The Sculpins of Baikal

Drawing of Leocottus kesslerii, one of the more plesiomorphic of Baikal's sculpins, from here.

In a post that appeared on this site some seven years ago, I briefly introduced you to the sculpins of Lake Baikal. Sculpins, to quickly recap, are a group of bottom-dwelling fish found in Eurasia and North America, both in marine and freshwater habitats. At some point, a representative of the freshwater sculpins entered the massive Siberian lake known as Baikal, where it gave rise to one of the world's classic adaptive radiations.

To date, about thirty species of sculpin have been described from Lake Baikal. The level of morphological divergence between these species is such that they have been classified in the past into three separate families: while some were placed in the widespread family Cottidae, others were placed in two families endemic to Baikal, the Abyssocottidae and Comephoridae. However, phylogenetic analyses indicate that all the Baikalian sculpins originated from a single ancestor, and the entire clade is nested not only within the Cottidae but also within the genus Cottus (Kontula et al. 2003). Some of the Baikalian sculpins, such as the relatively basal Leocottus kessleri, retain a habitus and lifestyle similar to those of other sculpins elsewhere. Others, such as the golomyankas of the genus Comephorus, have become remarkably modified.

Specimens of Abyssocottus korotneffi, copyright Muséum National d'Histoire Naturelle.

The greatest diversity of Baikalian sculpins has resulted from their radiation into the lake's deep waters, which reach over 1600 metres (Sideleva 1996). This is a habitat unparalleled in any other freshwater lake. The only other great lakes reaching even comparable depths are the rift lakes Malawi and Tanganyika in Africa (the great lakes of North America, in contrast, are reasonably shallow). In the African lakes, the water quickly becomes anoxic below a fairly shallow top layer, and so the depths are devoid of multicellular life. Baikal, in contrast, is oxygenated all the way down (in this post, I speculated that this was due to Baikal's hydrothermal vents; it seems I was wrong. Baikal is oxygenated because the change in surface water temperature between summer and winter results in water circulating between layers and drawing oxygen down; in the tropical great lakes, where surface temperature remains fairly constant all year round, this circulation doesn't happen). The bulk of Baikal's deep-water sculpins make up the prior family Abyssocottidae, and exhibit adaptations similar to those seen in many marine deep-water fish. Their retinal structure has become simplified as a result of low light conditions. Their scales are reduced, and the lateral line system is composed of neuromasts exposed directly on the surface of the skin rather than contained in sub-surface canals and exposed to the outside environment via pores. The convergences between 'abyssocottids' and marine deep-sea fishes are so marked that some authors previously used them to argue for a direct marine ancestry of the Baikal fish (perhaps through a direct connection between Baikal and the sea that was once thought to have existed in the past), but this has been firmly quashed by the more recent molecular analyses. Instead, the majority of Baikal's deep-waters sculpins form a single clade that originated from shallower-water ancestors; the only exception is the genus Procottus, which includes both shallow-water and deep-water species (Kontula et al. 2003).

Golomyanka Comephorus dybowskii, from here.

Possibly sister to this deep-water clade are the aforementioned two species of golomyanka in the genus Comephorus. The golomyankas are without question the most bizarre members of the Baikalian sculpin radiation. They have become adapted to a pelagic mode of life, swimming in the open water column and feeding on Baikal's similarly remarkable pelagic amphipod Macrohectopus branickii (and as remarkable as Lake Baikal's sculpins are, they are nothing compared to its amphipods). It is not a simple matter for a sculpin to swim freely: they lost their swim bladders at an earlier stage in their evolution, so their native position is quite closely associated to the water's bed. To correct for this ancestral lack of buoyancy, golomyankas have lost their covering of scales and developed a low-density body structure that contains a high proportion of oil, about one-third of their total mass. Their pectoral fins have become greatly enlarged, covering about twice the area of the remainder of the body. The end result is that golomyankas are close to neutral buoyancy, and able to simply float in water column, waiting to ambush passing prey.

