Conodonts: They Just Got Scarier

Reconstructed apparatus of Besselodus arcticus, from Dzik (1991).

I've told you before about conodonts, Palaeozoic microcarnivores with impressive tooth arrays. In the earlier post, I referred mostly to ozarkodinids, later conodonts that had grasping teeth in the front of their mouths and crushing plates towards the back. In this post, I'll be referring to panderodontids, an earlier group that lacked the crushing plates of ozarkodinids and had a tooth apparatus made up of simpler fang-like elements, similar to the reconstruction above. Apparatus of panderodontids have been found preserved in association, but we don't yet have preserved examples as good as available for the ozarkodinids.



With such different apparatus, panderodontids were obviously capturing and processing prey differently to ozarkodinids, and a paper just out by Szaniawski (2009) suggests one of those differences. Panderodontids and many other conodonts with coniform teeth had long grooves on the inner surface of some of their teeth (as seen in the photo of a Dapsilodus mutatus element above from Szaniawski, 2009) and Szaniawski points out that these grooves are extremely similar to those seen in the fangs of many venomous fish, lizards and snakes. He therefore infers that panderodontids were similarly venomous. As well as making conodont apparatus even more impressive than they already were, this would make panderodontids the earliest known venomous chordates*.

*Szaniawski refers to them as the "oldest known venomous animals". However, cnidarians had already been around for some time, and while the cnidarian venom delivery system doesn't fossilise, the fact that these were crown-group cnidarians makes it a pretty sure bet that they had it by then.

Earlier suggestions that the groove provided an anchoring point for muscles were couched in the belief that conodont elements were permanently internal, a view that is no longer standard*. Other forms of conodont lacked the venom groove, further evidence of the conodonts' ecological diversity.

*Conodont elements grew as new layers were put down over the outer surface, which is admittedly a little difficult to reconcile with their current interpretation as grasping teeth (which would require the absence of tissue cover). It seems likely that conodont teeth were only exposed when being actively used; at other times they would have been retracted into a covering pocket, in the same manner as the grasping spines of modern chaetognaths.

REFERENCES

Dzik, J. 1991. Evolution of oral apparatuses in the conodont chordates. Acta Palaeontologica Polonica 36 (3): 265-323.

Szaniawski, H. 2009. The earliest known venomous animals recognized among conodonts. Acta Palaeontologica Polonica 54 (4): 669-676.

Re-opening the Door (Taxon of the Week: Clausilioidea)

The latest Name the Bug challenge has been successfully identified as Apalopteron familiare, the Bonin honeyeater. I'll put up a descriptive post for that species in the next few days, but for now it's Taxon of the Week time.


A fantastic shot of the clausiliid Cochlodina laminata by Dietrich Meyer. The shell of this individual was 17 mm long.

The new Taxon of the Week is a repeat performance - Clausilioidea or door snails were featured here earlier this year and I'd recommend reading that post before this one. As mentioned there, one of the characteristic features of Clausiliidae (the family including all living clausilioids) is that they have very narrow, tall shells. Cain (1977) found that the distribution of shell shapes among terrestrial snails tended to be bimodal - shells were usually very long and thin or short and flatter, with very few being only slightly elongate. At least one factor in this difference in shell shapes appears to be choice of feeding territory* - elongate snails prefer grazing on very steep or vertical surfaces while shorter snails prefer more horizontal surfaces (Goodfriend, 1986). Studies on the behaviour of the clausiliid Cristataria genezarethana by Heller & Dolev (1994) found that they spent most of their lives sheltering in crevices - an individual snail would only be active for about six to twelve days of the year. Growth was estimated to be correspondingly slow - it may take eleven years for a Cristataria to reach maturity.

*In discussing possible reasons for the bimodality of shell shapes, Cain (1977) provided a very pithy summary of the problem in studying snail behaviour - "It is of course true that most species of snail when active are nocturnal or at least crepuscular, while conchologists are largely diurnal".


