The Diversity of Neosauropods, or Pity Poor Camarasaurus

Articulated skeleton of juvenile Camarasaurus lentus in the Carnegie Museum of Natural History, photographed by Daderot.

Let me just get the obvious out of the way first: sauropods were huge. Mind-bendingly huge. In some cases, big enough to reduce a human to a sticky puddle under foot and not even break their stride. For close to 150 million years, they were the largest land animals anywhere in the world, and no other terrestrial animal at any time has come even close to rivalling their largest representatives in size. Being around for so long, it should also be no surprise that they were diverse: a large number of sauropod genera have been named, representing a wide variety of forms. Nevertheless, most people's idea of sauropods is encompassed within just four genera from the late Jurassic of North America: Diplodocus, Apatosaurus, Brachiosaurus and Camarasaurus.

These four genera all belong to the clade Neosauropoda, which has been defined as the smallest clade containing the genera Diplodocus and Saltasaurus (a late Cretaceous South American genus). Upchurch et al. (2004) diagnosed the neosauropods by a number of cranial features, together with a reduction in the fourth hind toe (part of a general trend towards toe reduction in sauropods as their feet became more columnar—see this article by Darren Naish for more on the subject). However, Upchurch et al. were writing before the recognition of the Turiasauria, a European clade that is probably the sister group of neosauropods (Royo-Torres et al. 2006), and I don't know how that clade would affect the synapomorphy distribution*. The neosauropods quickly became the dominant sauropod group after their appearance in the middle Jurassic, and the only non-neosauropod sauropods to make it into the early Cretaceous were the aforementioned turiasaurs and possibly Jobaria, an African genus that varying analyses place either just inside or just outside the Neosauropoda.

*There has been an annoying tendency in recent years for many papers featuring phylogenetic analyses to present us with the character data matrix and the final trees from the analysis, but not do anything to map character changes onto the tree. The only way to find that out would be by transcribing the entire matrix and re-running the analysis yourself.

Mounted skeleton of Amargasaurus cazaui in the Melbourne Museum.

The famed North American genera include representatives of each of the two main lineages within the Neosauropoda. Diplodocus and Apatosaurus belong the Diplodocoidea, and Brachiosaurus and Camarasaurus belong to the Macronaria. Diagnostic features of the diplodocoids according the Upchurch et al. (2004) include restriction of teeth to the front of the jaw and a subrectangular snout. This last feature reaches an extreme in the middle Cretaceous Nigersaurus, which was another of those animals that serves to remind us that, if God did indeed create all of nature, then he was taking the piss. Ludicrous diplodocoids also include the late Jurassic South American Brachytrachelopan, which took one look at the graceful, elongate necks of all the other sauropods and decided that it simply couldn't be having with all that.

Reconstruction of Saltasaurus loricatus, by Lady of Hats.

The name of the other lineage, Macronaria, means 'big nostrils', and one of the notable features of this clade is, indeed, a great expansion in the size of the nares, the opening for the nostrils in the skull (though whether the size of the nares directly corresponds to the size of the actual nostrils is, I suppose, another question). As well as Brachiosaurus and Camarasaurus, this clade includes the Titanosauria, a very successful group that included the last surviving sauropods, but whose significance was overlooked for many years because they had the poor judgement not to achieve their main diversity in North America. At least some titanosaurs, such as Saltasaurus pictured above, sported a skin reinforced by nodules of bone.

Which brings us to Camarasaurus. For some reason, of the 'Big Four' genera, this is the one that gets the least love. While the other three have each in their turn enjoyed roles as stars of stage and screen, I'm not aware of a single film in which Camarasaurus has even been given a name-drop*. In John Sibbick's illustration of Brachiosaurus and Camarasaurus together (scroll down a bit at the link) in Norman's (1985) The Illustrated Encyclopedia of Dinosaurs, Brachiosaurus marches confidently towards the front bearing a goofy leer, while Camarasaurus is forced to sulk towards the back. It's all blatantly unfair. It's not as if Camarasaurus is rare: in fact, Camarasaurus may just be the best known of all sauropods, represented in the Morrison Formation by a whole whack of remains, including what is perhaps the single most beautiful sauropod specimen ever found (the one with which I opened this post). It was no slouch in the size department, either: its maximum length of about 23 metres is similar to that of Apatosaurus, despite it having proportionally shorter appendages than the latter. Nor does it lack distinctiveness: the short, bulldog-like face of Camarasaurus instantly stands out in any neosauropod line-up. So after all this time, doesn't Camarasaurus deserve to be given the spotlight?

