Field of Science

Predatory Ribbons

Some of you may have seen something like this doing the rounds:

The animal in the clip is called a nemertean. The Nemertea, commonly known as ribbon worms, are a group of more than 1200 known species of mostly predatory worm-like animals. The majority of nemerteans are marine, but there are also species found in freshwater or even terrestrial environments (the clip above shows a terrestrial species). As their vernacular name suggests, the majority of ribbon worms are flattish, slender animals with little in the way of external elaborations. Most are small and unassuming, but there are exceptions: one ribbon worm species from coasts of northern Europe, Lineus longissimus, grows to estimated lengths of over 30 m and may even be the longest animal in existence*. The most characteristic feature of ribbon worms is a long proboscis that they use in capturing prey; when not being deployed, this proboscis is retracted within a cavity called the rhynchocoel that runs much of the animal's length. Other than this, nemerteans have little in the way of internal body cavities other than the gut. They do have a simple blood-vascular system consisting of a few blood vessels but no actual heart; instead, the blood just kind of sloshes back and forth as a result to the animal's body contractions as it moves.

*Some uncertainty over the exact lengths of Lineus longissimus specimens is inevitable because, despite their remarkable length, they are still only a centimetre or so wide. When you're trying to extract something like that from among a bunch of rocks, it's gonna stretch and break. Still, thirty metres is a fairly conservative estimate of its length; Wikipedia cites a supposed maximum nearly twice that. These mega-nemerteans are definitely one of those animals that make me wonder, how does this thing even exist? I mean, what is the point of being so incredibly long and slender? How does it collect enough food at the front end to nourish itself all the way to the back end? How does it not just fall apart of its own accord, let alone when subjected to any external pressure?

Lineus longissimus, from here.


The relationships of nemerteans to other animals are rather uncertain, and they have generally been classified as their own independent phylum. Because of their simple body plan, many early authors compared them to flatworms, at least on a grade level, but this fell out of favour as it became accepted that the rhynchoel and blood-vascular system probably correspond to anatomical structures in more complex animals. More recent evidence from molecular and other sources has converged on a position within the Lophotrochozoa, the major animal clade that also includes molluscs, brachiopods and annelids, but their exact placement within this clade remains open to debate.

Molecular data have also influenced our understanding of relationships within the Nemertea. An influential classification of the group divided them between the Enopla, in which the proboscis is usually armed with a stabbing stylet or stylets, and the Anopla, in which the proboscis is unarmed (members of this latter group often have the proboscis branched as in the clip above; I'm guessing that in the absence of a stylet the proboscis probably works through adhesion). The two groups also differ in that Anopla always have the proboscis emerging from a separate pore to the mouth, whereas in many Enopla the mouth and proboscis pore share a common opening (Kvist et al. 2014). The Anopla were further subdivided into the Heteronemertea, which have a distinctive tissue layer called the dermis underneath the outer epidermis, and the Palaeonemertea which lack such a differentiation of skin layers. However, one need not be an expert in nemerteans to spot that the Anopla and Palaeonemertea were mostly defined by their lack of derived features (no stylets, no dermis) and so it should come as little surprise that molecular studies of the group have failed to offer resounding support for their monophyly. Instead, a number of studies have suggested that the Heteronemertea and Enopla together form a clade that Thollesson & Norenburg (2003) dubbed the Neonemertea. When they did so it was on the basis of molecular data only, but later authors have identified possible synapomorphies of the Neonemertea in features of the nervous and blood-vascular systems. One family of 'Palaeonemertea', the Hubrechtidae, has been suggested to also belong within the Neonemertea as sister-taxon to the Heteronemertea. This is of interest because the Hubrechtidae and Heteronemertea share a distinctive type of ciliated planktonic larva called a pilidium (other nemerteans either develop directly or have a creeping planula-type larva). Ciliated planktonic larvae are known a number of groups of animals, such as the veliger of molluscs, the trochophore of annelids, or the tornaria of acorn worms, and there has been a lot of discussion over the years as to whether similarities between these larvae represent a shared ancestry, or whether they might have evolved independently. In the case of nemerteans, at least, the current evidence seems to favour the latter. As for the other 'palaeonemerteans', there seems to be less of a consensus as to whether they form a single clade or a paraphyletic series relative to the Neonemertea.

A polystiliferan, Drepanogigas albolineatus, copyright Peter Wirtz.