Golomyankas are also distinctive in their reproductive biology. Other sculpins lay their eggs in nests among stones, where they are tended by the male until they hatch. This includes the Baikalian genus Cottocomephorus, which has adopted a partially pelagic life comparable to that of Comephorus, but not to the same extent (Cottocomephorus species resemble Comephorus in having enlarged pectoral fins, but are otherwise more typically sculpin-like). Golomyankas, in contrast, are viviparous, releasing active larvae directly into the water column. Golomyankas are by far the most abundant fish in Lake Baikal, and a major component in the diet of other fish species (including, when young, other golomyankas). They are one of the key components in making Lake Baikal what it is, the world's only freshwater sea.

REFERENCES

Kontula, T., S. V. Kirilchik & R. Väinölä. 2003. Endemic diversification of the monophyletic cottoid fish species flock in Lake Baikal explored with mtDNA sequencing. Molecular Phylogenetics and Evolution 27 (1): 143–155.

Sideleva, V. G. 1996. Comparative character of the deep-water and inshore cottoid fishes endemic to Lake Baikal. Journal of Fish Biology 49 (Suppl. A): 192–206.

The Acrotretids: Micro-brachiopods from the Dawn of... Brachiopods

Ventral valve of Acrotreta sp., copyright Ivo Paalits / TÜ geoloogiamuuseum.

When brachiopods have been featured on this site before, they have generally been representatives of the group known as the articulates. Today's subjects, the Acrotretidae, are instead members of the inarticulate brachiopods. Whereas the shells of articulate brachiopods have a hinge connecting the two valves, the shells of inarticulates do not. Instead, the valves of inarticulates are held together purely by the muscle and tissue around them. Fewer of the living brachiopods are inarticulates than articulates, and the inarticulates have been less diverse over most of brachiopod history.

The Acrotretidae are one of the earliest known families of brachiopods in the fossil record, first appearing in the early Cambrian. They were most diverse in the later Cambrian and early Ordovician, becoming less so in the later Ordovician. Only a single genus is known to have survived into the Silurian (Holmer & Popov 2000). This may be something of a pseudo-extinction: the 'Acrotretidae' as currently defined is probably ancestral to other families of the order Acrotretida that post-dated it. Nevertheless, the acrotretid lineage as a whole became extinct during the Devonian. At one time it was thought that some living brachiopod families (the craniids and discinids) might be descendants of the acrotretids; they are now believed to not be closely related.

Reconstruction of the anatomy of the acrotretid Linnarssonia constans (with a boring parasite at lower left) from Bassett et al. (2004).

The first feature that springs to attention about the acrotretids is that they were tiny. In general, their shells were only one or two millimetres across. The two valves of the shell were generally quite distinct for each other. The dorsal valve was generally low and convex, whereas the ventral valve was more or less a deep lop-sided cone. A rounded or oval opening was present in the ventral valve, usually just behind the point of the cone. In life, this would have been the opening through which extended the pedicel, the fleshy stalk that would have attached the stalk to its substrate. In brachiopods as small as acrotretids, the lophophore would have been fairly simple. Living forms with such simple lophophores open the shell wide when feeding and hold the lophophore filaments in a bell-shape; water containing food particles is drawn into the centre of the 'bell' and pushed out laterally through the filaments (Rudwick 1965).

