Deep Pleurotomella

The type species of Pleurotomella, P. packardi, copyright Forum Natura Mediterraneo.

'Turrid' time again! Though the disassembly of the enormous mass that was the old gastropod family Turridae (now several families of the superfamily Conoidea) has left the subject of today's post, the genus Pleurotomella, as a member of the Raphitomidae rather than the Turridae. Pleurotomella is a widespread genus, with species found in deeper parts of ocean basins around the world. As with many deep-water animals, we know relatively little about their lifestyles, though they are undoubtedly predators like other conoids. Like other conoids, Pleurotomella species have a radula with the teeth modified into hypodermic syringes for the injection of toxins. At least some species (including the type) are blind (Bouchet & Warén 1980) and I can imagine that they attack relatively sedentary prey such as worms.

Taxonomically speaking, Pleurotomella has one of those histories that can make a grown taxonomist just want to sit down and cry. I've already mentioned this horrible genus in my earlier post on Asperdaphne as a player in one of those scenarios where a misunderstood type species leads a genus to jettison almost all of the species previously associated with it and pick up a whole bunch of new ones that it never held before. An inordinate number of deep-water 'turrid' species seem to have been dumped into Pleurotomella at some time or other, many of which are probably only remotely related to the true Pleurotomella. However, since Bouchet & Warén (1980) redescribed the type species Pleurotomella packardi as part of a revision of north-east Atlantic 'turrids', we have much better grounds for the genus' recognition (Beu 2011). Species of Pleurotomella have strongly inflated whorls that are evenly rounded except for a concave 'ramp' below the suture between whorls. The shell contracts rapidly to a narrow base, and has prominent, sharp and often curved axial ridges.

Multispiral (left) and paucispiral (right) protoconches of Mangelia species, from Bouchet (1990). Scale bars = 200 µm.

Again as was the case in the Asperdaphne post, a notable factor in the taxonomic complications of Pleurotomella has been matters relating to the protoconch, the larval shell that remains perched throughout development at the tip of the post-larval shell, the teleoconch. Because the features of the protoconch such as ornamentation may often differ from those of the teleoconch, it can often be of significance in gastropod taxonomy. A lead proponent of the importance of the protoconch in 'turrid' taxonomy was the New Zealand malacologist A. W. B. Powell who produced an influentiall classification of turrids between the 1940s and 1960s. Nevertheless, Powell did note an interesting phenomenon: the common existence of 'genus pairs' that were all but indistinguishable in teleoconch morphology but very distinct in their protoconches. Because Powell regarded the teleoconch as phylogenetically less significant than the protoconch (in accord with Ernst Haeckel's old dictum that ontogeny should recapitulate phylogeny), he concluded that these 'genus pairs' must represent separate lineages converging on a single adult morphology.

More recent authors agree that, in this, Powell was wrong (Bouchet 1990). As noteworthy a source of taxonomic characters it may be, protoconch development is subject to selective and evolutionary pressures just as much as teleoconch development. The most regular difference between Powell's 'genus pairs' is that one would have a conical protoconch with a number of whorls (say three or four, referred to as multispiral) whereas the other would have a stubby round protoconch with at most about one-and-a-half whorls (paucispiral). This difference in protoconch morphology reflects a difference in how the larval shell is fed. In the original development path for gastropods, eggs hatch out to planktic larvae that feed themselves on other plankton (planktotrophy) before eventually settling and developing to maturity. However, many conoids (and other gastropods) have evolved eggs that have a large yolk; the developing embryos obtain their energy from the reserves in the yolk (lecithotrophy) and bypass the planktic stage, hatching directly as benthic crawlers. Because planktotrophs need their larval shell for longer than lecithotrophs, it becomes more developed; planktotrophs are multispiral, lecithotrophs are paucispiral. Powell's 'genus pairs' did not represent separate lineages evolving similar adult lifestyles, but members of the same lineage tackling early development different ways. As such, and because of the possibility that the change between planktotrophic and lecithotrophic development may have occured multiple times within a single group, most recent authors would not automatically recognise multispiral and paucispiral species as separate genera. Pleurotomella species mostly have multispiral protoconches, but some (including P. packardi and a number of Pacific species) have paucispiral ones.