As for the Enopla, it appears to form a valid clade. Previous authors divided the enoplans between the Hoplonemertea, including the majority of species, and the Bdellonemertea, including the single distinctive genus Malacobdella. The Hoplonemertea were in turn divided between the Monostilifera, in which the proboscis has a single long stylet, and the Polystilifera, in which it bears a pad of small stylets, and molecular analyses support the separation of these groups. Malacobdella (which lacks proboscis stylets but has the conjoined mouth-proboscis pore) has a sucker at the posterior end of its body, by which it lives attached to the gills of a mollusc. Malacobdella is not a parasite of the mollusc, per se: instead, it feeds on food particles drawn in by water flowing through the mollusc's gills. However, the recent analyses have indicated that Malacobdella is in fact a derived monostiliferan, and a number of recent authors have used the Hoplonemertea as an equivalent name to the old Enopla.

Live individual of the pelagic nemertean Dinonemertes shinkaii (head towards the right), from here.


Also distinctive within the Hoplonemertea are two clades, the polystiliferan Pelagica and the monostiliferan Korotkevitschiidae, that have left the ocean floor and adopted a pelagic life style. Members of both these groups are gelatinous and eyeless; the Pelagica have lost further internal organs such as nephridia. The Korotkevitschiidae (which also lack a proboscis stylet) are found towards the surface of the ocean; the Pelagica are found in much deeper waters (Chernyshev 2003). The pelagic nemerteans are among the most poorly known of all ribbon worms; they are rarely encountered (about half of the 100 or so described species of Pelagica are known only from single specimens) and their relatively simple morphology makes them difficult to compare to other nemerteans. If the individual in the photograph is any indication, however, they are beautiful animals.

REFERENCES

Chernyshev, A. V. 2003. Classification system of the higher taxa of enoplan nemerteans (Nemertea, Enopla). Russian Journal of Marine Biology 29 (Suppl. 1): S57–S65.

Kvist, S., C. E. Laumer, J. Junoy & G. Giribet. 2014. New insights into the phylogeny, systematics and DNA barcoding of Nemertea. Invertebrate Systematics 28: 287–308.

Thollesson, M., & J. L. Norenburg. 2003. Ribbon worm relationships: a phylogeny of the phylum Nemertea. Proceedings of the Royal Society of London Series B—Biological Sciences 270: 407–415.

Tachytes: Crickets Face Death from Above

A Tachytes species feeding, copyright Stephen Cresswell.


Most people who are not entomologists assume that 'ants', 'bees' and 'wasps' are all mutually exclusive groups of animals. But while bees and ants are distinct from each other, they are both really distinctive subgroups of wasps. It is not difficult to find guides on the interwebs purporting to tell you the differences between a bee and a 'wasp', but many of the points usually cited will not apply to all wasps (usually what is intended is the differences between a bee and a social wasp, which is the type of non-bee, non-ant wasp most likely to make itself known to humans). Some wasps can look very similar to bees indeed.

Diagram of Tachytes ocelli, from here.


Tachytes is a large, cosmopolitan genus of not-quite-bees, including more than 250 species found on all continents except Antarctica. Its members are robust and hairy, and one would have to look very closely to spot the features marking it as a non-bee (such as the point that its hairs, though numerous, are not branched in the manner of a true bee). Tachytes is classified in the Crabronidae, the family of wasps believed to be most closely related to true bees (other crabronids have been featured in earlier posts: here, here and here). Like other crabronids, adult Tachytes are pollinators, feeding on nectar from flowers. Tachytes species can be separated from most other crabronids by the shape of the ocelli. As well as the two large compound eyes with their multiple lenses on either side of the head, many insects have three small single-lens eyes, called the ocelli, on the top of the head (some insects have fewer or no ocelli). When all three ocelli are present, they are arranged in a triangle with one at the front and two at the rear. In Tachytes, the rear ocelli are present but deformed (I presume non-functional, though I don't actually know). They are reduced to a pair of scar-lines shaped roughly like a comma, or the upper part of a question mark without the dot.

Female Tachytes sinensis sinensis with katydid prey, from here.