An alternate model of the acrotretid anatomy was proposed by Chuang in the early 1970s. He compared acrotretids to the living inarticulate brachiopod Lingula, in which the pedicel does not pass through an opening in the ventral valve but instead is positioned in the centre rear of the animal, passing between the two valves. Chuang suggested that the acrotretid pedicel did likewise, and that the opening in the conical valve (which he interpreted as dorsal rather than ventral) was used to expel water after it was drawn over the lophophore. In support of this model, he conducted an experiment in which he drilled holes in a comparable position in the dorsal valve of living craniid brachiopods (demonstrating once again the concept that one can get away with anything so long as one is experimenting on 'lower lifeforms'), through which the brachiopods did indeed expel water. However, Chuang's model was dismissed by Rowell (1977) who identified a number of features confirmed that the perforate valve of acrotretids was indeed ventral. Lingula, despite being the best-known inarticulate in the modern brachiopod fauna, is a poor model for acrotretids due to its adaptations to an infaunal lifestyle buried in mud, including the modification of the pedicel into a supersized structure for digging and anchoring itself. As for Chuang's experimental observations, Rowell argued that the only thing they demonstrated was that "a system under pressure leaks when perforated", noting that "This relationship... applies equally to bicycle tires and brachiopods".

So how did acrotretids make their living? The impression I've gotten while researching this post is that they are common in deposits that would have been part of the outer continental shelf. In particular, they are often found in black shales, a rock type that was originally formed from anoxic mud. Obviously, few animals are actually able to make a living in an environment lacking oxygen. Some do, such as the "rat-tailed maggot" larvae of hoverflies that possess a long breathing tube with which to obtain air, but it is difficult to imagine acrotretids functioning in this way. The other animals found fossilised in black shales alongside acrotretids are planktonic and nektonic forms, such as graptolites or cephalopods. It is possible that many acrotretids were pseudoplankton, living attached to other organisms or objects floating in the water, such as floating seaweeds (not floating wood, though, because wood didn't exist yet). When the acrotretid died, or its host substrate disintegrated, then it would begin the long descent towards eventual fossilisation in the black muds deep below.

REFERENCES

Bassett, M. G., L. E. Popov & L. E. Holmer. 2004. The oldest-known metazoan parasite? Journal of Paleontology 78 (6): 1214–1216.

Holmer, L., & L. Popov. 2000. Lingulata. In: Kaesler, R. L. (ed.) Treatise on Invertebrate Paleontology pt H. Brachiopoda, Revised vol. 2. Linguliformea, Craniiformea and Rhynchonelliformea (part) pp. 30–146. Geological Society of America: Boulder, and University of Kansas: Lawrence.

Rowell, A. J. 1977. Valve orientation and functional morphology of the foramen of some siphonotretacean and acrotretacean brachiopods. Lethaia 10: 43-50.

Rudwick, M. J. S. 1965. Ecology and paleoecology. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt H. Brachiopoda vol. 1 pp. H199–H214. The Geological Society of America, Inc., and The University of Kansas Press.

The Phaeogenini: Widdle Icky Newmans

Female Diadromus collaris on a pupa of a diamondback moth Plutella xylostella, from here.

The ichneumons are perhaps the best-known family of parasitic wasps. Most people will have come across a description of the classic ichneumon lifestyle at at least some point: a female lays an egg in the larva of another insect, which then hatches into a wasp larva that eats out its hosts insides before emerging at maturity, leaving an empty husk behind. It is easy to see why ichneumons have become the poster children for parasitoid wasps everywhere: not only are they one of the most diverse wasp families, they can often be dramatic in appearance, growing to remarkable sizes. However, not all ichneumons are giants.

Female Eparces quadriceps, copyright Tom Murray.

The tribe Phaeogenini includes some of the smallest ichneumons, with some species being only a few millimetres long (Rousse et al. 2013). They belong the the subfamily Ichneumoninae, within which they are distinguished from most other tribes by their possession of round rather than elongate spiracles on the petiole. They are otherwise quite diverse in appearance, and Gauld (1984) suggested that they may be a polyphyletic assemblage of species that had convergently evolved their common features as a result of their small size. However, molecular phylogenetic analyses have largely supported the monophyly of the Phaeogenini (e.g. Quicke et al. 2009). One genus, Lusius, tends to be placed elsewhere among the ichneumons, but this is probably due to its having an anomalous 28S rDNA sequence with a number of deletions; Quicke et al. (2009) implied that they thought it more likely to still be a true phaeogenin. Some authors have suggested a relationship between phaeogenins and another unsual small ichneumon genus Alomya (in which case, due to the vagaries of priority, the name of this tribe becomes the Alomyini), but molecular analysis does not support this association.

