Hypno-Moss

Recent decades have seen a great deal of shifting around in the classification of mosses. As molecular data have become de rigeur in phylogenetic studies, a number of features previously used to distinguish higher groupings have proven to be more labile than previously appreciated. This has lead to a hunt to discern whether other features may be more reliable.

Hypnum cupressiforme, from Andrew's Moss Site.

The Hypnales are one of the major moss groups: as currently recognised, about a third of mosses are Hypnales. They are a major subgroup of the clade of pleurocarpous mosses, i. e. those in which the reproductive sporophytes arise from the sides of gametophyte stems, as explained earlier in this post. In the past, the pleurocarpous mosses have been divided between three orders, the Hypnales, Hookeriales and Leucodontales, on the basis of features of branching habit and the peristome, the array of teeth surrounding the opening of the spore capsule. In the Hookeriales, the teeth of the endostome (the inner ring of the peristome) are connected by a high basal membrane, and molecular phylogenetic analyses have generally supported this order as monophyletic. The Leucodontales were defined by having reduced peristome teeth, and usually sympodial growth (as the primary shoot produces a side-branch, it ceases growing itself and the new branch becomes the new primary shoot). The Hypnales had well-developed peristome teeth, and their growth was generally monopodial (the primary shoot continues growing even after it produces side-branches). The distinction between these latter two orders also correlated with their choice of niches: Leucodontales were mostly epiphytes, whereas Hypnales mostly grew on the ground. However, molecular phylogenetic analyses have not supported the distinction between the Hypnales and Leucodontales, with features such as reduced peristome teeth apparently evolving multiple times with the united clade combining the two orders (Buck et al. 2000). As a result, recent authors have treated the Hypnales as including most members of both the prior orders Hypnales and Leucodontales. A smaller number of pleurocarpous mosses have been placed outside the clade including Hookeriales and Hypnales in the broad sense; there are now known as the Ptychomniales and Hypnodendrales. The broader Hypnales is less well defined morphologically, but its members tend to have differentiated alar cells (distinctly formed cells at the basal corners of the leaves) and smooth spore capsules (Huttunen et al. 2012).

A mat of Leucodon, from here.
This shuffling is not restricted to the higher levels, either. Relationships within the Hypnales remain poorly resolved; indications are that at some point this group went through a quite rapid diversification, resulting in a fairly high level of convergence between lineages and low support for molecular branches. Huttunen et al. (2012) found support for a large clade within the Hypnales including the majority of its Northern Hemisphere members, with a paraphyletic grade outside this containing mostly Southern Hemisphere taxa. Huttunen et al. suggested a Gondwanan origin for the Hypnales, with their diversification in the Northern Hemisphere (where the other pleurocarpous orders never made many inroads) related to the break-up of the Laurasian landmasses. Within the Northern Hemisphere clade, many previously recognised families appear to be polyphyletic; even the type genus of the order, Hypnum, contains species that seem to occupy widely separate places in the hypnalean family tree.

The Azores-endemic moss Echinodium renaudii, copyright Paulo A. V. Borges.

A good example of all this mess is the genus Echinodium, a small genus of six living species whose distinctive appearance lead to it being placed in a family all of its own. Echinodium species grow as fairly stiff plants with long leaves that taper to a narrow point and have thickened margins (the margins are two cell layers thick whereas the body of the leaf is only one cell thick). Echinodium mosses also have a very unusual distribution: two species are found in southeastern Australia and New Zealand, but the other four are restricted to the Macaronesian islands in the Atlantic (that is, the Canaries, the Azores and Madeira). When fossil Echinodium species were discovered in eastern Europe, it was suggested that the genus' current distribution could be a relict of a previously much wider one. However, a molecular analysis of the genus by Stech et al. (2008) identified another explanation: not only were the Australasian and Macaronesian Echinodium species widely separated geographically, they were widely separated phylogenetically. The Australasian species were placed in the family Neckeraceae, whereas the Macaronesian species were related to mosses of the family Lembophyllaceae. What is more, the Macaronesian species did not form a single clade within the Lembophyllaceae: at least one of the species was placed separately from the rest. The supposedly distinctive 'Echinodium' features, it seems, have evolved independently, possibly as an adaptation for wet habitats.

