The Tuna-Lizards

The classic ichthyosaur Ichthyosaurus communis, from here.

Ichthyosaurs have long been one of the most famous examples of convergent evolution. These Mesozoic marine reptiles, as any textbook will tell you, evolved a body form similar to that of modern dolphins and sharks, and presumably held a similar niche as fast-swimming apex predators. But interesting as that might be, it's certainly not all there is to be said about ichthyosaurs.

The classic ichthyosaurs that said textbooks will usually depict are members of the clade Thunnosauria that first appeared in the upper Triassic (Thorne et al. 2011). Thunnosaurs differ from other ichthyosaurs in having a relatively short tail, shorter than the trunk, and hindfins that are much shorter than (usually less than half as long as) the forefins (Maisch & Matzke 2000). The name 'Thunnosauria' appropriately means 'tuna-lizards': as with modern tunas, the compact body of the thunnosaurs indicates greater specialisation for more powerful, tail-driven swimming.

Cast of the short-beaked Ichthyosaurus breviceps, from Charmouth Heritage Coast Centre.

In the Lower Jurassic, thunnosaurs are represented by the genera Ichthyosaurus and Stenopterygius, though the known fossil record for the former is earlier than that of the latter. Both genera are represented by hundreds (if not thousands in the case of Stenopterygius) of known specimens from Europe (Motani 2005): primarily England for Ichthyosaurus, Germany for Stenopterygius. Stenopterygius grew up to 4 m in length; Ichthyosaurus would have been somewhat smaller (Maisch & Matzke 2000). One species of Ichthyosaurus, I. breviceps, stands out for its particularly short and robust rostrum in comparison to other species. Another potential Lower Jurassic thunnosaur is Hauffiopteryx typicus, which also has a distinctively small rostrum, but in this case a particularly fine and slender one (Maisch 2008).

Mounted skeleton of Ophthalmosaurus icenicus, from the British Natural History Museum.

During the Lower Jurassic, the thunnosaurs were among a number of ichthyosaur lineages present. By the time of the Upper Jurassic, all surviving ichthyosaurs (with one possible exception*) belonged to a single thunnosaur lineage, the Ophthalmosauridae. Unfortunately, for most of the Middle Jurassic the ichthyosaur fossil record is missing, and a gap of more than ten million years separates Stenopterygius from Ophthalmosaurus. The only break in this gap is the Argentinan Chacaicosaurus cayi, which sits a few million years later than Stenopterygius. Intriguingly, Chacaicosaurus is not only intermediate in age, it is intermediate in morphology: while its skull is similar to that of Ophthalmosaurus, its forefin is more similar to that of Stenopterygius. As noted by Maisch & Matzke (2000), "It appears as if Chacaicosaurus cayi is one of the rare forms that are true structural intermediates".

*The possible exception is the Upper Jurassic Nannopterygius enthekiodon, some features of which suggest that it occupies a more basal Stenopterygius-grade position (Maisch & Matzke 2000). Unfortunately, it has not yet been adequately described and included in a formal phylogenetic analysis. This is rather frustrating: Nannopterygius promises to be a quite distinctive animal, with greatly reduced fins and long spinal processes on the anterior tail vertebrate.

Reconstruction of Platypterygius bannovkensis, by Olorotitan. Platypterygius was the latest surviving ichthyosaur genus.

The ophthalmosaurids survived from the late Middle Jurassic to the early Upper Cretaceous. Ophthalmosaurus had a slender rostrum with reduced dentition, while other genera such as Brachypterygius and Platypterygius had higher, more robust rostra with their full complement of teeth. Some ophthalmosaurids grew very large: Platypterygius reached up to 9 m. The name Ophthalmosaurus means 'eye lizard', and reference to the large eyes of this ichthyosaur seems to be de rigeur for any popular book in which it features, together with some speculation that it may have been a nocturnal hunter. However, a quick scan through the various ichthyosaur skulls illustrated by Maisch and Matzke (2000) indicates that ichthyosaur eyes were generally large. Those of Ophthalmosaurus were not the largest; the eyes of Eurhinosaurus longirostris are particularly ridiculous, with orbits filling almost the entire side of the cranium! So perhaps the question should not be why Ophthalmosaurus had large eyes, but why those ichthyosaurs without large eyes had reduced them.

