  |    |    `--*Q. cuyana Rusconi 1952F62
    |  |    `--Lapworthella Cobbold 1921F62
    |  |         |--*L. nigra Cobbold 1921F62
    |  |         |--L. bella Missarzhevsky 1966D94
    |  |         |--L. corniforma Meshkova 1969WS93
    |  |         |--L. lucida Meshkova 1969WS93
    |  |         |--L. ludwigseni Landing 1984SB09, L95
    |  |         |--L. marginata Meshkova 1969WS93
    |  |         `--L. tortuosa Missarzhevsky 1966D94
    |  `--+--KennardiidaeSB09
    |     |    |--Kennardia reticulata Laurie 1986WS93
    |     |    `--DailyatiaSB09
    |     `--Camenella Missarzhevsky 1966 (see below for synonymy)SB09
    |          |--*C. garbowskae Missarzhevsky 1966SB09
    |          |--‘Tommotia’ admiranda (Missarzhevsky 1966)WS93
    |          |--C. baltica (Bengtson 1970)L95
    |          |--‘Tommotia’ kozlowskii (Missarzhevsky 1966)WS93
    |          |--C. parilobataSB09
    |          |--‘Tommotia’ plana (Missarzhevsky 1966)WS93
    |          |--C. reticulosa Conway Morris 1990SB09
    |          `--‘Tommotia’ zonata (Missarzhevsky 1969)WS93
       |  `--TannuolinidaeSB09
       |       |--Tannuolina multifora Fonin & Smirnova 1967SB09, WS93
       |       `--+--Micrina etheridgei (Tate 1892)SB09, WS93
       |          `--+--+--LingulosacculusMSC17
       |             |  `--HyolithaMSC17
       |             `--+--YuganothecaMSC17
       |                `--BrachiopodaSB09
          |    |--Kulparina Conway Morris & Bengtson in Bengtson et al. 1990L95
          |    |    `--K. rostrata Conway Morris & Bengtson in Bengtson et al. 1990L95
          |    |--Jaycea Landing 1995 [=Jaycea]L95
          |    |    `--*J. deltaformis Landing 1995 [=Jayceia deltaformis]L95
          |    |--Sunnaginia Missarzhevsky 1969L95
          |    |    |--*S. imbricata Missarzhevsky 1969L95
          |    |    `--S. angulata Brasier 1986WS93
          |    `--Eccentrotheca Landing, Nowlan & Fletcher 1980L95
          |         |--E. guanoSB08
          |         |--E. heleniaVP17
          |         `--E. kanesia Landing, Nowlan & Fletcher 1980 [incl. E. grandis Brasier 1986]L95
               |--Phoronopsis Gilchrist 1908H75
               |    |--P. harmeriMS98
               |    `--P. viridisSBM11
               `--Phoronis Wright 1856H75
                    |--P. architectaGD00
                    |--P. australisGD00
                    |--P. hippocrepiaFVC73
                    |--P. ijimaMS98
                    |--P. ovalis Wright 1856GO78
                    |--P. psammophilaWT06
                    `--P. vancouverensisPH06
Brachiozoa incertae sedis:
  Novocrania anomalaCV16
    |--Kelanella altaica Missarzhevsky 1966WS93
    `--Sonella rostriformis Missarzhevsky & Grigor’yeva 1981WS93
  Tentaculitoidea [Cricoconarida]S12
    |  i. s.: Variatellina pseudogeinitzianaN79
    |    |--TrypanoporaW93
    |    `--TorquaysalpinxW93
         |--ParanowakiaH79 [ParanowakiidaeS12]
         |    |--P. bohemicaH79
         |    `--P. intermediaH79
         |    |--DneprovskitesS12
         |    `--Polycylindrites Lyashenko 1955F62
         |         `--*P. nalivkini (Lyashenko 1954) [=Tentaculites nalivkini]F62
         `--Homoctenidae [Annulosi, Homoctenida]F62
              |--Denticulites Lyashenko 1957F62
              |    `--*D. lyashenkoi (Lyashenko 1957) [=Tentaculites lyashenkoi]F62
              `--Homoctenus Lyashenko 1955F62
                   |--*H. krestovnikovi Lyashenko 1955F62
                   |--‘Tentaculites’ deubeli Zagora 1964S12
                   |--H. hanusi Bouček 1964W93
                   |--H. nanusF62
                   `--H. tenuicinctus (Reomer 1950)S12

Camenella Missarzhevsky 1966 [incl. Camena Missarzhevsky 1966 nec Martens in Albers & Martens 1860 nec Hewitson 1865, Tommotia Missarzhevsky 1970; Tommotiidae]SB09

*Type species of generic name indicated


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[MS98] Margulis, L., & K. V. Schwartz. 1998. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth 3rd ed. W. H. Freeman and Company: New York.

[MSC17] Moysiuk, J., M. R. Smith & J.-B. Caron. 2017. Hyoliths are Palaeozoic lophophorates. Nature 541: 394–397.

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[PH06] Passamaneck, Y., & K. M. Halanych. 2006. Lophotrochozoan phylogeny assessed with LSU and SSU data: evidence of lophophorate polyphyly. Molecular Phylogenetics and Evolution 40 (1): 20–28.

[S12] Schindler, E. 2012. Tentaculitoids—an enigmatic group of Palaeozoic fossils. In: Talent, J. A. (ed.) Earth and Life: Global biodiversity, extinction intervals and biogeographic perturbations through time pp. 479–490. Springer.

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