REFERENCES

Buck, W. R., B. Goffinet & A. J. Shaw. 2000. Testing morphological concepts of orders of pleurocarpous mosses (Bryophyta) using phylogenetic reconstructions based on trnL-trnF and rps4 sequences. Molecular Phylogenetics and Evolution 16(2): 180–198.

Huttunen, S., N. Bell, V. K. Bobrova, V. Buchbender, W. R. Buck, C. J. Cox, B. Goffinet, L. Hedenäs, B.-C. Ho, M. S. Ignatov, M. Krug, O. Kuznetsova, I. A. Milyutina, A. Newton, S. Olsson, L. Pokorny, J. A. Shaw, M. Stech, A. Troitsky, A. Vanderpoorten & D. Quandt. 2012. Disentangling knots of rapid evolution: origin and diversification of the moss order Hypnales. Journal of Bryology 34 (3): 187–211.

Stech, M., M. Sim-Sim, M. G. Esquível, S. Fontinha, R. Tangney, C. Lobo, R. Gabriel & D. Quandt. 2008. Explaining the ‘anomalous’ distribution of Echinodium (Bryopsida: Echinodiaceae): independent evolution in Macaronesia and Australasia. Organisms Diversity & Evolution 8 (4): 282–292.

Sea Bass, Mutant or Otherwise

...though to the best of my knowledge, none of them have fricking lasers on their heads.

Painted comber Serranus scriba, copyright Roberto Pillon.

The Serranidae are a group of marine fish that go by vernacular names such as sea bass, rock bass or rock cod. They are carnivores, and are found mostly around reefs in tropical and subtropical waters around the world. In appearance, they are fairly generalised (these are fish that look like fish) with a body shape that is longer than high, but not too long, and relatively big jaws with the lower jaw often jutting forward a bit beyond the upper. Some of them are quite colourful (as befits a tropical reef fish) and some of the smaller ones turn up in marine aquaria as a result. As used in the past, the Serranidae has been quite a broad grouping of fish united by having three spines on the margin of the opercle (the gill cover) and the maxilla in the upper jaw not hidden by the cheekbone when the mouth is closed. Members of this broad Serranidae were commonly divided between three subfamilies: the Serraninae (including the sea basses), Epinephelinae (including the groupers) and Anthiinae (basslets and goldies), though some authors further subdivided the Epinephelinae. However, recent molecular studies have indicated the polyphyly of this grouping, with the Serraninae and Epinephelinae occupying distinct positions within the clade known as the Serraniformes or Perciformes sensu stricto (see this old post), and so have cut the latter out of the Serranidae. As for the Anthiinae, their position remains uncertain, with some analyses placing them with the Serraninae and others with the Epinephelinae (Lautredou et al. 2013). As a result, a monophyletic Serranidae is probably to be restricted to the old 'Serraninae'.

Shy hamlet Hypoplectrus guttavarius, copyright Florent Charpin.

There are over eighty species listed for this restricted Serranidae on FishBase, but new ones continue to be described. As is common among reef fishes, it can be hard to determine exactly what counts as a species (whatever your preferred definition). A prime example of this is the genus Hypoplectrus, small serranids known as hamlets (no, I don't know why either) found in the Caribbean and the Gulf of Mexico. Hamlets come in a range of different colours and patterns, but structurally speaking the various forms are otherwise indistinguishable. As a result, some authors have regarded them as all colour morphs of a single species. Others have recognised close to twenty different species. Domeier (1994), conducting field observations on hamlets together with breeding experiments in the laboratory, found that different colour morphs would usually only mate with partners sharing their own colour pattern, though hybrid matings could be produced if no more suitable mate was provided. These hybrid matings produced offspring bearing intermediate colour patterns, and the rarity of such intermediates in the field led Domeier to infer that the different morphs were mostly acting as good species.

Kelp bass Paralabrax clathratus, photographed by Steve Lonhart.

Most sea basses are simultaneous hermaphrodites: they have both male and female reproductive organs functional at the same time. Though they are capable of fertilising their own eggs, they still usually breed in pairs with each individual alternating the release of male and female gametes. Not all serranids follow this reproductive template: members of the genera Chelidoperca and Centropristis are protogynous, starting their mature lives as females before switching over to males. Two species of Serranus, the lantern bass Serranus baldwini and the barred serrano S. psittacinus, are mostly simultaneous hermaphrodites like other species in the genus, but the largest individuals resorb their female organs and become exclusively males. Finally, many species of the genus Paralabrax have entirely separate males and females. Phylogenetic analysis suggests that protogyny may be the original mode of sexual development in the serranids, with separate sublineages developing simultaneous hermaphroditism vs separate sexes (Erisman & Hastings 2011). In correlation with this, individuals of Paralabrax that are functionally single-sexed have been found to retain non-functional remnants of the other sex's organs.