Like many other wasps, female Tachytes dig burrows in which they they sequester other insects in a paralysed state to provide food for their young (the big change between bees and other wasps was the provision of their larvae with a plant-derived food source such as honey or pollen instead of animal matter). The size of the burrow varies from species to species, but some are quite extensive: the North American species T. praedator digs a burrow with a centimetre-wide entrance about a metre long, with a 70 cm down-shaft followed by a horizontal run of about a foot (Lin 1967). This species digs at night, thus presumably both avoiding the heat of the day and reducing the risk of detection by predators or parasites. The top of the burrow is usually marked by a heap comprised of the removed soil from its digging; in some species, the female will close over the top of the burrow when not actively digging. A series of cells are constructed branching from the end section of the burrow; each of these cells is filled with enough prey to feed one larva, then an egg is laid in the cell and its entrance sealed. Most Tachytes species feed their larvae with Orthoptera such as crickets, grasshoppers or katydids; the exact type preferred differs between species, though I get the impression of a general correlation between the size of the Tachytes species and the size of its preferred prey. At least two exceptional Tachytes species from central Asia (T. ambidens and T. bidens) are known to supply their larvae with small caterpillars rather than orthopterans. The prey is carried by the female held by her legs, with the antennae clasped by the mandibles. Hunting behaviour has been described for two species from North America, T. intermedius and T. mergus, that stock their nest with pygmy mole crickets (Tridactylidae). Females of these species walk along the ground tapping at it with their antennae. When one locates a cricket in its underground burrow, she quickly digs downwards in an attempt to grab the cricket by its head with her mandibles and haul it out. Even if the cricket hears her coming and attempts to flee from its burrow, it may not escape. The wasp moves so quickly that she is often able to grab the cricket in mid-air as it leaps for freedom. These two Tachytes species also differ from others in the genus in that the crickets are sequestered in the nest not fully paralysed: it has been noted that mole crickets recovered from their nests are still quite mobile and even able to jump.

REFERENCES

Bohart, R. M., & A. S. Menke. 1976. Sphecid Wasps of the World: A generic revision. University of California Press.

Lin, C. S. 1967. Nesting behavior of Tachytes (Tachyplena) praedator Fox, with a review of the biology of the genus (Hymenoptera: Sphecidae: Larrinae). American Midland Naturalist 77 (1): 241–245.

Mites from a Land of Ice and Snow

Mycobates sarekensis, from Siepel & Dimmers (2010).


The Arctic tundra is not an inviting place. Cold winds sweep through a forebidding landscape, unhindered by forest. In places, patches of bare rock can be seen, with no vegetation able to retain a foothold other than hardy lichens. And yet even here you can find an entire ecosystem in place if you look closely enough.

The animal shown at the top of this post is an oribatid mite of the genus Mycobates, a group of about 35 species belonging to the family Punctoribatidae (sometimes referred to as Mycobatidae). These are sturdy, stocky oribatids with a body that is oval in cross-section, with a length generally around the half-millimetre mark (Seniczak et al. 2015). Characteristic features of Mycobates include pteromorphs (triangular outgrowths of the body wall that hang down over the bases of the legs) that are hinged by a line of weaker cuticle so they can be moved up and down, a convex pedotectum I (another protruding shelf, this time on the underside of the body shielding the base of the first pair of legs) and overlapping lobes at the back of the body where the dorsal shield (the notogaster) overhangs the edge of the venter (Behan-Pelletier 1994). Their legs are curved inwards towards the body, and the dorsal setae are usually smooth, without barbs, and flexible (Seniczak et al. 2015).

SEM of Mycobates beringianus from Behan-Pelletier (1994). Note the cluster of pores visible as a lighter patch on the side of the notogaster; these are secretory or respiratory structures.


Many of these features are suited to the preferred habitat of a number of species in the genus: burrowing through the thalli of lichens (which are both home and food). Mycobates species are found in cooler boreal and alpine habitats. Species found in more Arctic habitats, such as the tundra-dwelling M. sarekensis, are found close to ground level (in the tundra, there's not many other levels to be found at). In more temperate regions, they are often arboreal, crawling about on the trunks and branches of trees. Some species have been found in association with mosses as well as lichens; many of the northern species are able, snuggled as they are in their moss or lichen hosts, to live in microhabitats too dry for many other invertebrates.

REFERENCES

Behan-Pelletier, V. M. 1994. Mycobates (Acari: Oribatida: Mycobatidae) of America north of Mexico. Canadian Entomologist 126: 1301–1361.

Seniczak, S., A. Seniczak & S. J. Coulson. 2015. Morphology, distribution and biology of Mycobates sarekensis (Acari: Oribatida: Punctoribatidae). International Journal of Acarology 41 (8): 663–675.

Mintbush Genus Limits

Victorian Christmas bush Prostanthera lasianthos, copyright Melburnian.