Dirophanes fulvitarsis encounters a smaller wasp (perhaps a figitid?). Copyright J. K. Lindsey.

Like other members of the Ichneumoninae, the Phaeogenini are parasitoids of Lepidoptera: specifically, in accord with their small size, micro-lepidoptera. However, identification of the hosts of phaeogenins can be difficult, as they tend not to attack them until after the host has formed a cocoon (Diller & Shaw 2014). Where hosts are known, they are often borers in plant stems or leaves. The phaeogenin Diadromus collaris attacks the diamondback moth Plutella xylostella, a significant pest on brassicas and related plants. As such, it has been widely introduced around the world to help in the control of this pest.

REFERENCES

Diller, E., & M. R. Shaw. 2014. Western Palaearctic Oedicephalini and Phaeogenini (Hymenoptera: Ichneumonidae, Ichneumoninae) in the National Museums of Scotland, with distributional data including 28 species new to Britain, rearing records, and descriptions of two new species of Aethecerus Wesmael and one of Diadromus Wesmael. Entomologist's Gazette 65: 109–129.

Gauld, I. D. 1984. An Introduction to the Ichneumonidae of Australia. British Museum (Natural History).

Quicke, D. L. J., N. M. Laurenne, M. G. Fitton & G. R. Broad. 2009. A thousand and one wasps: a 28S rDNA and morphological phylogeny of the Ichneumonidae (Insecta: Hymenoptera) with an investigation into alignment parameter space and elision. Journal of Natural History 43 (23–24): 1305–1421.

Rousse, P., S. van Noort & E. Diller. 2013. Revision of the Afrotropical Phaeogenini (Ichneumonidae, Ichneumoninae), with description of a new genus and twelve new species. ZooKeys 354: 1–85.

Metabiantes

Metabiantes leighi, from Schönhofer (2008).

The handsome fellow in the photo above represents a species of the genus Metabiantes, currently the largest recognised African genus of harvestmen in the family Biantidae. Metabiantes species are currently recognised from the greater part of sub-Saharan Africa, with species known from as far north as Kenya on the east coast and the Ivory Coast in the west (Staręga 1992). The bulk of study on this genus, however, has focused on the South African species. Kauri (1961) divided the southern African species between two groups based on genital morphology. Schönhofer (2008) recorded that the widespread Transvaal species M. leighi inhabits leaf litter in evergreen forests. It seems to inhabit slightly drier microhabitats than other harvestmen in the area.

Compared to many other harvestmen, Metabiantes species do not show a high degree of sexual dimorphism. The males' chelicerae tend to be a bit larger, and consequently the front of the carapace tends to be a bit broader (Schönhofer 2008). In some species, the metatarsus of the males' second leg bears a series of teeth (Lawrence 1937). While I haven't found any explicit investigation of the role that such modifications play, the fact that the second legs in harvestmen fill a sensory function (being used much like the antennae in insects) provides a likely suggestion.

REFERENCES

Kauri, H. 1961. Opiliones. In: Hanström, B., P. Brinck & G. Rudebeck. South African Animal Life: Results of the Lund University Expedition in 1950–1951 vol. 8 pp. 9–197. Almqvist & Wiksell: Uppsala.

Lawrence, R. F. 1937. The external sexual characters of South African harvest-spiders. Transactions of the Royal Society of South Africa 24 (4): 331–337.

Schönhofer, A. L. 2008. On harvestmen from the Soutpansberg, South Africa, with description of a new species of Monomontia (Arachnida: Opiliones). African Invertebrates 49 (2): 109–126.