Congregation of aestivating individuals of the clausiliid snail Albinaria caerulea. This is one of the more widespread species of Albinaria, found in coastal western Turkey and the Cyclades islands of Greece, while an isolated population in Attica in Greece may have resulted from human transportation. Photo by Aydin Örstan.

Perhaps the most extensively studied clausiliid group is the Mediterranean genus Albinaria, found in southern Greece and Turkey. Albinaria shows a high level of apparent diversity - over 200 species and subspecies have been described. Many of these taxa occupy highly restricted distributions, and sympatry within the genus is rare. However, narrow hybrid zones exist between many "species", and they can often be interbred readily in the laboratory (Douris et al., 1998), leading to the suggestion that the number of true species involved may be much lower and that many supposed "species" may instead represent ecotypes. Comparable patterns of diversity are known from other land snail genera; one of the most notorious examples is the West Indian genus Cerion, the taxonomy of which was an early research topic of Stephen Jay Gould and a strong influence in the development of his opinions on developmental constraints and evolutionary contingencies. Molecular studies of Albinaria have supported the recognition of certain species, but failed to distinguish between others (Giokas, 2000). In some cases, two morphologically distinct "species" might form a clade together but remain intermixed within the clade, suggesting that they are indeed ecological variants of a single species.

REFERENCES

Cain, A. J. 1977. Variation in the spire index of some coiled gastropod shells, and its evolutionary significance. Philosophical Transactions of the Royal Society of London Series B - Biological Sciences 277 (956): 377-428.

Douris, V., R. A. D. Cameron, G. C. Rodakis & R. Lecanidou. 1998. Mitochondrial phylogeography of the land snail Albinaria in Crete: long-term geological and short-term vicariance effects. Evolution 52 (1): 116-125.

Giokas, S. 2000. Congruence and conflict in Albinaria (Gastropoda, Clausiliidae). A review of morphological and molecular phylogenetic approaches. Belg. J. Zool. 130 (Suppl. 1): 93-100.

Goodfriend, G. A. 1986. Variation in land-snail shell form and size and its causes: a review. Systematic Zoology 35 (2): 204-223.

Heller, J., & A. Dolev. 1994. Biology and population dynamics of a crevice-dwelling landsnail, Cristataria genezarethana (Clausiliidae). Journal of Molluscan Studies 60 (1): 33-46.

Hints for Name the Bug # 7

Yesterday's Name the Bug challenge is still sitting there without a successful identification, so here's some hints:

1. Phylogenetic wanderer, perhaps not so fond of honey after all.

2. Mama's still got it; Papa's lost it.

Name the Bug #7

After the last entry was obviously so difficult, I've decided to go a little easier on you all this time:



Attribution, as always, to follow.

Update: Identity now available here. Photo by Outsuka Hiroyuki.

Voice of the Iron Lady

Margaret Thatcher, bane of an entire generation of British liberals*, recently (Correction: not that recently, but in 1990) spoke at the 2nd World Climate Conference, and you can read a transcript of her speech online. Climate change isn't something I mention often at this site - it's really not my field of expertise. But I thought that I would like to share one excerpt with you:

Many of the precautionary actions that we need to take would be sensible in any event. It is sensible to improve energy efficiency and use energy prudently; it's sensible to develop alternative and sustainable and sensible ... it's sensible to improve energy efficiency and to develop alternative and sustainable sources of supply; it's sensible to replant the forests which we consume; it's sensible to re-examine industrial processes; it's sensible to tackle the problem of waste. I understand that the latest vogue is to call them ‘no regrets’ policies. Certainly we should have none in putting them into effect.