*Though it is a pity that the name 'Camarasaurus' won out in the priority stakes over its competitor 'Morosaurus', which to those of us from a New Zealand background suggests a dinosaur made out of chocolate.

REFERENCES

Norman, D. 1985. The Illustrated Encyclopedia of Dinosaurs. Salamander Books: London.

Royo-Torres, R., A. Cobos & L. Alcalá. 2006. A giant European dinosaur and a new sauropod clade. Science 314: 1925-1927.

Upchurch, P., P. M. Barrett & P. Dodson. 2004. Sauropoda. In: Weishampel, D. B., P. Dodson & H. Osmólska (eds) The Dinosauria, 2nd ed., pp. 259-322. University of California Press: Berkeley.

Sleepers

Hawaiian sleeper Eleotris sandwicensis, from the Hawaii Biological Survey.

Fishes of the genus Eleotris are a group of gobioids commonly known as the spinycheek sleepers. I haven't found a definite statement as to why they're called sleepers, but presumably it's because, as sit-and-wait ambush predators, they spend a lot of time lying around on the bottom. Eleotris species are found in tropical and subtropical waters around the world, mostly in estuaries and freshwater. They are smallish fish, with most species seeming to be in the ten to twenty centimetre size range. The 'spinycheek' part of the vernacular name refers to the presence of a hook-like spine on the lower corner of the preoperculum (the bone running between the cheek and the gill cover on the side of the head). This spine may be covered with tissue and so not always readily visible, but Pusey et al. (2004) note that it 'can be easily detected by running a thumbnail lightly, and carefully, along the preoperculum margin'. Carefully, I think, is the operative word here.

Eleotris oxycephala, from Chinese Academy of Fishery Sciences.

The species of Eleotris are mostly a conservative bunch appearance-wise, and the genus seems to have gotten a reputation for being difficult to work with taxonomically (it doesn't help matters that for a long time 'Eleotris' was something of a dumping ground for generalised gobioids). The Japanese species were revised in 1967 by Akihito (yes, that Akihito), the West African species have been revised by Miller (1998), and the North and South American species by Pezold & Cage (2002), but species from the remainder of the Indo-Pacific remain unrevised. There has been some disagreement over the status of a group of New World species classified in the genus Erotelis, which resemble Eleotris species but are generally more elongate and have higher numbers of fin rays (Pezold & Cage 2002). Miller (1998) felt that this genus should be synonymised with Eleotris, but Pezold & Cage (2002) argued that its members were distinct enough to be kept separate. A molecular phylogenetic analysis of the gobioids by Thacker & Hardman (2005) suggested that 'Erotelis' is nested within Eleotris, which may support their synonymisation.

Dusky sleeper Eleotris fusca, photographed by C. Appleby.

The sleepers are amphidromous, meaning they spend part of their life in the sea. Sleepers enter the sea as larvae, returning to fresher waters as they mature. As a result of this marine stage in the life cycle, individual species of Eleotris may be widespread and can often be found in places such as oceanic islands that lack populations of permanently freshwater species. It has even been suggested they may cross oceans: Miller (1998), noting similarities between species on either side of the Atlantic, suggested that this may be the result of trans-Atlantic dispersal. Among the evidence cited in favour of this possibility was the record in 1987 of a specimen of the northern South American species Eleotris pisonis from the island of St Helena in the mid-Atlantic. However, Miller also noted that the amount of time it would take to disperse across the Atlantic is greater that the time it would take for the larva to develop to maturity (and mature Eleotris are not known from the open sea). Pezold and Cage (2002) were more skeptical about the possibility of trans-Atlantic dispersal, even though they admitted to being unable to identify any characters distinguishing the Caribbean E. amblyopsis from the West African E. daganensis. They queried whether the St Helena record may have been an individual transported in ship ballast water, rather than an unaided dispersal.