Which is not to say that protoconch morphology has become irrelevant. Bouchet & Warén (1980) did maintain the genus Neopleurotomoides as separate from Pleurotomella on the basis of protoconch morphology, despite these two genera having very similar teleoconches. In this case, the difference is not just the number of spirals in the protoconch, but its ornamentation. Pleurotomella species with a multispiral protoconch have a cancellate (cross-hatch) pattern of ridges covering it, but Neopleurotomoides has a sparser ornament of one or two spiral keels crossed by axial ribs. The distinction between the two genera remains problemematic: species with a paucispiral protoconch (which is usually more or less unornamented) cannot be readily assigned to either genus, and there are many 'Pleurotomella' species for which the protoconch remains undescribed. But the take-away lesson, as so often in taxonomy, is this: no source of characters should be ignored, but nor should it be fetishised.

REFERENCES

Beu, A. G. 2011. Marine Mollusca of isotope stages of the last 2 million years in New Zealand. Part 4. Gastropoda (Ptenoglossa, Neogastropoda, Heterobranchia). Journal of the Royal Society of New Zealand 41 (1): 1–153.

Bouchet, P. 1990. Turrid genera and mode of development: the use and abuse of protoconch morphology. Malacologia 32 (1): 69–77.

Bouchet, P., & A. Warén. 1980. Revision of the north-east Atlantic bathyal and abyssal Turridae (Mollusca, Gastropoda). Journal of Molluscan Studies, Supplement 8: 1–119.

Tully as a Vertebrate

Reconstruction of Tullimonstrum gregarium by Sean McMahon, from McCoy et al. (2016).

McCoy, V. E., E. E. Saupe, J. C. Lamsdell, L. G. Tarhan, S. McMahon, S. Lidgard, P. Mayer, C. D. Whalen, C. Soriano, L. Finney, S. Vogt, E. G. Clark, R. P. Anderson, H. Petermann, E. R. Locatelli & D. E. G. Briggs (in press, 2016) The ‘Tully monster’ is a vertebrate. Nature.

Several years ago, I included the 'Tully monster' Tullimonstrum gregarium in a list of some of the most phylogenetically mysterious organisms on the planet. Multiple suggestions have been made as to its affinities: mollusc, annelid, nemertean (nemerteans and sea cuumbers both having weird histories of problematic fossils assigned to them for little apparent reason), some sort of de-chitinised arthropod relative by way of Opabinia, the Loch Ness monster... A new publication just out by McCoy et al. (2016) adds a further interpretation to the mix.

Tullimonstrum is represented by literally thousands of specimens from the Carboniferous Mazon Creek deposit of Illinois. The organisms preserved in this deposit are contained within nodules, each individual at the centre of a mineral ball that precipitated around it after its death. It had a somewhat elongate, torpedo-shaped body, at the front of which was an elongate proboscis ending in a pincer-like structure. Towards the front of the main body was a dorsal cross-bar with a dark round body at each end; these bodies have most commonly been seen as eyes on the end of stalks but alternative interpretations include statocysts, solid structures that many aquatic animals possess for sensing balance. A fin-like structure was present at the tail end of the animal. Many specimens also show regularly spaced dark cross-lines suggesting some sort of segmental division of the body.

Another structure commonly visible in the Tullimonstrum fossils is a pale, flattened linear structure running down the length of the animal. Most authors have presumed that this represents the gut but McCoy et al. argue that it does not resemble the gut as preserved in other Mazon Creek fossils. In these other fossils, the gut is dark-coloured and is not flattened. Some authors have tried to explain this difference between the 'gut' of Tullimonstrum and that of its associates by suggesting that the Tully monster fed on soft prey such as jellyfish whose remains did not preserve after death, but the dark colour in most Mazon Creek guts does not represent the actual gut contents themselves but minerals that precipitated around the gut contents during the fossilisation process. Presumably, such minerals would be just as likely to condense around jellyfish remains as any other organic tissue. Even more damning, McCoy et al. identified a handful of Tullimonstrum specimens in which the gut was indeed preserved as in other Mazon Creek fossils, and as a separate structure from the pale line that was also present in these same specimens.