REFERENCES

Domeier, M. L. 1994. Speciation in the serranid fish Hypoplectrus. Bulletin of Marine Science 54 (1): 103–141.

Erisman, B. E., & P. A. Hastings. 2011. Evolutionary transitions in the sexual patterns of fishes: insights from a phylogenetic analysis of the seabasses (Teleostei: Serranidae). Copeia 2011 (3): 357-364.

Lautredou, A.-C., H. Motomura, C. Gallut, C. Ozouf-Costaz, C. Cruaud, G. Lecointre & A. Dettai. 2013. New nuclear markers and exploration of the relationships among Serraniformes (Acanthomorpha, Teleostei): the importance of working at multiple scales. Molecular Phylogenetics and Evolution 67: 140–155.

Stygophalangium: Harvestman or Mite?

The original illustration of Stygophalangium karamani, from Oudemans (1933).

In 1933, the Dutch zoologist Anthonie Oudemans described what he believed to be a remarkable new species of harvestman. Based on two specimens collected from an underground spring in modern-day Macedonia and dubbed Stygophalangium karamani, Oudemans regarded this as a highly degenerate form as a result of its habitat: small, soft-bodied, and eyeless. It exhibited some significant differences to other harvestmen: in particular, the body lacked obvious signs of external segmentation. Also, its apparent aquatic collection point stood in direct contrast to the otherwise terrestrial habitats of other species. Nevertheless, Oudemans placed this unusual animal in a new family, the Stygophalangiidae, and suggested that its reduced morphology compared to other harvestmen might be compared to the position of Eriophyes (a plant-feeding, four-legged genus) among the mites. However, due to its anomalous character, subsequent authors have not paid much attention to little Stygophalangium. Mello-Leitão (1944) briefly suggested that it might represent a primitive form, placing it at the base of a branch of the phylogenetic tree leading to the Cyphophthalmi (mite-like harvestmen) and Palpatores (long-legged harvestmen). A number of online sources, such as Wikipedia, refer to Stygophalangium as being classified with the Eupnoi (a subgroup of the Palpatores), but this claim seems to be baseless. It seems to be derived from Joel Hallan's online list of harvestman species (which no longer appears to be available) but while Oudemans did compare Stygophalangium to the eupnoin Phalangium opilio (the common field harvestman) in his original description, he did not actually classify his new species with any particular subgroup of harvestmen. Eventually, Kury (2011) dismissed Stygophalangium from consideration in his summary of harvestman classification, stating that it 'is probably a member of the Acari'.

Unfortunately, as much as Stygophalangium might not be a convincing harvestman, it is also not a very convincing mite. One of the primary features that lead Oudemans to see Stygophalangium as a harvestman was its possession of three-segmented chelicerae. Most arachnids have chelicerae with only two segments (the basal segment and an opposing mobile claw or fang); three-segmented chelicerae are only found in two groups, the harvestmen and the mite group Parasitiformes. Of the four main groups (Opilioacarida, Holothyrida, ticks and Mesostigmata) within the Parasitiformes, none are similar to Stygophalangium. The ticks have distinctly modified (and kind of terrifying) blood-sucking mouthparts. The Holothyrida and Mesostigmata are both armoured to varying degrees, and mesostigs also bear a branched structure called the tritosternum underneath the mouthparts that is not described for Stygophalangium. The Opilioacarida are large, superficially harvestman-like mites that also have visible indications of external segmentation. And while there are a number of known lineages of aquatic mites, none of them really looks anything like Stygophalangium. It would be surprising if Oudemans, one of the leading mite researchers of his time, failed to recognise a mite when he had one in front of him! It is true that Oudemans' work underwent a precipitous decline in his last years as a result of problems with his mental health (Southcott 1961), but at the time of Stygophalangium's publication Oudemans remained alert and well.

Ventral view of Stygophalangium, with close-ups of chelicera, terminal pedipalp segments, and leg claw, from Oudemans (1933).