REFERENCES

Maisch, M. W. 2008. Revision der Gattung Stenopterygius Jaekel, 1904 emend. von Huene, 1922 (Reptilia: Ichthyosauria) aus dem unteren Jura Westeuropas. Palaeodiversity 1: 227-271.

Maisch, M. W., & A. T. Matzke. 2000. The Ichthyosauria. Stuttgarter Beiträge zur Naturkunde Serie B (Geologie und Paläontologie) 298: 1-159.

Motani, R. 2005. True skull roof configuration of Ichthyosaurus and Stenopterygius and its implications. Journal of Vertebrate Paleontology 25 (2): 338-342.

Thorne, P. M., M. Ruta & M. J. Benton. 2011. Resetting the evolution of marine reptiles at the Triassic-Jurassic boundary. Proceedings of the National Academy of Sciences of the USA 108 (20): 8339-8344.

The Source

I was taking some photos today of the new house to send to my parents in New Zealand, when I thought I might take some extras to put up here and demonstrate the current state of my office. You never know, someone might be interested. I'm somewhat anachronistic in that I do still largely work from printed material rather than pdfs, and the ghosts of a thousand trees probably haunt my workspace.

The person who is able to identify the most of the books visible in these photos wins the grand prize of having identified the most books in these photos. Of course, most of my reference collection is not quite so photogenic:

Most of my papers lurk in large filing cabinets, while the boxes contain copies of particularly lengthy papers and out-of-print or otherwise unobtainable books that I haven't yet gotten ring-bound like the ones on the shelves.

Hat-tip to Darren Naish, of whom this post is something of a blatant rip-off.

Tetragraptines

Colonies of Tetragraptus quadribrachiatus, from the University of Oslo.

In preparation for this post, I have been attempting to develop an understanding of graptolite branching patterns. This is not something that should be attempted lightly, if at all. If anything in this post seems confused, it's because it is.

The Tetragraptinae were a group of graptolites that lived during the Lower Ordovician, and formed part of the early radiation of planktonic graptoloids. In one of the earlier phylogenetic (or at least quasi-phylogenetic) classifications of graptolites, that of Fortey & Cooper (1986), the tetragraptines (including the genera Tetragraptus and Pseudophyllograptus) were recognised on the basis of what was called the 'Tetragraptus serra proximal type'. In an earlier post, I explained how graptolite colonies grew as a series of branching zooids (individuals). The colony section for each individual zooid is called the theca, and graptolite workers usually refer to the thecae in discussions rather than the zooids (as the zooids are generally not preserved in fossils). A developing colony starts with the initial larval zooid, called the sicula. Out of the side of the sicula grows the first mature theca, which is referred to as th1¹ (the sicula is not included in the thecal count because it has a different growth pattern from the sequential thecae). The second theca, th1², then buds off from th1¹. The third theca to arise is th2¹, then th2², then th3¹, and so on and so forth. If all these bud in a simple sequence, the colony is not branching. However, if one or more of these basal thecae is what is known as a dicalycal theca (it produces two daughter thecae instead of just one), the colony branches. In most tetragraptines, th1² is a dicalycal theca, as are its two daughter thecae, so the mature colony has four branches. The basal canals of th1² and th2¹ crossing over the sicula, plus the proximal part of th2², make the lower part of the proximal region very robust: this massiveness is what characterises the Tetragraptus serra proximal type. Other characters listed by Fortey & Cooper (1986) as synapomorphies for the Tetragraptinae, reclined colony branches and a reduction in the number of branches, were also found in other lineages.

Proximal region of Tetragraptus bigsbyi, showing robust morphology, together with diagrammatic representation of thecal connections in early colony. From Bulman (1970).