The thing that has always confused me about the climate change 'debate' is that I don't really see why there needs to be a debate in the first place. Ultimately, the best methods being proposed to combat climate change seem to come down to reducing pollution, and reducing waste. Irrespective of whether anthropogenic climate change is happening or not, these are good things in themselves. It strikes me as being a bit like the idea that bathing regularly reduces the likelihood that you will get sick. Maybe there's a chance that you won't become ill even if you don't wash yourself for a fortnight - but you would still feel better if you had, nonetheless.

*Does anyone else here remember The Tin-Pot Foreign General and the Old Iron Woman?

Soft yet Scaly (Taxon of the Week: Coccidae)

The stellate scale Vinsonia stellifera (Coccidae). Scales are insects that have abandoned motility for most of their lives to become sedentary plant suckers. Photo from here.

The truly bizarre insects known as scales have been covered at this site previously, including a brief description of the scale life cycle. In that post I referred to the ensign scales or Ortheziidae; in this post I'll cover the soft scales or Coccidae. The Coccidae include about 1000 species, some of which produce a dorsal covering of wax while others lack a dorsal covering (Williams, 1991). While ortheziids belong to the group of scale families known as archaeococcids, coccids belong to the more derived grouping known as neococcids. Neococcids are distinguished from archaeococcids by the absence of spiracles on the abdomen, and of compound eyes in the adult males (instead, male neococcid eyes have become reduced to dissociated ocelli). Coccids are distinguished from other neococcid families by the presence of a pair of rounded or triangular plates at the base of the anal cleft (Williams, 1991).


While female scales remain immotile for the rest of their lives once they have found a host, males regrow their legs and usually develop wings at maturity to find females. This is the Kuno scale Eulecanium kunoense. Photo by Joyce Gross (and very impressive it is too - photographing something as minute as a male scale would not be an easy call.

Another distinctive feature of neococcids is something referred to as Paternal Genome Loss (PGL - also known as Paternal Genome Elimination). In most neococcid families, males are technically diploid but early in development the chromosomes a male has inherited from its father are all inactivated so that it becomes functionally haploid. When the male produces sperm, these inactivated chromosomes are eliminated from sperm production and only the maternally-inherited chromosomes are passed on to its offspring. The reason for the evolution of PGL remains unknown*, but it appears likely to have evolved among neococcids on a single occasion (Yokogawa & Yahara, 2009). True haplodiploidy as found in Hymenoptera, where males are truly haploid as opposed to functionally haploid, has also evolved in scales of the archaeococcid family Margarodidae but is as yet unknown among neococcids despite suggestions that PGL may be a precursor to the origin of haplodiploidy. It is worth noting that, while an origin of haplodiploidy from PGL may seem reasonably intuitive, there is the small problem that there are more than twice as many known cases of taxa evolving haplodiploidy as PGL.

*Endosymbiotic bacteria such as Wolbachia have been shown to cause PGL in some insects (and the presence or absence of endosymbionts has been shown to affect PGL in at least one neococcid); alternatively, it could result from genetic factors on the animal's own X chromosome promoting the transmission of maternal chromosomes.

REFERENCES

Williams, D. J. 1991. Superfamily Coccoidea. In The Insects of Australia, 2nd ed. vol. I pp. 457-464. Melbourne University Press.

Yokogawa, T., & T. Yahara. 2009. Mitochondrial phylogeny certified PGL (Paternal Genome Loss) is of single origin and haplodiploidy sensu stricto (arrhenotoky) did not evolve from PGL in the scale insects (Hemiptera: Coccoidea). Genes Genet. Syst. 84: 57-66.

Name the Bug: Alaskiella medfraensis

Alaskiella medfraensis (from Frýda & Blodgett, 1998)

No-one successfully identified this one. I guess Palaeozoic gastropods just don't have the same following as other animals.

Alaskiella medfraensis is a member of the Porcellioidea, a superfamily of gastropods containing the Palaeozoic Porcelliidae and the Mesozoic Cirroidea. Porcellioids are distinguishable from other gastropods by virtue of being the only heterostrophic vetigastropods. I'll explain what that means, and I apologise in advance if it's a little hard to follow. I know it confuses the hell out me.