REFERENCES

Miller, P. J. 1998. The West African species of Eleotris and their systematic affinities (Teleostei: Gobioidei). Journal of Natural History 32 (2): 273-296.

Pezold, F., & B. Cage. 2002. A review of the spinycheek sleepers, genus Eleotris (Teleostei: Eleotridae), of the western hemisphere, with comparison to the West African species. Tulane Studies in Zoology and Botany 31: 19–63.

Pusey, B., M. Kennard & A. Arthington. 2004. Freshwater Fishes of North-eastern Australia. CSIRO Publishing: Collingwood.

Thacker, C. E., & M. A. Hardman. 2005. Molecular phylogeny of basal gobioid fishes: Rhyacichthyidae, Odontobutidae, Xenisthmidae, Eleotridae (Teleostei: Perciformes: Gobioidei). Molecular Phylogenetics and Evolution 37: 858-871.

Raphitoma histrix

The shell in the photo above (from G. & Ph. Poppe) belongs to Raphitoma histrix (Bellardi 1847)*, a small marine gastropod (the shell shown is just over 6 mm long). Raphitoma histrix was first described as a fossil from the Pliocene of north-western Italy, but the name has also been used for Recent shells from the Mediterranean and western Africa. Rolán et al. (1998) implied (but did not state) that the identity of the modern specimens could do with a double-check. Raphitoma histrix is found in soft-bottomed, reasonably calm waters.

*There has been a bit of confusion over the exact name for this species. Many authors have referred to it as 'Raphitoma hystrix (Cristofori & Jan 1832)'. However, as noted by Pusateri et al. (2012), Cristofori & Jan's name was a nomen nudum (meaning that it lacked an accompanying description), making it not validly published. The name was not validated until it was used by Bellardi (1847), who used the spelling 'histrix'.

Raphitoma histrix is the type species of the genus Raphitoma, which is in turn the type genus of the subfamily Raphitominae. Past authors have classified this genus within the family Turridae, and the Raphitominae broadly corresponds with the group that has often been called the 'Daphnellinae' (other 'turrids' have been featured on this site before: Comitas, Antiplanes, Kuroshioturris, Asperdaphne and Paradrillia). However, with the recognition that the old 'Turridae' was largely just a dumping ground for less differentiated members of the superfamily Conoidea, the Raphitominae is now usually treated as part of the Conidae, and so a member of the same family as the cone shells. Like other members of this family, including other species of Raphitoma, R. histrix is probably a predator, possibly feeding on other marine invertebrates such as worms.

REFERENCES

Pusateri, F., R. Giannuzzi-Savelli & M. Oliverio. 2012. Revisione delle Raphitomidae mediterranee 1: su Raphitoma contigua (Monterosato, 1884) e Raphitoma spadiana n. sp., specie sorelle (Gastropoda, Conoidea). Sociedad Espanola de Malacologia—Iberus 30 (1): 41-52.

Rolán, E., J. Otero-Schmitt & F. Fernandes. 1998. The family Turridae s.l. (Mollusca, Gastropoda) in Angola (West Africa), 1. Subfamily Daphnellinae. Sociedad Espanola de Malacologia—Iberus 16 (1): 95-118.

The Araeolaimida: We Barely Know Ye

Axonolaimus sera, from here.

Overall, the nematodes cannot be considered one of the best-known groups of animals. This is not because they are at all uncommon: there is the oft-cited factoid that nematodes are so abundant in every corner of the world that, if everything other than them was somehow instantaneously removed, the ghostly shadow of the planet Earth would supposedly still be visible as a cloud of microscopic worms. Nematodes are even found in places other animals are not: they have been found further beneath the Earth's surface than any other multicellular organism. There are some nematode species that attract attention, such as those that cause diseases, or are notable crop or animal pests. The nematode Caenorhabditis elegans has been a workhorse of developmental biology for many a year. But these well-studied taxa represent only a small proportion of the full nematode diversity out there.