An actual fossil of Tullimonstrum in the Museo di Storia Naturale di Milano, copyright Ghedoghedo.

So what was this structure, if not a gut? McCoy et al. note that at least one other fossil from the Mazon Creek preserves a similar structure: the hagfish-like Gilpichthys, in which it represents the notochord. The structure's preservation is consistent with this interpretation: being a fluid-filled tube, the notochord would flatten readily during fossilisation, and it does not accumulate minerals like the gut because it lacks an external connection. And if Tullimonstrum also possesses a notochord, then that makes it also a chordate. And with that in mind, McCoy et al. interpret other structures as supporting chordate, and specifically vertebrate, affinities: the fin-like structures are indeed fins, paired stains bordering the notochord in a few specimens appear to be gill pouches, tooth-like structures within the 'pincer' at the end of the proboscis are keratinous teeth similar to those of lampreys and hagfish, and the apparent 'segments' in some specimens represent vertebrate myomeres (muscle blocks). Including Tullimonstrum in a phylogenetic analysis of basal vertebrates, coded according to these and other interpretations, places it within the stem-lineage of modern lampreys.

So how strong is this re-assignment? The problem with the structural analysis of any problematic fossil is that it is ultimately dependent on finding the right comparative framework, and the more distinct the problematicum is from any living organism the harder it is to be sure you're making the right comparison. That's not a criticism of this particular paper; that's simply the limitation its authors have to work with. In this case, I kind of suspect that the identification of Tullimonstrum as a vertebrate all hinges on whether they've correctly identified that notochord. None of the other 'vertebrate' features identified is sufficiently distinct to clinch the deal on their own. A tail-fin could indicate a vertebrate, or it could indicate a mollusc like a squid. The famous Tullimonstrum proboscis (which, offhand, McCoy et al. interpret as a cartilage-supported structure rigidly bending at set points like an arm rather than curling like a tentacle, based on the regular aspect of its preservation) is unlike anything known from any other vertebrate, but nor does it strongly resemble anything found in any other animal (the aforementioned Opabinia suggestion is right out: as I mentioned in an earlier post on Nectocaris, the Opabinia proboscis contains no direct part of the digestive tract itself). Certainly the placement of Tullimonstrum as a stem-lamprey is the weakest part of the whole deal, as the specific features cited as synapomorphies are either convergently present in other vertebrates (e.g. keratinous teeth) and/or dependent on some admittedly more tentative structural interpretations (e.g. tectal cartilages). There may be a certain element here of Tullimonstrum's intractable weirdness conflicting with the phylogenetic analysis' need to put it somewhere. I also wonder if I should be criticising Sean McMahon's reconstruction (reproduced at the top of this post) for presenting Tullimonstrum as somewhat laterally flattened: the majority of Tullimonstrum specimens are preserved dorsoventrally rather than laterally, which I would suspect indicates that they were probably flatter top-to-bottom than side-to-side.

Those criticisms aside, McCoy et al. have certainly presented one of the more robust reconstructions of Tullimonstrum to date. Most of what I've said comes under the heading of intrigued enquiries rather than actual disagreements, and if they're right about that notochord then they're on pretty firm ground. After all, even if the Tully monster is not specifically a stem-lamprey doesn't exclude it from being any sort of chordate. There are few (if any) problematica as well represented in the fossil record as Tullimonstrum, and we have not heard the last word on it yet.

Omorgus: A Beetle with a Taste for Hair

A group of Omorgus clambering over what looks like a scat, copyright Stephen Cresswell.