The Tetragraptinae were one of a number of groups of Ordovician graptolites with four-branched colonies, though other taxa lacked the T. serra proximal region. In a phylogenetic analysis of graptoloids, Maletz et al. (2005) identified four-branched graptoloids as a single clade that they called the Tetragrapta. This is in contrast to Fortey & Cooper (1986), who placed these taxa at a number of places in the graptoloid tree. The analysis of Maletz et al. (2005) differed from that of Fortey & Cooper (1986) in being a computational analysis rather than being constructed 'by hand'. Some characters given high weight by Fortey & Cooper (1986), such as the presence of a structure called a virgella, were found to be less significant by Maletz et al. (2005). However, in some regards the coverage of the latter study was less complete than the earlier. Most notable for the present post is that Fortey & Cooper (1986) had also included 'Dichograptus' solidus in the Tetragraptinae. This species apparently also has the T. serra proximal region, but also has more than four branches in the colony. It is possible that its inclusion in a computational analysis would weaken the association of four-branched graptoloids as a clade.

By the end of the Ordovician, the graptoloid lineages with multi-branched colonies were extinct. There have been numerous suggestions for why this may have happened—buoyancy issues or competition between zooids are among the front runners—but for the rest of graptoloid history, simplicity would become the watchword.

REFERENCES

Bulman, O. M. B. 1970. Graptolithina with sections of Enteropneusta and Pterobranchia. In Treatise on Invertebrate Paleontology Part V 2nd ed. (C. Teichert, ed.) pp. V1-V149. The Geological Society of America, Inc.: Boulder (Colorado), and the University of Kansas: Lawrence (Kansas).

Fortey, R. A., & R. A. Cooper. 1986. A phylogenetic classification of the graptoloids. Palaeontology 29: 631-654.

Maletz, J., J. Carlucci & C. E. Mitchell. 2009. Graptoloid cladistics, taxonomy and phylogeny. Bulletin of Geosciences 84 (1): 7-19.

Life on Mars: the Cambrian terrestrial environment

The question of when life first moved onto the land has been the subject of speculation for as long as anyone has realised that there was a 'first' to speculate about. Established terrestrial communities were clearly present by the latter part of the Silurian, but was there anything earlier? The reasonable expectation is that there was, at least on some level. Pretty much as soon as there was life inhabiting the oceans in prokaryote form, weather cycles would have been carrying bacteria and their spores onto their land. It is not unreasonable to assume that some of them may have been able to acquire a toehold in some attainable niche, and from there diversify to the surrounding environment. Later, other microbial and simple organisms may have joined them. But such organisms leave little trace in the fossil record. What were they like, how did they live? A paper that has just been published in Palaeontology (Retallack 2011) has described simple terrestrial fossils preserved from the Middle Cambrian, and may provide a rare glimpse of the early Earth.

Reconstruction of Cambrian terrestrial biota from Retallack (2011).

The remains described by Retallack (2011) are extremely simple: flat, thallose impressions called Farghera, subterranean threads known as Prasinema and buried ovoid structures called Erytholus. All of these are described as form taxa: that is, they represent a particular recognisable fossil structure whose relationship to other such fossils is unknown. Different form taxa may even represent different parts of a single organism.

The linear, branching Farghera thalli were an average of just under 2 mm wide, though they could get much wider, and preserved thalli are often several centimetres in length. The living thalli would have been similar to an alga or lichen, either of which they could have been. The thread-like Prasinema are preserved as a central filament less than 1 mm in diameter, surrounded by a dark halo up to about 2.5 mm across. It seems likely that only the central filament represents the original central organism; the halo would have formed by microbes growing around the filaments as they decayed. Prasinema filaments could apparently grow to 30 cm beneath the original soil surface, and probably represent structures similar to fungal hyphae.

Most unusual are the Erytholus, globose structures up to 2 cm in diameter, divided into internal layers with a broad central column. Retallack (2011) suggests a number of possible interpretations for Erytholus: vendobiont or xenophyophore (unlikely because of the terrestrial location), alga (again unlikely, because it is both terrestrial and buried beneath the surface), or fungal or slime mold reproductive structures, comparable to truffles. However, the truffle interpretation is problematic because truffles are produced to disperse spores through being eaten by animals. Obviously, this could not have been the case in the terrestrial Cambrian! A further possibility that I can think of is that Erytholus may have been some sort of resting structure, analogous to a plant bulb or tuber (though note that this interpretation would not necessarily exclude a reproductive function).