Being very small and soft-bodied, nematodes do not usually present taxonomists with a great variety of clearly defined morphological features. As a result, dividing nematodes into well-supported groupings has not been an easy task (there are some notable exceptions: try looking up the Desmoscolecida one of these days). Take, as an example, the group known as the Araeolaimida. This name spent many years as a bit of a wastebasket for various non-parasitic nematode families. Eventually, it was restricted by Ley & Blaxter (2002) to just four families: the Axonolaimidae, Comesomatidae, Diplopeltidae and Coninckiidae, with many taxa previously treated as Araeolaimida included in a separate order Plectida. Fonseca & Bezerra (2012) include a fifth family, the Bodonematidae, that was not mentioned by Ley & Blaxter. Even in this restricted sense, the Araeolaimida may not represent a coherent group. There is no single feature shared by all araeolaimidans that is not found in other nematodes, and a molecular phylogenetic study of nematodes by van Megen et al. (2009) did not recover a monophyletic araeolaimidan clade. Nevertheless, Araeolaimida normally have the ovaries outstretched within the bodies of females (in many other nematode taxa, they are folded back on themselves), and the amphids (sensory grooves on the sides of the head) are usually spiral or looped in shape. The majority of araeolaimidans are marine, with freshwater and terrestrial environments being home to two genera of Diplopeltidae, and a few species of Axonolaimidae (Fonseca & Bezerra 2012). We don't know much about their diet, but they are probably grazers on micro-algae or bacteria. About 400 species of Araeolaimida have been described, but it would be very surprising if there weren't more out there.

Head end of the freshwater diplopeltid Cylindrolaimus, photographed by Peter Mullin. Note the dark circle near the end: this is the amphid.

The separate families are a bit easier to define (Fonseca & Bezerra 2012). The single known species of Bodonematidae, Bodonema vossi, stands out by having a pharynx with the mid-part differentiated into a series of muscular bulbs, as opposed to the fairly simple pharynxes of other Araeolaimida. Coninckia, the only genus of Coninckiidae, has the amphids sitting on differentiated plaques that are not present in other taxa. The Comesomatidae have spiral amphids, while the Axonolaimidae and Diplopeltidae have simpler looped amphids. The last two families are distinguished by the shape of the buccal cavity, which is larger and more strongly sclerotised in the Axonolaimidae.

One detail which caught my eye when researching this post is that males of some axonolaimids produce two different forms of spermatozoa (Riemann 1986). The sperm cells produced in the anterior testis of Nicascolaimus punctatus are more than three times the size of those produced in the posterior testis. In another axonolaimid species, Axonolaimus helgolandicus, it is the posterior testis that produces the larger cells. Both types of sperm were shown in N. punctatus to be transferred to females, but the reason for the two different sperm types is unknown. Pomponema, a genus belonging to a separate group of nematodes from the Araeolaimida, produces dimorphic sperm in which the larger cells seem to break down before they are transferred to the female, and it is possible that only one sperm type functions to fertilise the female in axonolaimids as well. Perhaps the other sperm type represent some sort of nuptial gift? Or could they somehow interfere with fertilisation by other males? We await the nematode enthusiast who will find out.

REFERENCES

Fonseca, G., & T. N. Bezerra. 2012. Order Araeolaimida De Coninck, 1965. Zoology Online. Berlin, Boston: De Gruyter. Retrieved 3 Jun. 2014, from http://www.degruyter.com/view/Zoology/HBZ-2011-000076.

Ley, P. de, & M. Blaxter. 2002. Systematic position and phylogeny. In: Lee, D. L. (ed.) The Biology of Nematodes, pp. 1-30. Taylor & Francis: Florence (Kentucky).