I still remember my first Omorgus. Pretty much as soon as I saw it in the pitfall trap, I knew that this was a different type of beetle from any I'd seen before. Large, knobbly, robust... it looked a picture of glorious ugliness. Which only made it all the more frustrating that, somewhere in the process of making it into the trap, this particular specimen appeared to have somehow lost its head. Without the ability to look it in the face, I might never know what I'd found.

It wasn't until later in the lab that I discovered my mistake: my beetle wasn't headless at all! Instead, the head was retracted back, hidden beneath the expanse of the pronotum (the dorsal shield of the first thoracic segment). And so I became acquainted with my first keratin beetle.

A similar Omorgus to the one I found, O. bachorum, to give some idea how I missed the head. Copyright Clare McLellan.

Omorgus is one of the handful of genera of keratin beetles, a group of relatives of the scarabs known as the Trogidae or Troginae (there has been some inconsistency as to whether trogids are treated as their own family or as a subfamily of the main scarab family Scarabaeidae). They have robust forelegs with large femora, and striate elytra that are often covered with tubercles and/or setae. Trogids vary in size from about half a centimetre in length up to three centimetres. They get their name of 'keratin beetles' from their unique diet: both as adults and larvae, trogids feed primarily on keratin such as animal hair. They are most commonly scavengers, feeding at animal carcasses (often arriving late in the process, taking the parts of the animal rejected as indigestible by other scavengers). However, they also feed on other animal foods such as insect larvae, eggs or guano, and some appear to be specialist associates of bird nests or animal burrows (Scholtz 1986). An Australian flightless species Omorgus rotundulus was found to have a gut full of other arthropods, particularly ants and termites, in quantities that lead to the suggestion that it might be an active predator rather than a scavenger (Houston et al. 2010).

An Omorgus chowing down on a dead lizard, copyright William Archer.

Earlier authors commonly treated all trogids as belonging to a single genus Trox, but more recent authors have recognised four or five genera in the family. Omorgus includes about 150 species (Strümpher et al. 2014) found mostly in arid regions. The most obvious feature separating Omorgus species from other trogids is that the pedicel (the second segment of the antennae) is attached to the scape (the first segment) subapically rather than apically. In all species but one, the scutellum (the little thoracic shield visible between the bases of the elytra) is hastate (shaped a bit like a spear-head, with a constricted base broadening out further down) rather than a more simple oval as in other trogids. The exception, T. batesi, is a South American species that is placed in its own subgenus Haroldomorgus. The remaining species are divided between two subgenera Omorgus sensu stricto and Afromorgus, distinguished by features of the male genitalia (Scholtz 1986). Afromorgus is found in Africa and Asia whereas the type subgenus contains the Australian and other American species.

Most trogids are fully capable of flight (many are attracted to lights at night) but, as alluded to above, a handful of species are flightless. In flightless species, the elytra become fused together into a sold carapace. The impression I get from scanning the literature is that flightlessness in trogids may not be so much a matter of conserving energy as it is of conserving water. For animals living on a dry diet in a dry habitat, such adaptations are only to be expected.

REFERENCES

Houston, T. F., J. Zhang & B. P. Hanich. 2010. Diet of the flightless trogid beetle Omorgus rotundulus (Haaf) (Coleoptera: Trogidae) in the Little Sandy Desert of Western Australia. Australian Entomologist 36 (4): 207–212.

Scholtz, C. H. 1986. Phylogeny and systematics of the Trogidae (Coleoptera: Scarabaeoidea). Systematic Entomology 11: 355–363.

Strümpher, W. P., C. L. Sole, M. H. Villet & C. H. Scholtz. 2014. Phylogeny of the family Trogidae (Coleoptera: Scarabaeoidea) inferred from mitochondrial and nuclear ribosomal DNA sequence data. Systematic Entomology 39: 548–562.

When Mayflies Last for Millions of Years

Mayfly Paraleptophlebia prisca preserved in amber, from Penney & Jepsen (2014).