I've previously explained the difference between dextral and sinistral gastropods and how to distinguish the two. The method that I described therein is the correct one for orthostrophic shells. Orthostrophic growth is the standard gastropod growth pattern with the shell growing in a downwards spiral. However, some types of gastropod are hyperstrophic which essentially means that from a developmental perspective they grow upwards rather than downwards. Because the shell's "correct" orientation is therefore rotated 180° from that of an orthostrophic shell, if you hold a hyperstrophic shell with the aperture downwards a dextral shell is going to appear sinistral and vice versa. In life, of course, the aperture will still be held downwards in a hyperstrophic gastropod, but they can still be distinguished because the positions of the organs (gastropods are bilaterally asymmetrical) will be reversed - the stuff that you'd expect to see on the left side will instead be on the right. In the case of fossil gastropods, where you can't look at the organs, distinguishing hyperstrophic shells becomes a lot more difficult and I have to confess that I really don't see how they do it (apparently the shape and orientation of the aperture may offer some indications).

Heterostrophic shells like Alaskiella start off life growing upwards like a hyperstrophic shell but then change the direction of growth to downwards. As a result of this, they also change the direction of spiralling - so Alaskiella starts off spiralling upwards dextrally before it spirals downwards sinistrally. Among living gastropods, heterostrophic coiling is characteristic of the Heterobranchia, the clade that includes opisthobranchs (sea slugs and related animals) and pulmonates (lung-breathing snails). However, porcellioids developed heterostrophy independently of heterobranchs; instead, they belong to the Vetigastropoda, the clade including trochids (top shells) and turbinids (cat's-eye or turban shells). The main characters showing porcellioids to be vetigastropods are the presence of a vetigastropod-type protoconch (the embryonic shell) and nacre (the shiny inner layer of, for instance, a paua shell [Haliotis iris]; Frýda et al., 2008). Neither of these features is really visible in the figure above (nacre has only been identified as preserved in one cirrid genus so far) but you can readily see the large selenizone, the groove running around the outer edge of the whorls, which tends to be a characteristic of vetigastropods*. Porcellioids can also be distinguished from heterobranchs in that while heterobranchs change the direction of coiling at the transition point between the protoconch and the teleoconch (the post-embryonic or post-larval shell), porcellioids change directly during the early part of the teleoconch (Frýda & Blodgett, 2004).

*To be scrupulously correct, not all vetigastropods have a selenizone and not every gastropod with a selenizone (which may be a groove or may be a row of openings) is a vetigastropod. However, in general, non-vetigastropods with selenizones have only small ones.

Alaskiella medfraensis differs from other porcellioids in that other porcellioids have the axis of coiling of the protoconch parallel to that of the teleoconch, but Alaskiella has the axes offset at an angle. The change in the axis of coiling is what gives Alaskiella its characteristic looped look where the shell changes direction, which always puts me in mind of the looped peak that tends to form at the top of a meringue when you spoon it out.

REFERENCES

Frýda, J., & R. B. Blodgett. 1998. Two new cirroidean genera (Vetigastropoda, Archaeogastropoda) from the Emsian (late Early Devonian) of Alaska with notes on the early phylogeny of Cirroidea. Journal of Paleontology 72 (2): 265-273.

Frýda, J., & R. B. Blodgett. 2004. New Emsian (Late Early Devonian) gastropods from Limestone Mountain, Medfra B-4 quadrangle, west-central Alaska (Farewell Terrane), and their paleobiogeographic affinities and evolutionary significance. Journal of Paleontology 78 (1): 111-132.

Frýda, J., R. B. Blodgett, A. C. Lenz & Š. Manda. 2008. New porcellioidean gastropods from Early Devonian of Royal Creek area, Yukon Territory, Canada, with notes on their early phylogeny. Journal of Paleontology 82 (3): 595-603.