Megen, H. van, S. van den Elsen, M. Holterman, G. Karssen, P. Mooyman, T. Bongers, O. Holovachov, J. Bakker & J. Helder. 2009. A phylogenetic tree of nematodes based on about 1200 full-length small subunit ribosomal DNA sequences. Nematology 11 (6): 927-950.

Riemann, F. 1986. Nicascolaimus punctatus gen. et sp.n. (Nematoda, Axonolaimoidea), with notes on sperm dimorphism in free-living marine nematodes. Zoologica Scripta 15 (2): 119-124.

Wasps with Fangs on their Feet?

Theronia septentrionalis, photographed by Stephen Cresswell.

The ichneumons are one of the more familiar groups of parasitoid wasps for the general public. The species in the photo above is a member of the Theronia group of ichneumons, which attain a reasonable size by wasp standards (a number of species seem to be in the range of 1.5 centimetres long) and are often brightly coloured in yellow or green. The Theronia group is primarily tropical in distribution, though some species are found in more temperate regions. Authors have differed on whether they treat this group as a single genus or divide it between about half a dozen genera; either option is complicated by the fact that both the group as a whole and some of its constituent restricted genera are doubtfully monophyletic (Gauld et al. 2002). Where their larval hosts are known, many members of the Theronia group are endoparasitic in moth cocoons (including some economically significant pests such as the gypsy moth), though at least some species are not parasites of the moth itself but are hyperparasites of other ichneumon larvae attacking the moth. One (sub)genus, Nomosphecia, includes parasites of vespid wasp larvae (Gauld 1984).

Male Theronia atalantae, photographed by Phil Huntley-Franck.

Bright colours are often a sign of danger in the animal kingdom, and the Theronia group seem to follow that trend. One of the group's distinctive features is larged, curved claws with an associated spatulate bristle. As noted by Gauld (1984), "When caught they sink their large claws into their captor." This sounds uncomfortable enough in itself, especially as said claws have a tendency to break and leave their tips embedded in the skin if the wasp is not allowed to remove them in her own time. But there's more: the inside of the claw bears a fluid-filled cavity, and the act of embedding the claws releases the contents of this cavity into the wound. In other words, the claws seem to function in much the same way as the fangs of a venomous snake.

Or do they? We know that the fluid injected by Theronia into would-be attackers can cause irritation to vertebrate epithelium (Gauld et al. 2002), but we don't seem to know just what it contains or how it acts. As such, we don't know how confident we can be that the fluid is indeed effective defensively. Theronia may have poison claws that act like fangs. Or it may just have big sharp claws, and that may be enough.

REFERENCES

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

Gauld, I. D., D. B. Wahl & G. R. Broad. 2002. The suprageneric groups of the Pimplinae (Hymenoptera: Ichneumonidae): a cladistic re-evaluation and evolutionary biological study. Zoological Journal of the Linnean Society 136: 421-485.

The Importance of Genitalia

Take a look at the figure above (taken from Mauriès 2003). What you're looking at is the intimate business of a male millipede, in this case a Bosnian millipede called Fagina silvatica. And if you ever had the pleasure of finding yourself working on millipede taxonomy, you'd be looking at a lot of these.

Fagina silvatica belongs to a superfamily of millipedes called the Neoatractosomatoidea, which is in turn part of the order Chordeumatida in the clade Helminthomorpha. Helminthomorph millipedes (as indicated by their name, which means 'worm-like') all cleave pretty closely to the classic image of their kind, with an elongate body bearing large numbers of relatively short legs. Chordeumatida are characterised by having silk-spinning glands on the telson, the very end segment of the body, and three pairs of strong bristles on the top of each body segment. Male chordeumatidans also have the eighth and ninth pairs of legs modified into the gonopods, the copulatory structures. Because millipedes are generally not extravagant animals in overall appearance, it is the gonopods that have become the primary structures for identifying them, and many millipede species cannot be reliably distinguished without examining them. In the Neoatractosomatoidea, the eighth pair of legs forms the gonopods proper that deliver the male's sperm to the female's vulvae, while the ninth pair form protective structures called paragonopods. The gonopods proper are divided into two branches that fold around each other, usually to guide a whip-like flagellum or other extended structure passing between them (one genus, Guizhousoma, lacks the flagellum—Mauriès 2005). One neoatractosomatoid genus, Osellasoma, also has the seventh pair of legs modified into protective structures (Mauriès 2003). Neoatractosomatoids have 28 or 30 body segments. Some neoatractosomatoids have the sides of the body extended into flattened processes called paraterga; others have the body more or less cylindrical. And no, I haven't been able to find a single photograph or illustration showing a neoatractosomatoid in its entirety. You'll have to content yourself with looking at their genitals (Wikipedia has photos of other Chordeumatida).