Staręga, W. 1992. An annotated check-list of Afrotropical harvestmen, excluding the Phalangiidae (Opiliones). Annals of the Natal Museum 33 (2): 271–336.

I Said Primrose-Willows, Darling

The plant shown in the photo above (copyright Forest and Kim Starr) is Ludwigia octovalvis, commonly known (along with other species in the same genus) as primrose-willow. This is a very common plant in tropical and subtropical regions around the world; indeed, it is so widespread that we have little idea where in the world it originated*. The name 'primrose-willow' derives, of course, from its combination of primrose-like flowers with willow-like leaves, but it is no close relation to either. Primrose-willows belong to the Onagraceae, the same plant family as evening primrose or fuchsias. Ludwigia octovalvis is a shrubby plant, sometimes growing up to four metres in height. Lower parts of the stem may become woody with age, but the greater part of the plant is herbaceous. It prefers to grow in damp habitats, in swampy soil or alongside streams, even rooted in ponds. The seeds are minute and easily spread by water or mixed in with other materials. They are also durable: Raven (1977) refers to the possibility of propagating Ludwigia from seeds preserved in herbarium specimens.

*Which, if one were of a panbiogeographical bent, might be taken to indicate that it has survived unchanged since the Triassic at least.

Ludwigia octovalvis may even grow as floating mats upon the surface of water. The floating roots then produce spongy, upright branches called aerophores. These have been interpreted as floatation devices, but the plant is apparently perfectly buoyant without them. It is more likely that they allow oxygen to reach the waterlogged roots. The lower part of the stem may also become covered in aerenchyma, porous tissue that also aids in the diffusion of gases. If conditions dry up and the plant becomes rooted in the ground, the aerophores disappear and the roots resume their normal rootly business.

Close-up of flower of Ludwigia octovalvis, copyright Bob Peterson.

Primrose-willows are generally toxic to humans. In the usual way, I have come across references to Ludwigia octovalvis being used folk-medicinally, mostly to help with ailments of the digestive tract such as diarrhoea and worms. A quick look through Google Scholar indicates that this has lead to a certain degree of pharmaceutical research, but so far this doesn't seem to have lead to much major commercial application. At the present point in time, the main economic impact of Ludwigia octovalvis is as a weed. It can grow mixed in with fields of crops such as rice and taro, or particularly lush patches of primrose-willow may clog up waterways. On the flipside, I did find this summary of the species that notes that, "Its yellow flowers add a splash of color to areas often devoid of colorfully flowering plants".

REFERENCES

Raven, P. H. 1977. Onagraceae. Flora Malesiana, ser. I, 8 (2): 98–113.

The Diprotodontids: Marsupials Go Large

Reconstruction of Diprotodon optatum by Anne Musser, from Long et al. (2002). Offhand, running a search for Diprotodon through Google Image brings up some true horrors of digital imagery.

Prior to the arrival of humans, the Australian fauna included many strange, and often dramatic, animals that are sadly no longer with us. Enormous python-like snakes, monitors that would have made a Komodo dragon look underwhelming, drop bears, and of course the notorious demon duck of doom. But among the most iconic of Australia's extinct fauna were the Diprotodontidae, heavyset herbivores that included the largest of all marsupials. Diprotodontids are sometimes referred to in the popular press as giant wombats, but this is a bit misleading: though more closely related to wombats than any other living marsupials, they were a quite distinct group of animals (besides, they shared their world with actual giant wombats that reached the size of a cow). A potentially more appropriate descriptor that has been suggested is 'marsupial rhinos', though at least some diprotodontids were decidedly not like rhinos either.

Skull of Zygomaturus trilobus in Museum Victoria, photographed by Nigel Waring.