So if Stygophalangium was not a harvestman, and not a mite, then what was it? It is possible, of course, that it represented some taxon that has never been recorded since, but such an agnostic interpretation simply leaves the question of its affinities open. We can still at least try and compare it to other animals as best we can. One quite important point that I have avoided mentioning so far is that Oudemans' specimens were apparently not mature: Oudemans was unable to find indications of either a genital or anal opening. Though he described the body as unsegmented, it should be noted that his illustration is a reconstruction of what was apparently a not so smoothly mounted animal. Oudemans did note that a number of creases were visible on the bodies of his specimens, though he interpreted these as artefacts of slide-mountaing rather than segment boundaries because they did not appear to be placed evenly (with some creases even crossing over each other). Also, the supposed aquatic habitat may be a red herring. Subterranean samples are commonly collected by lowering sampling devices down a borehole, and it is not unknown for surface-dwelling organisms to fall in the borehole or be picked up when the traps are raised or lowered. So is Stygophalangium a larval harvestman or mite?

Again, we can rule out any arachnid except harvestmen or parasitiform mites due to the three-segmented chelicerae. The objections given above to adult ticks or Mesostigmata apply equally well to their juveniles, so they're also out. Larval Holothyrida lack the heavy armour of the adults, but these large litter-dwelling mites are not found anywhere near Europe. On the harvestman side of things, most harvestmen as both adults and nymphs have the second pair of legs particularly long and filamentous, functioning in a similar manner to the antennae of insects. The only harvestmen to lack this feature are the Cyphophthalmi, and together with the Opilioacarida they are the only real candidates for comparison with Stygophalangium. Both are soil-dwelling animals, and both are known from the Balkan region.

Larva of Opilioacarus texanus, from Klompen (2000).

One point in favour of an opilioacarid identity is that Oudemans described the chelicerae of Stygophalangium as inserted more dorsally than in other harvestmen. Opilioacarids have similarly inserted chelicerae, with a hypostome extending underneath the chelicerae. Oudemans also described Stygophalangium as lacking setae dorsally (instead having a somewhat scaly texture); opilioacarids have dorsal setae on the prosoma only. The opiliacarid prelarva (the earliest stage of its life cycle) has a scaly texture very similar to Stygophalangium (Klompen 2000), but mite larvae and prelarvae have only three pairs of legs. If Stygophalangium is an opilioacarid, it would have to be one of the later nymphal instars in which the fourth pair of legs has developed. Other features of opilioacarid juveniles conflict with Stygophalangium, such as the two pairs of large eyes on the opilioacarid prosoma. Also, Oudemans illustrated the venter of Stygophalangium with the coxae (the basalmost leg segment) integrated with the underside of the body, whereas opilioacarids (like other Parasitiformes) have the coxae free from the venter and attached by sockets. As Oudemans indicated the coxae of Stygophalangium with dotted lines only, it is possible that he inferred their position under the assumption of harvestman affinities. However, even if we assume this to be the case and that what Oudemans took to be the trochanters (the second leg segment) were actually the coxae, then Stygophalangium is left with one leg segment too few.

Larva of Siro rubens, from Juberthie (1964).

The only information on the juvenile stages of Cyphophthalmi is a brief description of the larva of Siro rubens by Juberthie (1964). Cyphophthalmi lack obvious eyes, and their legs do have the right number of segments for Stygophalangium. Juberthie described the cyphophthalmid larva as lacking a developed anus, which correlates with Oudeman's description of Stygophalangium (opilioacarid nymphs, in contrast, have a well-developed anal cone). He also recorded the presence of a pair of egg-teeth in the midline of the prosoma near the front of the body, in the same position where Oudemans described a distinctive pigmented spot on Stygophalangium. Points against a cyphophthalmid identification include the non-dorsal insertion of the chelicerae (though, again, one can't help wondering about the possibility of distortion through slide-mounting) and the presence of sparse but distinct dorsal setae. Especially difficult are the pairs of large setae marking the positions of the repugnatorial tubercles on either side of the prosoma. Unfortunately, Juberthie did not describe the venter of the cyphophthalmid larva, or comment on the degree of sclerotisation (mature cyphophthalmids are heavily sclerotised, whereas Stygophalangium is explicitly soft-bodied).