As defined by Mauriès (2003, 2005), the Neoatractosomatoidea only includes about 25 known species, mostly found in southern Europe. A single species, the aforementioned Guizhousoma latellai, is known from caves in China. Mauriès (2003) separated three families previously placed in the Neoatractosomatoidea into a separate superfamily Mastigophorophylloidea; if the mastigophorophylloids are included with the neoatractosomatoids, then the group includes further species found in northern Asia. Mauriès separated the two superfamilies on the basis that mastigophorophylloids possessed a flagellum on both the gonopods and the paragonopods, instead of only on the gonopods. The subsequent discovery of the entirely flagellum-less Guizhousoma could raise questions about the significance of this character, and the flagellum appears much reduced if not entirely absent on the paragonopods of at least one putative mastigophorophylloid, Kirkayakus pallidus, as illustrated by Mikhaljova (2004)*. However, I have to admit to having absolutely zero experience with interpreting millipede gonopods, so I am hardly one to be voicing an opinion.

*Mikhaljova (2004) illustrates this species under the name of Altajella pallida, but it has since been renamed by Özdikmen (2008) (yes, that Özdikmen) due to the original genus being preoccupied).

REFERENCES

Mauriès, J.-P. 2003. Schizmohetera olympica sp.n. from Greece, with a reclassification of the superfamily Neoatractosomatoidea (Diplopoda: Chordeumatida). Arthropoda Selecta 12 (1): 9-16.

Mauriès, J.-P. 2005. Guizhousoma latellai gen.n., sp.n., de Chine continentale, type d'une nouvelle famille de la superfamille des Neoatractosomatoidea (Diplopoda: Chordeumatida). Arthropoda Selecta 14 (1): 11-17.

Mikhaljova, E. V. 2004. The Millipedes (Diplopoda) of the Asian Part of Russia. Pensoft: Sofia.

Özdikmen, H. 2008. New family and genus names, Kirkayakidae nom. nov. and Kirkayakus nom. nov., for the millipedes (Diplopoda: Chordeumatida). Munis Entomology & Zoology 3 (1): 342-344.

Arthropods in the Precambrian?

The Ediacaran animal Spriggina floundersi, from here.

The Ediacaran biota has been touted as one of the great mysteries of palaeontology. Comprising the latest part of the Precambrian era, the Ediacaran is generally believed to have given us the earliest known animal fossils. However, palaeontologists have disagreed on just how the Ediacaran fossils relate to modern animals (see McCall 2006 for an exhaustively detailed review). Some see the Ediacarans as including the ancestors of groups that remain with us today: jellyfish, corals, comb jellies, sponges. Others see Ediacarans as outside the modern lineages: ancient animal groups that were swept aside by more modern animals at the beginning of the Cambrian. And some have even questioned whether the Ediacarans were even animals at all, suggesting links instead to fungi or Foraminifera, or even that they were an entirely independent lineage unrelated to any modern multicellular organisms.

In 1996, Benjamin Waggoner proposed the name 'Cephalata' for a clade uniting the arthropods with two groups of Ediacaran organisms: the Sprigginidae and the Vendiamorpha. These are among the most undeniably animal-like of the Ediacarans. The sprigginids (including Spriggina shown at the top of the post) have an undivided 'head' followed by a long segmented body. The vendiamorphs are shield-like organisms that also show evidence for segment-like divisions behind the 'head', such as branching internal structures that may represent side-branches of an internal gut.