As with the Silurian, I think it is important to remember that the environment would have been very different in those days in more ways than one might immediately think. There are parts of the world today where lichens and algae remain the primary ground cover, but we should be careful in assuming that such spots are close analogues of the Cambrian terrestrial environment. Such areas are today arid and/or highly eroded, but in the Cambrian lichens and algae would have also been able to dominate areas in which vascular plants would overshadow them today. I also find myself again wondering what effect the absence of a complex vegetation profile might have had on weather patterns at the time. Would winds have been stronger if there were less low-level wind breaks? Would the effects of rain events have been more catastrophic if water flow was less impeded by ground-cover (if Erytholus was indeed a sort-of-tuber, perhaps it functioned as a source of regrowth if the above-ground component of the organism was destroyed by weather?) If we could see the Cambrian environment for ourselves, there could be no doubt that we would find it utterly alien.

REFERENCE

Retallack, G. J. 2011. Problematic megafossils in Cambrian palaeosols of South Australia. Palaeontology 54 (6): 1223-1242.

Nerites Old and New

Four-toothed nerites Nerita versicolor, from here. Some tropical nerite species can be vary variable in their patterning.

Like many New Zealand kids, I spent a large number of my early days at the beach (my great-grandparents and great-uncle lived beside a bay near Pataua north of Whangarei, and we used to camp there most summers). Most of my time at the beach tended to be occupied with the search for animals under rocks: mud crabs, snapping shrimp, whelks, even the occasional worm. The tops of the rocks would be home to oysters and nerites, and if you pulled a nerite off the rock you could see it close itself up, hiding behind its green and white operculum.

At the time, I wasn't aware of much difference between the nerites and any other marine snail, but there is one. Nerites and their allies, the Neritimorpha (sometimes called Neritopsina) are one of the major basal lineages among gastropods. They have a distinctive protoconch (larval shell), with closely convolute whorls (Frýda & Heidelberger 2003). Most living neritimorphs also dissolve out the columella, the central whorl of the shell, as they grow, so the interior of the shell is a single open cavity. Their shells lack nacre, and are closed with an operculum.

The fossil record of neritimorphs stretches back to the Palaeozoic, though the crown group probably originated close to the Permian-Triassic boundary (Nützel et al. 2007). Among taxa identified as stem neritimorphs in the Palaeozoic are the Naticopsidae (named for their superficial resemblance to the living moon snails, Naticidae) and the Platyceratidae, open-whorled forms that include species found in apparent symbiotic associations with other invertebrates such as crinoids. However, the discovery of preserved protoconches in some 'platyceratids' have demonstrated that, while some species had protoconches comparable to those of modern neritimorphs, others had distinctive open hook-like protoconches unlike those of any other gastropod (Frýda et al. 2009). Those species with the hook-shaped protoconches have been separated out as the Cyrtoneritimorpha, while the modern neritimorphs and those with comparable protoconches are called the Cycloneritimorpha. Despite the similarities in adult shell form between cyrtoneritimorphs and cycloneritimorphs, the distinct protoconch form suggests that the two lineages may not be closely related. However, no features have been identified as yet aligning cyrtoneritimorphs with any other gastropod group, and their true affinities are a mystery.

Neritopsis radula, from here.

Among the crown group neritimorphs, the two living species of the genus Neritopsis are distinctive in being the only species to retain the columella, and molecular analysis corroborates this morphological distinction in identifying Neritopsis as basally divided from most other neritimorphs (Kano et al. 2002). Neritopsis does form a clade with the genus Titiscania, but questions of columella retention are irrelevant for that genus, as its two species lack a shell entirely (instead, they protect themselves from predators by discharging white threads from glands on their back). Neritopsids were abundant during the Mesozoic, but became progressively rarer from about the mid-Cenozoic. The living species of Neritopsis and Titiscania are found in secluded habitats such as submarine caves and crevices under rocks.

The terrestrial neritimorph Helicina clappi, photographed by Robert Pilla.