And that is about as far as we can go without looking at the original specimens. Personally, I suspect the issues with a cyphophthalmid identification are easier to overcome than those with an opilioacarid one (perhaps Oudemans did indeed mistake segment boundaries for mounting artefacts, and perhaps the dorsal setae had been lost post-mortem and Oudemans overlooked their sockets) but any such judgement requires the original description to be at least partially erroneous. Oudemans said that his type specimens were deposited in the Rijksmuseum van Natuurlijke Historie in Leiden; I wonder if they're still there?

REFERENCES

Juberthie, C. 1964. Recherches sur la biologie des opilions. Annales de Spéléologie 19 (1): 5–244.

Klompen, J. S. H. 2000. Prelarva and larva of Opilioacarus (Neocarus) texanus (Chamberlin and Mulaik) (Acari: Opilioacarida) with notes on the patterns of setae and lyrifissures. Journal of Natural History 34 (10): 1977–1992.

Oudemans, A. C. 1933. Ein neuer Stygobiont, Stygophalangium karamani Oudms. Zoologischer Anzeiger 103: 193–198.

Southcott, R. V. 1961. Studies on the systematics and biology of the Erythraeoidea (Acarina), with a critical revision of the genera and subfamilies. Australian Journal of Zoology 9: 367–610.

Serpularia: A Rightly Forgotten Problematicum

I think it may be time to rock out something that hasn't been seen on this site for a while. Horns at the ready...

(Credit, again, to Neil from Microecos). And I'm afraid that may just be the most excitement that we get in this post. While some fossils are problematic because they're so strange that they can't be easily compared to living animals, others are problematic simply because they're rubbish.

In 1840, the palaeontologist Georg Graf zu Münster ('Graf' being a German title that generally gets translated as 'Count') published his Beiträge zur Petrefakten-Kunde, in which he described a number of fossils held in his collection. This book included a section on fossils from the Ordovician Orthoceratite Limestone of the Fichtel Mountains in Bavaria. Which, close to the end, included this little tidbit:

Unter mehreren Bruchstücken einiger mir noch unbekannten Versteinerungen kommen auch einige röhrenformige Korper vor, welche ich anfänglich für den von Murchison aus der 27sten Tafel abgebildeten Myrianites hielt, allein genaue Untersuchung zeigte, dass diese Korper formliche Schalen hatten und daher vielleicht zu den Serpuliten gehört hatten, daher ich sie vorläufig Serpularia genannt habe. Aus der Taf. IX. Fig. 14 und 15 sind zwei Arten von dergleichen Bruchstücken abgebildet; Fig. II. Serpularia crenata; glatt gebogene Röhre, aus dem Rücken crenulirt. Fig. 15. Serpularia bicrenata; glatte etwas zusammengedrückte ganz grade Röhrchen, die an beiden Seiten crenulirt sind.

Translated with the help of Google Translate, I think this means: "Among several fragments of fossils unknown to me occured a tube-like body, which I initially took for Myrianites as figured by Murchison in the 27th plate, until close examination showed that this body had distinct signs of segmentation and was therefore perhaps one of the Serpulidae. Therefore, I have provisionally called it Serpularia. On Plate IX Figs 14 and 15 are shown two types of the like fragments; Fig. 14, Serpularia crenata: smooth curved tube crenulated from the back. Fig. 15, Serpularia bicrenata: smooth, slightly compressed, quite straight tubes that are crenulated on both sides".

Münster's (1840) original figures of the two Serpularia.

As perfunctory as it was, that seems to be all there was to say on the matter. The good Graf's Serpularia has pretty much never been mentioned again*, beyond being cited to cause a name change in a later homonymous gastropod genus, and a brief listing in Howell's (1962) coverage of worm fossils for the Treatise on Invertebrate Paleontology that adds nothing to the original description.

*Though if it were to be mentioned again, it would probably have to be under a different name. The name 'Serpularia' had earlier been used by Fries in 1829 for a genus of slime moulds. At the time, slime moulds were treated as fungi, and hence fell under the purview of botanical rather than zoological names, but with the recognition that they are amoebozoans an increasing number of authors would move them into the field of the Zoological Code.

Münster believed that his fossils belonged to the Serpulidae, a family of annelid worms. Annelids, being mostly soft and squishy things that do not stand up well to decay, have a pretty deplorable fossil record, but serpulids are a bit of an exception. These are sessile worms that secrete a calcareous tube in which they live their lives (modern serpulids appeared on this site in this post). Unfortunately, while these tubes are eminently fossilisable, they are also a bit nondescript, and have little to mark them as uniquely serpulid.