Of all the media available for the preservation of fossils, none approaches perfection anywhere near as close as amber. There is little structural that amber does not preserve: external apperance, soft tissues, even cellular structure may potentially be examined. Amber offers us a window into the past unlike any other. The first amber deposits to go on the record (going back as far as the ancient Greeks), and the largest deposit yet discovered, was the Baltic amber of northern Europe, formed in the Eocene or Oligocene epoch* from the sap of a relative of the modern pines.

*Calculating the age of amber deposits is not easy. The amber itself cannot be dated directly (it is too old to be carbon-dated, of course, and it usually contains no mineral sediments that can be dated by other means) so aging it depends on indirect methods such as comparison of enclosed fossils with other aged samples, or dating of the deposits in which the amber is buried. Fossil comparisons suffer from the fact that the types of organisms that tend to be found preserved in amber are usually different from those preserved by other means (so we may know that two amber deposits are of similar age as each other, but we may still not know what that age actually is). Dating of surrounding deposits may be more straightforward, but is complicated by the fact that amber's relative buoyancy makes it prone to reworking (when a geological specimen becomes eroded out of its original formation and reburied in a younger one, thus making it appear younger than it is). Of course, if an amber deposit was produced over a long period of time, it may be impossible to tell if a particular piece comes from early or late in that period. Current consensus seems to indicate an Eocene age for the Baltic amber, but older references may refer to it as Oligocene or even Miocene.

Most of the insects we find preserved in Baltic amber are similar to those we find today (though differences in climate between now and then mean that the amber contains a number of groups that we would not expect to find so far north today). The specimen shown at the top of this post is assigned to a genus of mayfly (of the diverse family Leptophlebiidae) that remains widespread in Europe and North America today, of which it represents the earliest record. Like other members of the genus, it is a relatively small mayfly with a wingspan of less than 15 millimetres (I came across this fly-fishing website referring to the frustration of anglers trying to handle such small flies). Several specimens of this species are known from the Baltic amber, representing the subimaginal and imaginal stages of both sexes (mayflies are unique among living insects in that their wings become functional before they are fully mature; when a mayfly first emerges from the water it is as a near-mature subimago, subsequently moulting to a fully mature imago). Presumably, like modern mayflies, Paraleptophlebia prisca emerged as synchronised swarms, many individuals of which may have found themselves landing on an unwisely chosen tree-trunk and trapped within weeping sap.

This species was first described in 1856 by F. J. Pictet-Baraban along with other species of 'Neuroptera' from Baltic amber (at the time, the order Neuroptera included a wide range of insects with relatively unspecialised wings such as mayflies, dragonflies, bark-lice and lacewings). Pictet assigned it to the genus Potamanthus, another Recent mayfly genus albeit one in a different family Potamanthidae. He did express some uncertainty about this placement, an uncertainty that was later borne out by Demoulin (1968) who re-identified it as Paraleptophlebia on the basis of wing venation and male genital characters. Subsequent authors have agreed with Demoulin's assessment, regarding the mayflies of today as little different from those you might have found flying about over 50 million years ago.

REFERENCES

Demoulin, G. 1968. Deuxième contribution à la connaissance des Éphéméroptères de l’ambre oligocène de la Baltique. Deutsche Entomologische Zeitschrift, N. F. 15 (1–3): 233–276.

Penney, D., & J. E. Jepson. 2014. Fossil Insects: An introduction to palaeoentomology. Siri Scientific Press: Manchester.

Pictet-Baraban, F. J., & H. Hagen. 1856. Die im Bernstein befindlichen Neuropteren der Vorwelt. In: Berendt, G. C. Die im Bernstein befindlichen organischen Reste der Vorwelt vol. 2 pp. 41–126. Nicolaischen Buchhandlung: Berlin.

We've Got a Thing that's called Foram Love

Pileolina patelliformis, from Brady (1884).

It's been a while since we last had a foram post, so why don't we have one today? Ladies and gentlemen, I present to you: the Glabratellidae.