Despite (or perhaps because of) the severity of Australia's climate over much of the continent, the country has become famed for its wildflower displays. At the right time of year, the otherwise bleak landscape becomes a riot of form and colour. The display shown above belongs to a species of the genus Prostanthera, an assemblage of about 100 species of bushy shrubs (very rarely small trees) known as mintbushes, endemic to yet ubiquitous around Australia (new species continue to be described at fairly regular intervals). As suggested by their vernacular name, mintbushes belong to the mint family Lamiaceae, the same family as many well-known garden herbs such as sage, rosemary or thyme. Like these relatives, mintbushes have strongly aromatic foliage, due to the presence of glands secreting volatile oils on the leaves. However, the edibility of most species is unknown; I did find a couple of references to culinary uses of the round-leaved mintbush Prostanthera rotundifolia though it is not common (and a couple of comments in this thread suggest that it may be a bit pungent for regular use). Certainly, to push the pun in this post's title far further than it deserves, there is no evidence of mintbush gin.

Prostanthera species are most readily recognised by their flowers (Wilson et al. 2012). The calyx at the base of the flower has the sepals fused so that it is shaped as two lips, an upper and a lower. The corolla of petals has five lobes, two in the upper lip and three in the lower. There are four anthers, which often (though not always) have a distinct basal appendage; it is this appendage that gives the genus its name (from the same Greek word that gives us the term 'prosthetic'). Different species have flowers in a wide range of colours, and many Prostanthera species have become popular ornamentals.

Flower of scarlet mintbush Prostanthera aspalathoides, copyright Patrick Kavanagh.


Prostanthera is a member of a tribe of Australian Lamiaceae known as the Westringieae, members of which have a dry fruit splitting into four sections (Conn 1984). The two-lobed calyx of Prostanthera separates it from most other genera that have been recognised in the Westringieae except for a small genus called Wrixonia. The only significant difference between Wrixonia and Prostanthera is that whereas the latter retains four fertile anthers, the former has one pair of anthers sterile and reduced. A molecular phylogenetic analysis by Wilson et al. (2012) found that Wrixonia species were nested within Prostanthera, raising doubt about whether Wrixonia should be recognised as a separate genus. Also of interest was the relationship between the two sections into which Prostanthera has been divided: section Prostanthera and section Klanderia. These sections differ in characteristics of their flowers. Prostanthera section Prostanthera has flowers that are white, mauve or blue, with a corolla in which the central lower lobe is longer than the others so the overall appearance is similar to an orchid (such as in the P. lasianthos at the top of this post). In section Klanderia, the flowers are green, yellow or red, and the two upper lobes of the corolla are the longest so the appearance of the flower is more tubular (such as in the P. aspalathoides just above). Some authors have regarded the difference between two sections as enough to warrant recognised section Klanderia as a separate genus (in which case it becomes known as Cryphia, because botanical nomenclature is complicated like that). The two sections differ in flower morphology because they differ in pollinator type: flowers of section Prostanthera are pollinated by insects, whereas flowers of section Klanderia are pollinated by birds. Again, Wilson et al. (2012) found that the larger section Prostanthera, which retains the ancestral pollinator type, is paraphyletic with regard to the derived section Klanderia.

REFERENCES

Conn, B. J. 1984. A taxonomic revision of Prostanthera Labill. section Klanderia (F.v.Muell) Benth. (Labiatae). J. Adelaide Bot. Gard. 6 (3): 207–348.

Wilson, T. C., B. J. Conn & M. J. Henwood. 2012. Molecular phylogeny and systematics of Prostanthera (Lamiaceae). Australian Systematic Botany 25: 341–352.

Look Away: Chameleons

Mediterranean chameleon Chaemaeleo chamaeleon, copyright Benny Trapp.


For my next post, I drew the topic of 'Chamaeleonidae', the chameleons*. This left me with a bit of a quandary because Darren Naish over at Tetrapod Zoology covered the chameleons recently in his usual exhaustive style in a series of three posts (part 1, part 2, part 3). I was somewhat tempted to simply tell you all to go read Darren's posts and consider my work done, but let's see if I can dig up anything he left out. You should still go read Darren's posts anyway.

*I'm using Chamaeleonidae in the restricted sense here, the one that will probably be familiar to most people. Some authors have suggested expanding Chamaeleonidae to also include members of the dragon family Agamidae, following the recognition that the latter in its traditional sense is paraphyletic. This suggested re-classification does not appear to have caught on widely.