Because of the dominance of annelids among modern worms, there has been a definite tendency in the past to assume that any given worm-like fossil represents an annelid. Howell's (1962) aforementioned list of annelids includes the Ediacaran Spriggina (identity still under debate, but probably not an annelid) and the Cambrian Pikaia (now generally regarded as an early chordate). Similarly, any worm-like tube has been assumed a serpulid. But even among annelids, serpulids are not the only tube-bearing worms. At least two other families, the Sabellidae and the Cirratulidae, include species producing calcareous tubes. There are also other groups of non-annelid worms that, though relatively uncommon or unprepossessing today, may have been more prominent in the past. After all, we are talking here about a period of hundreds of millions of years. We know that vertebrates have gone through a great deal of evolutionary change over that period; why should we assume that worms have not?

So while fossils have been assigned to the serpulids going back as far as the Cambrian (if not beyond), there is little reason to take those assignations at face value. When so-called Palaeozoic serpulids have been examined critically in recent years, they have so far proven to lack features that would definitely confirm their identification (Vinn & Mutvei 2009). Weedon (1994) found that Palaeozoic fossils that had been assigned not only to the Serpulidae, but to the modern genus Spirorbis, had a shell microstructure that suggested a relationship to bryozoans or brachiozoans rather than to annelids. Without a similar close analysis, we could not assume a priori that Münster's Serpularia were not serpulids, but odds would currently be against it.

REFERENCES

Howell, B. F. 1962. Worms. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt W. Miscellanea: Conodonts, Conoidal Shells of Uncertain Affinities, Worms, Trace Fossils and Problematica pp. W144–W177. Geological Society of America and University of Kansas Press.

Münster, G. 1840. Beiträge zur Petrefacten-Kunde von Herm. v. Meyer und Georg Graf zu Münster vol. 3. In Commission der Buchner'schen Buchhandlung: Bayreuth.

Vinn, O., & H. Mutvei. 2009. Calcareous tubeworms of the Phanerozoic. Estonian Journal of Earth Sciences 58 (4): 286–296.

Weedon, M. J. 1994. Tube microstructure of Recent and Jurassic serpulid polychaets and the question of the Palaeozoic 'spirorbids'. Acta Palaeontologica Polonica 39 (1): 1–15.

More on Spider-Hawks

A couple of years ago, I presented a bit of an abortive post on wasps of the family Pompilidae, the spider-hawks. Despite their striking appearance and relatively high visibility, I noted, it was nigh on impossible to find reliable taxonomic information on them.

Diagram of the forewings of Cryptocheilus australis (above) vs Heterodontonyx bicolor (below) from Wahis (2008), showing the differences in the shape of the marginal cell (the large cell along the top margin of the wing).

This question came back to the fore for me recently when I had to attempt to identify a number of spider-hawks for work. With no recent key available for Australian pompilids, I had to try and piece together clues. As it turns out, a large part of the difficulty in identifying spider-hawks is that they are, overall, a conservative bunch. Though coming in a range of sizes and colours, they tend to be structurally uniform. This makes it difficult to find reliably key-able characters, and means that evolutionarily quite distinct species can look superficially quite similar. Take, for example, one of the most 'familiar' of the Australian pompilids, the black-and-orange Cryptocheilus bicolor. Recently, Wahis (2008) established that this was not a true species of Cryptocheilus, but belonged to a distinct (albeit related) genus as Heterodontonyx bicolor. The two genera can be distinguished by the shape of the marginal cell in the forewing, which is distally pointed in Heterodontonyx but rounded in Cryptocheilus. The thing is, many of the photos one may find online labelled as 'Cryptocheilus bicolor' are true Cryptocheilus, not Heterodontonyx. Those on Wikipedia may be correctly identified, but these here are not. Not every large orange-and-black spider-hawk in Australia is Heterodontonyx bicolor.

Specimen of Telostegus inermis, copyright Josef Dvořák.