The clade formed by the remaining neritimorphs was more successful, containing about 450 living species. As well as marine species, they include a number of brackish- or fresh-water taxa, and two families (the Hydrocenidae and Helicinidae) of terrestrial snails. Members of the family Phenacolepadidae are mostly limpet-shaped inhabitants of low-oxygen, sulphide-rich environments underneath rocks or sunken logs. Other families such as the Neritidae have remained mostly more conservative (though the Neritidae also include a limpet-like genus, Septaria), but are widespread throughout the world. The species I encountered as a child, offhand, was Nerita melanotragus—and if anyone out there can tell me why a small snail should have been given a name that appears to mean 'black goat', I'd be interested to know.

REFERENCES

Frýda, J., & D. Heidelberger. 2003. Systematic position of Cyrtoneritimorpha within the class Gastropoda with description of two new genera from Siluro-Devonian strata of Central Europe. Bulletin of the Czech Geological Survey 78 (1): 35-39.

Frýda, J., P. R. Racheboeuf, B. Frýdová, L. Ferrová, M. Mergl & S. Berkyová. 2009. Platyceratid gastropods—stem group of patellogastropods, neritimorphs or something else? Bulletin of Geosciences 84 (1): 107-120.

Kano, Y., S. Chiba & T. Kase. 2002. Major adaptive radiation in neritopsine gastropods estimated from 28S rRNA sequences and fossil records. Proc. R. Soc. Lond. B 269: 2457-2465.

Nützel, A., J. Fŕyda, T. E. Yancey & J. R. Anderson. 2007. Larval shells of Late Palaeozoic naticopsid gastropods (Neritopsoidea: Neritimorpha) with a discussion of the early neritimorph evolution. Paläontologische Zeitschrift 81 (3): 213-228.

What to do with a Dead Hummingbird

We've all been there: that dead hummingbird is just cluttering things up, you don't really know what to do with it, but you don't really want to throw it out because, hey, you never know when that sort of thing might come in handy. Well, fear not! A dead hummingbird can be a very practical thing:

You need never be without a scale bar again!

The above figure, from Archibald et al. (2011), shows a rufous hummingbird Selasphorus rufus alongside the newly described early Eocene giant ant Titanomyrma lubei. This fossil comes from the American Green River Formation, in present-day Wyoming. At 51 mm in length, this is one of the largest known ants, rivalled in the modern fauna only by the marginally longer but possibly less robust driver ant Dorylus wilverthi (I wrote about driver ants in an earlier post). The title of largest ant ever goes, so far as we know, to Titanomyrma giganteum (or Formicium giganteum*) from the Messel Formation of Germany.

*There's a bit of skullduggery in Archibald et al.'s paper viz. the relative status of the pre-existing genus Formicium and their new genus Titanomyrma, whereby Titanomyrma is not diagnostically different from Formicium, but Formicium is relegated to the status of a form taxon for wing fossils only. This is all above board, ICZN-wise, but I'm not sure I'd condone it.

Living giant ants (which, except for Dorylus, are all under 35 mm) are mostly tropical in distribution, but the locality from which Titanomyrma lubei hails would have been within the Arctic Circle when it was alive (Update: Neil has corrected me: the Green River Formation was not Arctic, but northern temperate). The Eocene was a much seamier time than today and, though not tropical, the Arctic would have been far from a frozen wasteland.

REFERENCE

Archibald, S. B., K. R. Johnson, K. W. Mathewes & D. R. Greenwood. 2011. Intercontinental dispersal of giant thermophilic ants across the Arctic during early Eocene hyperthermals. Proceedings of the Royal Society of London Series B—Biological Sciences 278 (1725): 3679-3686.

Ants Go Out in the Noonday Sun

Furnace ants Melophorus carrying a dead earwig back to the colony. Despite the size range visible in the photo, all represent a single species. Photographed by Alex Wild.

If there is one group of organisms that you are guaranteed to see anywhere you go in Australia, it is ants. Especially in the inland arid parts of the country, ants are generally the most prominent insects to remain active and visible during the daylight hours, and they are perhaps the best-studied group of Australian insects (mind you, I work in a lab inhabited primarily by ant specialists, so my impression may be biased).