The most famous of the diprotodontids was also the first to be described, and indeed the first fossil mammal of any kind described from Australia. Diprotodon optatum, named by Richard Owen in 1838, was the largest of the diprotodontids, sometimes standing more than six feet tall at the shoulder, and reaching estimated weights of around two and a half tonnes. At the time of human arrival, Diprotodon would have been one of the dominant herbivores in the arid central region of Australia. A number of species of Diprotodon have been named over the years, but a review of the genus by Price (2008) recognised only a single species, with the two different size classes present probably representing the different sexes. In the less arid coastal regions, Diprotodon was replaced by various species of the slightly smaller (but still formidably sized) genus Zygomaturus (Long et al. 2002). The best known species in this genus, Z. trilobus, bore a distinctive large bony boss on the snout, giving its skull a profile reminiscent of a cartoon bear. Two other diprotodontid species that would have come into contact with humans are known from the Pleistocene of montane New Guinea, Hulitherium tomasettii and Maokopia ronaldi. Both these species were smaller than the mainland Australians, being about the 100 kg mark. Maokopia has been interpreted as a grazer, while Hulitherium has been seen as a browser, and suggested as a direct analogue of the Asian giant panda (Long et al. 2002).

Reconstruction of Hulitherium tomassettii as a panda analogue, by Peter Schouten.

The broader record of diprotodontids goes back to the Oligocene, with two main lineages being recognised, the Diprotodontinae and Zygomaturinae. Of the species referred to above, all but Diprotodon optatum are zygomaturines. The two groups are primarily distinguished by their dentition, with the premolars being generally more complex in zygomaturines than diprotodontines. In both lineages, the earlier members were smaller: Long et al. (2002) describe a number of genera as 'sheep-sized'. The smallest known diprotodontid, the late Oligocene Raemeotherium yatkolai, they describe as 'lamb-sized'. Black et al. (2012) estimated the weight of the middle Miocene Nimbadon lavarackorum as abut 70 kg. They also suggested that it was an adept climber, in a similar manner to the modern koala, making it the largest known arboreal mammal from Australia. It might seem odd to picture an animal of this size up in a tree, even allowing for the higher density of the canopy in Australia's Miocene rainforests. However, there are larger arboreal mammals alive even today: male orangutans, for instance, may weigh over 100 kg.

Reconstruction of a climbing pair of Nimbadon lavarackorum (adult and juvenile) by Peter Schouten, from Black et al. (2012).

Interestingly, Nimbadon is not placed as a particular basal diprotodontid in the phylogeny of zygomaturines presented by Mackness (2010). As other related marsupial families, such as koalas or thylacoleonids (marsupial lions), also include climbers, it would not be unreasonable to consider such habits plesiomorphic for diprotodontids as a whole. The 'rhino-like' appearance of the later giants would then be something of a novelty, an adaptation to the drier conditions and more open woodlands that arose at the end of the Miocene. If we are to regard the diprotodontids as marsupial rhinos, then we must consider the possibility of rhinos in trees.

REFERENCES

Black, K. H., A. B. Camens, M. Archer & S. J. Hand. 2012. Herds overhead: Nimbadon lavarackorum (Diprotodontidae), heavyweight marsupial herbivores in the Miocene forests of Australia. PLoS ONE 7 (11): e48213. doi:10.1371/journal.pone.0048213.

Long, J., M. Archer, T. Flannery & S. Hand. 2002. Prehistoric Mammals of Australia and New Guinea: One hundred million years of evolution. University of New South Wales Press: Sydney.

Mackness, B. S. 2010. On the identity of Euowenia robusta De Vis, 1891 with a description of a new zygomaturine genus. Alcheringa 34 (4): 455–469.

Price, G. J. 2008. Taxonomy and palaeobiology of the largest-ever marsupial, Diprotodon Owen, 1838 (Diprotodontidae, Marsupialia). Zoological Journal of the Linnean Society 153: 389–417.

The Cancellothyridids: A Modern Success Story

Northern lamp shels Terebratulina septentrionalis, from Oceana.