So what of Telostegus, the genus that I was complaining about being unable to find the diagnostic characters for in my earlier post? Evans (1972) describes it as having bifid tarsal claws, and a vena spuria in the forewing. A vena spuria ('spurious vein') is a fold in the wing that might be mistaken at first glance for a wing vein. In the images above, it can be seen as a dark line along the middle of the wing in the dorsal view. Evans (1972) separated two genera of spider-hawks, Telostegus and Elaphrosyron, on the basis of the number of submarginal cells in the forewing (two in Telostegus, three in Elaphrosyron) but more recent authors have not regarded this distinction as valid.

REFERENCES

Evans, H. E. 1972. A review of the Australian species of Elaphrosyron and Telostegus, with notes on other genera (Hymenoptera: Pompilidae). Breviora 386: 1–18.

Wahis, R. 2008. Contribution à la connaissance des Pompilides d’Australie (Hymenoptera : Pompilidae). 2. Sur quelques spécimens récoltés par G. Else (Natural History Museum, London) avec descriptions de deux espèces nouvelles des genres Auplopus et Ctenostegus. Faunistic Entomology 61 (1–2): 23–31.

Hadromeros: A Trilobite Survivor

Reconstruction of Hadromeros subulatus, from Kielan-Jaworowska et al. (1991).

Trilobites of the genus Hadromeros were widespread in the Late Ordovician and Early Silurian of Eurasia and North America. They are classified in the Cheiruridae, the same family that includes another trilobite genus that has been featured on this site, Sphaerexochus. However, Hadromeros differs from Sphaerexochus in that its glabella (its 'nose') is not as large. Whereas in my earlier post I suggested that Sphaerexochus may have been a predator, Hadromeros was probably a less aggressive feeder. It was possibly a detritivore, picking bits of nutritious material out of the sand and mud. This interpretation is supported by the known leg morphology of a closely related genus, Ceraurus, in which the legs are fairly generalised and show little adaptation for food processing (Bergström 1973).

Hadromeros and Ceraurus are placed in the Cheirurinae, a distinct subfamily from the Sphaerexochinae that includes Sphaerexochus. One characteristic feature of many members of the Cheirurinae is that the bases of the pleura, the plates that run down each side of the thorax, are swollen in dorsal view. The purpose of these swellings remains unknown. You might expect that, trilobites being as abundant in the fossil record as they are, we would know a great deal about them, and in many respects that is quite true. However, in other respects our knowledge is also frustratingly incomplete. Trilobites have an extensive fossil record because their dorsal exoskeleton was mineralised, and it is this that is usually preserved. The ventral section of the body, on the other hand, was not mineralised, and is only preserved under exceptional circumstances. This includes such significant features as the legs and mouthparts (as indicated above, we have some knowledge of the leg morphology of Cerarurus, but no direct evidence for Hadromeros). It is possible that the cavities underneath the cheirurine pleural bases housed some modification of the gills, if the gills in trilobites were comparable to those of living crustaceans. But how or to what purpose the gills were modified can only be speculated upon.

Morphologically, Hadromeros was a fairly unremarkable trilobite, but it stands out from the other genera of the Cheirurinae in one important respect (that has, indeed, already been alluded to in this post). The end of the Ordovician saw a mass extinction in marine life, by some measures the second largest to have ever occurred. Few groups of animals made it through unscathed, and the cheirurids were no exception. Of the eight subfamilies of Cheiruridae recognised by Přibyl et al. (1985), only three made it through to the Silurian: the Cheirurinae, Sphaerexochinae and Deiphoninae. Within the Cheirurinae, Hadromeros is the only genus currently known from both sides of the Ordovician-Silurian boundary, and may have been ancestral to all other post-Ordovician cheirurines. While other genera were whisked away, Hadromeros became the Trilobite that Lived.

REFERENCES

Bergström, J. 1983. Palaeoecologic aspects of an Ordovician Tretaspis fauna. Acta Geologica Polonica 23 (2): 179-206.

Kielan-Jaworowska, Z., J. Bergström & P. Ahlberg. 1991. Cheirurina (Trilobita) from the Upper Ordovician of Västergötland and other regions of Sweden. Geologiska Föreningen i Stockholm Förhandlingar 113 (2-3): 219-244.

Přibyl, A., J. Vaněk & I. Pek. 1985. Phylogeny and taxonomy of family Cheiruridae (Trilobita). Acta Universitatis Palackianae Olomucensis Facultas Rerum Naturalium Geographica-Geologica XXIV 83: 107-193.