Melophorus, the subject of today's post, is a genus of ants unique to Australia. It is a member of the Formicinae, the clade of ants distinguished by the production of formic acid through an acidopore at the end of the abdomen, and its species are distinguished from related genera by their slit-shaped propodeal spiracle, metapleural gland and antennae inserted close to the posterior margin of the clypeus. About thirty species have been described to date, but this number is undoubtedly low: for instance, of the 30+ morphospecies identified from the south-west corner of Western Australia, only about a quarter represent named species (Heterick 2009). Estimates of species numbers are complicated by the fact that most, if not all, Melophorus species are polymorphic, though variation is continuous rather than into discrete castes.

Emerging young queen (the large red winged individual) and males (smaller, black) of Melophorus bagoti, from Ken Cheng.

The highest diversity of Melophorus species is found in arid environments, where they forage during the daytime. The best-studied species, the Australian honeypot ant Melophorus bagoti, has the highest recorded heat tolerance of any ant, and forages in air temperatures above 50°C, with ground temperatures in excess of 70°C (Christian & Morton 1992). Foragers of M. bagoti mostly collect the carcasses of dead insects that have expired in the heat, though they will also collect plant material such as seeds and liquid foods such as nectar (other Melophorus species may focus on the latter food supplies). Despite their high heat tolerance, the M. bagoti workers are working on the knife-edge: about one-fifth of a colony's foragers will die each day in these punishing conditions, and the average life expectancy for a forager is only about five days (Muser et al. 2005). Indeed, Muser et al. (2005) estimated that the average forager only makes one successful food collection during its working life, and suggested that the bulk of a colony's food supply was derived from a relatively small core of foragers that managed to beat the odds (the longest forager career they recorded was at least 27 days).

Repletes of Melophorus hanging from a colony ceiling, photographed by Sarah Tahourdin.

Melophorus bagoti is known as the honeypot ant* because, in addition to the normal workers, the colony is home to specialised workers called repletes. The repletes do not leave the colony to forage; in fact, they probably barely move at all. Foragers collecting liquid food will, upon returning to the colony, pass their collections on to a replete. The replete's abdomen swells enormously as it fills with food, transforming the replete into a living food store, ready to pass its cache back to hungry workers that approach it for feeding. Needless to say, honeypot repletes are also of interest to predators, including humans, who are quite happy to take advantage of these sweet pre-packaged morsels when they find them.

*Melophorus species as a whole have been called 'furnace ants' due to their high heat tolerance.

Repletes, frozen and used to add flair to desserts. Photograph by Peter Menzel.

It has been suggested that Melophorus species became specialists in high temperatures as this avoided competition with the Iridomyrmex meat ants that dominate the Australian daytime ant fauna. Iridomyrmex species are large and aggressive, and effectively exclude most other species from competing with them. However, a small number of Melophorus species not only do not seem to avoid Iridomyrmex, but actually seek out their company. Melophorus anderseni mingles with workers of Iridomyrmex sanguineus entering and exiting their nest, from which it steals larvae and carries them back to its own nest, located alongside the Iridomyrmex nest but having smaller entrances that the large Iridomyrmex cannot enter. Melophorus anderseni workers are apparently able to avoid detection by the Iridomyrmex as they rub against them to pick up the meat ant's scent. Though the Iridomyrmex have not been observed taking action agains the Melophorus themselves, they have been observed using pebbles to block off the entrances to the Melophorus nest (Agosti 1997).

REFERENCES

Agosti, D. 1997. Two new enigmatic Melophorus species (Hymenoptera: Formicidae) from Australia. Journal of the New York Entomological Society 105 (3-4): 161-169.

Christian, K. A., & S. R. Morton. 1992. Extreme thermophilia in a central Australian ant, Melophorus bagoti. Physiological Zoology 65 (5): 885-905.

Heterick, B. E. 2009. A guide to the ants of south-western Australia. Records of the Western Australian Museum Supplement 76: 1-206.

Muser, B., S. Sommer, H. Wolf & R. Wehner. 2005. Foraging ecology of the thermophilic Australian desert ant, Melophorus bagoti. Australian Journal of Zoology 53: 301-311.