As has been noted on this site more than once before, brachiopods are a group of animals probably more familiar to the student of palaeontology than of zoology. From the brief gloss that tends to be their only coverage in textbooks, one might be forgiven for thinking them all but inconsequential in the modern fauna. But where conditions suit them (usually sheltered locations where low levels of light and water flow favour their slow metabolisms over the higher energy requirements of bivalves), brachiopods can still be abundant, and even dominant.

One of the most diverse families of brachiopods in the modern fauna is the Cancellothyrididae. Cancellothyridids first make their appearance in the Jurassic, becoming widespread in the Cretaceous (Cooper 1973). Members of this family have shells with a large foramen (the opening at the rear of the shell through which passes the pedicel or stalk by which the brachiopod is attached to its substrate), usually with the deltidial plates surrounding the foramen greatly reduced. The main defining feature of the Cancellothyrididae is the structure of the brachidium, the skeletal structure that provides the support for the base of the lophophore, the tentacle-like feeding structures. In cancellothyridids, the two sides of the brachidium coalesce in the middle to form a tube.

Dorsal valve of the Cretaceous cancellothyridid Cricosia filosa in (A) lateral, (B) ventral and (C) posterior views, from Cooper (1973), showing the tubular brachidium.

The brachidium does not extend into the arms of the lophophore, which are instead strengthened by unattached spicules. The tubular shape of the brachidium distinguishes the Cancellothyrididae from the closely related family Chlidonophoridae, whose members share the large posterior foramen but have the two sides of the brachidium open in back. Cooper (1973) recognised two subfamilies of cancellothyridids, the living Cancellothyridinae and the Cretaceous Cricosiinae; the cricosiines have the tubular section of the brachidium longer and narrower than the cancellothyridines.

Modern cancellothyridids are found in the Indo-Pacific and the North Atlantic, but seem to be absent from the South Atlantic. The majority of living species are included in the widespread genus Terebratulina, with the other living genera all having restricted distributions in the Indo-Pacific. However, a molecular phylogenetic analysis of species of Terebratulina and the Australian genus Cancellothyris by Lüter & Cohen (2002) indicated that both Atlantic Terebratulina and Cancellothyris were nested within Pacific Terebratulina. Paraphyly of the widespread genus would also correlate with its palaeontological distribution: while the other genera are known only from the Recent fauna, Terebratulina has a fossil record dating right back to the origins of the cancellothyridids in the Jurassic (Muir-Wood 1965). Lüter & Cohen (2002) tentatively suggested the possibility of a North Pacific origin for Terebratulina (and, by implication, for Cancellothyrididae as a whole), with dispersal to the North Atlantic occurring through the gap between North and South America before formation of Central America. Their preference for this option rather than the alternative of dispersal through the Tethys (the seaway that once separated Africa from Eurasia) was based on their estimate via molecular clock of a separation of about 100 million years between the Atlantic and Pacific species, supposedly too early for the Tethys option. However, it must be stressed that their sampling of even modern cancellothyridid diversity was not comprehensive. A trans-Tethys dispersal of cancellothyridids may also be indicated by the presence of the fossil genus Rhynchonellopsis in the lower Oligocene of northern Europe (Muir-Wood 1965). Of course, there is no inherent reason why cancellothyridids could not have travelled in both directions!

REFERENCES

Cooper, G. A. 1973. Fossil and recent Cancellothyridacea (Brachiopoda). Tohoku Univ., Sci. Rep., 2nd Ser. (Geol.), Special Volume 6: 371–390.

Lüter, C., & B. L. Cohen. 2002. DNA sequence evidence for speciation, paraphyly and a Mesozoic dispersal of cancellothyridid articulate brachiopods. Marine Biology 141: 65–74.

Muir-Wood, H. M. 1965. Mesozoic and Cenozoic Terebratulidina. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt H. Brachiopoda vol. 2 pp. H762–H816.