Glabratellids are a group of forams found in littoral habitats, first appearing in the fossil record in the Eocene (Loeblich & Tappan 1964). They secrete a calcareous test with a hyaline (glass-like) microstructure. By foram standards, glabratellids can be quite small: the smallest are well under 100 µm in diameter. They have a trochospiral body shape—that is, the body chambers are arranged in such a way that they spiral like a trochus or top shell—with a flat base. At the centre of the underside is an aperture or umbilicus. The spire may be fairly low, giving them what I always think of as a 'jelly mould' shape, or it may be high so their overall appearance is conical. In the genus Schackoinella, the test bears a spine on the outside of each of the body chambers.

The glory that is Schackoinella sarmatica, from the Geological Survey of Austria.

The most distinctive feature of glabratellids, perhaps, is their life cycle. We know the life cycles of relatively few foram species but as a rule they show a clear alternation of generations, with both well-developed haploid and diploid individuals. Haploid individuals (gamonts) produce gametes by nuclear mitosis that fuse to form zygotes that grow into mature diploid individuals (schizonts or agamonts); these latter produce haploid embryos via meiosis. The two generations may differ somewhat in appearance, and many foram species have had their gamonts and schizonts mistaken in the past for separate species. The most consistent difference between generations in all chambered forams is that the gamonts have a larger first chamber as a result of growing from larger embryos than the schizonts. In glabratellids, gamonts are also smaller and relatively higher-spired than schizonts, and the former are sinitrally coiled (to the left) while the latter are dextrally coiled (to the right).

The life cycle of Glabratellidae was described in detail by Loeblich and Tappan (1964) (the figure to the the left from therein shows the lifecycle of Pileolina patelliformis). Schizonts herald the production of offspring by wrapping themselves in a protective cover of dead diatoms and other rubbish. Young gamonts are formed by nuclei dividing in the test and each becoming surrounded by their own individual cell membranes. After they form, the embryonic offspring crawl around in the parent test feeding on any leftover cytoplasm and also on the test itself. By the time they grow to about two or three chambers in size, the gamonts dissolve the umbilical wall of the parent test and escape through the aperture.

As the gamonts themselves reach maturity, their thoughts no doubt turn to their own posterity. Whereas in some other forams the haploid generation simply releases their gametes into the water column to find their own way to fusion, sexual reproduction in glabratellids is a somewhat more intimate affair. Mature gamonts form into pairs, joined to each other via their umbilical surfaces from which they will resorb the test. Locked in their embrace, the pair become cemented to the substrate. Gametes, again, are formed by the production of plasma membranes around individual nuclei; these gametes move by means of three flagella instead of by pseudopodia. The two parents exchange gametes of which only about a tenth fuse to form zygotes; the remainder provide a food source for their developed siblings. Again, the young schizonts grow to about two or three chambers in size before being released by the dissolution of the cement holding the parent tests together.

This cosy mode of reproduction means that glabratellids may have the potential for greater population differentiation than other broadcast-spawning Foraminifera. Tsuchiya et al. (2003), in a study genetic diversity in representatives of the genus Planoglabratella collected around Japan, found evidence for cryptic speciation in P. opercularis. Some individuals of this 'species' were closer genetically to individuals of another species P. nakamurai than to other P. opercularis, and closer inspection revealed certain details of their morphology that were more nakamurai-like than opercularis-like. It may be that we have underestimated the diversity of glabratellids, and many more species of this group remain to be discovered.

REFERENCES

Loeblich, A. R., Jr & H. Tappan. 1964. Treatise on Invertebrate Paleontology pt C. Protista 2. Sarcodina: chiefly "thecamoebians" and Foraminiferida. The Geological Society of America, and The University of Kansas Press.

Tsuchiya, M., H. Kitazato & J. Pawlowski. 2003. Analysis of internal transcribed spacer of ribosomal DNA reveals cryptic speciation in Planoglabratella opercularis. Journal of Foraminiferal Research 33 (4): 285–293.