The vendiamorph Vendia sokolovi, from Ivantsov (2004).

It is difficult to see these taxa as anything other than mobile animals. One supporter of non-animalian affinities for the Ediacarans, Adolf Seilacher, did suggest that Spriggina was a sessile organism, maintaining that the 'head' was in fact a holdfast while the 'body' extended upwards like the frond of a sea pen (I have seen a memorable reconstruction, though unfortunately I can't recall where, showing an individual of mobile Spriggina crawling past a cluster of sessile Spriggina). However, the numerous Spriggina specimens that have been found in Australia and Russia are invariably preserved lying flat, while sessile organisms from the same locations are preserved with the holdfast below the level of the body. Vendiamorphs, on the other hand, are simply not shaped in a way that allows them to be seen as anything other than lying flat. An immobile sprigginid or vendiamorph lying flat below the water would have been vulnerable to being buried by sediment, without any way of digging itself back out.

But if sprigginids and vendiamorphs were definitely animals, what kind of animals were they? It is at this point that things get a bit more vague. Their segmented appearance immediately suggests arthropods (and onychophorans) or annelids, but there is not a great deal to suggest one or the other. The differentiated head of sprigginids suggests the head of an arthropod, while vendiamorphs have been compared to the larvae of arthropods such as trilobites. However, it is unclear whether the Ediacaran taxa possessed anything like the limbs of arthropods and related taxa. The segments of sprigginids may be separated at the edges, and some have argued that folds in vendiamorph fossils are suggestive of limbs underneath a dorsal shield, but there is nothing that one would call unequivocal. Lateral outgrowths of sprigginids may correlate to annelid parapodia instead of arthropod limbs, and folds in the bodies of vendiamorphs may be nothing more than that. We recognise relationships between fossil and extant animals on the basis of whether they have features in common, but our assessment of what features they have may be coloured by what features we expect to see.

Another possible vendiamorph, Parvancorina minchami, from here. Note the fine parallel lines on the body, which some have interpreted as the outlines of limbs.

Some authors have drawn attention to a feature of both vendiamorphs and sprigginids that is visible in the image of Vendia above: their so-called 'glide reflectional symmetry'. Though their bodies appear segmented, the segments do not go straight across the body as one might expect. Instead, the left and right sides of the body are slightly offset from each other. For this reason, some authors have claimed that these animals do not show true bilateral symmetry and hence argued for placing them outside the Bilateria crown group, along its stem. However, others have suggested that the offset between sides may be an artefact of preservation. Even if it was indeed a feature of the living animal, glide reflectional symmetry may not necessarily force the sprigginids outside the Bilateria: a number of living bilaterians also show a certain degree of symmetry offset either as adults or during development, including basal chordates (Waggoner 1996).

During the period of the Cambrian, directly after the Ediacaran, we have access to beautifully preserved fossil deposits that have allowed us to characterise many animals from that period in exquisite detail. No such fossils exist for the Ediacaran; instead, Ediacaran animals are mostly preserved in coarse sediments that preserve only relatively broad features of the fauna. This can turn the Ediacarans into tantalising shadows, and what we see in them can say more about our assumptions than the animals themselves.

REFERENCES

Ivantsov, A. Yu. 2004. New Proarticulata from the Vendian of the Arkhangel’sk region. Paleontologicheskii Zhurnal 2004 (3): 21–26 (transl. Paleontological Journal 38 (3): 247–253.

McCall, G. J. H. 2006. The Vendian (Ediacaran) in the geological record: enigmas in geology's prelude to the Cambrian explosion. Earth-Science Reviews 77: 1-229.

Waggoner, B. M. 1996. Phylogenetic hypotheses of the relationships of arthropods to Precambrian and Cambrian problematic fossil taxa. Systematic Biology 45 (2): 190-222.