Chameleons are certainly one of the most distinctive of lizard groups, with their clasping zygodactylous foot structure, periscopic eyes and projectile tongues. They are most diverse in Africa, with only the genus Chamaeleo extending into southern Europe (just barely) and south-western Asia to India. The name is Greek in origin and can be read as 'ground lion'; presumably someone thought that the European chameleon's hissing threat display looked a bit like an imitation of a lion's roar. The most familiar chameleons are primarily arboreal, creeping slowly along branches, but the smaller leaf chameleons are terrestrial and this may represent the ancestral habit for the family (Tolley et al. 2013).

The tiny leaf chameleons, some of which are less than two centimetres long when mature, have a morphotype that is best described as 'completely daft'. This is a bearded leaf chameleon Rieppeleon brevicaudatus, copyright Fridtjof Busse.


Chameleons are most famous, of course, for their colour-changing abilities. Most of you will probably be aware that said colour changes are related to social signalling rather than camouflage... except that, in a sense, they are related to camouflage too, because a chameleon that drops its signal colours becomes a lot better concealed. The colour is managed through guanine crystals in the integument: changing the distance between crystals changes the wavelength reflectance of the light. The degree to which chameleons change colour varies from species to species. Some merely change their overall shade from darker to lighter (which I suspect may be related to body temperature regulation as much as anything else) whereas others reveal bright lurid patterns of flouro stripes and blotches. I haven't come across any indications whether there are any chameleon species that don't change colour at all, nor do I know what the distribution of colour-changing is like in other lizard groups. I do have a vague memory that thorny devils Moloch horridus (belonging to the related Agamidae) become duller in colour when they are colder, but I'm not certain about this. And I'm going to cite one recent paper on colour signalling in chameleons by Ligon (2014) purely for its epic title: "Defeated chameleons darken dynamically during dyadic disputes to decrease danger from dominants".

Male Malagasy giant chameleon Furcifer oustaleti, copyright Drägüs.


Because of their Africa-centric distribution, chameleons have generally been assumed to have originated on that continent. The fossil record of chameleons is pretty abysmal (probably because they tend not to frequent habitats conducive to fossilisation) though Miocene fossils do indicate a wider distribution in Europe during warmer times. The European fossil species are included in the genus Chamaeleo, though I'm not sure if this indicates a close relationship with the modern European species or is simply an artefact of when most larger chameleons were included in this genus. The eastern islands of Africa are home to three independent clades of chameleons: the leaf chameleons Brookesia and a clade of larger chameleons containing the genera Furcifer and Calumma in Madagascar, and the Seychelles endemic Archaius tigris. Though Brookesia represents the sister clade to the remaining chameleons, current directions make it likely that each of these clades represents a dispersal from African ancestors* (it is unlikely that chameleons are old enough for the clades to have been separated by plate tectonics). One announcement that was recent enough to have missed Darren Naish's posts (though it made it into the ensuing comments) was the discovery of a close relative of the chameleons preserved in Burmese amber from the Cretaceous period. This specimen (which has not yet been given a scientific name beyond the collection number of JZC Bu154) is, at less than 11 mm long, probably a very young juvenile, though even when adult it may have only been in the size range of the tiny Brookesia leaf chameleons. JZC Bu154 retains a number of ancestral features relative to modern chameleons, such as a non-clasping foot structure, so if it is related to the chamaeleonids it is undoubtedly in the stem group (Daza et al. 2016). As such, its Burmese provenance does not contradict an African origin for crown-group chameleons.

*At least one chameleon has dispersed in the opposite direction by other means: thanks to human transportation, the Malagasy giant chameleon Furcifer oustaleti established a small population in the vicinity of Nairobi, though it appears doubtful whether this still survives. Two other species, the veiled chameleon Chamaeleo calyptratus and Jackson's chameleon Trioceros jacksonii, native to the Arabian Peninsula and east Africa, respectively, have been introduced to some parts of the United States, most notably Hawaii.

REFERENCES

Daza, J. D., E. L. Stanley, P. Wagner, A. M. Bauer, & D. A. Grimaldi. 2016. Mid-Cretaceous amber fossils illuminate the past diversity of tropical lizards. Science Advances 2: e1501080.

Tolley, K. A., T. M. Townsend & M. Vences. 2013 Large-scale phylogeny of chameleons suggests African origins and Eocene diversification. Proceedings of the Royal Society B 280: 20130184.

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.