Return of the Water Bears (Taxon of the Week: Tardigrada)

False colour SEM image of two tardigrades, from here.

If you're a long-time reader of this site, you're probably already aware of the existence of tardigrades or water bears, microscopic stumpy-legged invertebrates. Previous posts on Tardigrada have given an overview of the main subgroups of tardigrades, and suggested how you might find your own specimens. The next logical step, I suppose, would be to say a few things about tardigrade ecology, and for that I shall draw heavily from the excellent reviews of Nelson & Marley (2000) and Nelson (2002).

Tardigrades may live in salt water, fresh water or terrestrially among mosses and leaf litter. However, because all tardigrades require at least a film of water to live in, the boundary between freshwater and terrestrial species is a trifle blurry and many species can be found in both. Tardigrades feed on plants and algae; their mouthparts have a piercing stylus through which they suck the cytoplasm out of cells. Different techniques are used for collecting marine and limno-terrestrial species, and I mention that solely because it gives me an opportunity to note that one of the methods for collecting marine tardigrades (and other sand-dwelling meiofauna) involves sieving material through a fine mesh net referred to as "Higgins' mermaid bra" (or, depending on author, "Gwen's mermaid bra", as it was Mrs Higgins who invented the tool used by her husband).


Close-up of the head of the tardigrade Macrobiotus. The stylet apparatus is visible inside the head; the stylets are everted when the animal is feeding. Photograph by Martin Mach.

In one of my earlier posts, I referred to the well-known ability of tardigrades to form resistant tuns when exposed to unfavorable conditions, a process called cryptobiosis. What I did not explain at that time was that five different types of cryptobiosis have been identified in tardigrades: encystment (production of a dormant phase without significant water loss), anoxybiosis (resistance to low oxygen levels), cryobiosis (resistance to freezing temperatures), osmobiosis (resistance to elevated salinity) and anhydrobiosis (resistance to desiccation). Not all tardigrades share all five resistances - for instance, anhydrobiosis (the best-known form) is only found among terrestrial tardigrades - and different species will have different degrees of resistance. Much has been made of the resilience of at least some tardigrade tuns, such as their ability to survive immersion for up to eight hours in liquid helium at -272°C (Rebecchi et al., 2007; for comparison, absolute zero is calculated to be -273.15°C) and even to survive exposure to the vacuum of space (Jönsson et al., 2008). However, the often-repeated claim that tardigrade tuns can survive for more than one hundred years seems to be unsupported (Jönsson & Bertolani, 2001, reviewed the 1948 report generally cited in support of this claim and found that the tuns tested in that report in fact failed to revive); tuns have not yet been definitely shown to survive for more than ten years.



Cryobiosis, the ability to withstand freezing, allows tardigrades to inhabit cryoconite holes like the one shown above in a photo from here. Cryoconite holes develop when darkly-coloured dust accumulates in patches on a sheet of ice; the increased heat absorption by the dark dust melts the surrounding ice, forming a small patch of liquid water. This water may then become home to bacteria, algae and other microscopic organisms released by the melting ice - a self-contained microscopic ecosystem where a nematode may be the most fearsome predator in town. The cryoconite hole may freeze up again when the winter comes, of course, but its inhabitants can wait in the ice for the sun to come again.

REFERENCES

Jönsson, K. I., & R. Bertolani. 2001. Facts and fiction about long-term survival in tardigrades. Journal of Zoology 255 (1): 121-123.

Jönsson, K. I., E. Rabbow, R. O. Schill, M. Harms-Ringdahl & P. Rettberg. 2008. Tardigrades survive exposure to space in low Earth orbit. Current Biology 18 (17): R729-R731.

Nelson, D. R. 2002. Current status of the Tardigrada: evolution and ecology. Integrative and Comparative Biology 42 (3): 652-659.

Nelson, D. R., & N. J. Marley. 2000. The biology and ecology of lotic Tardigrada. Freshwater Biology 44 (1): 93-108.

Rebecchi, L., T. Altiero & R. Guidetti. 2007. Anhydrobiosis: the extreme limit of desiccation tolerance. Invertebr. Survival J. 4: 65-81.

Hints for # 6

A quick reminder that no-one has yet made a guess for Name the Bug #6. So a couple of hints:

(1) Devonian

(2) Aydin's comment that the animal is sinistral is incorrect, or at least not entirely correct.

    (2a) Take a close look at the protoconch.

Name the Bug # 6

(Attribution to follow)

It may be a couple of days before I identify this one (assuming that it doesn't follow the usual pattern and have someone successfully identify it within a matter of minutes). I'm not entirely sure of the actual size of the organism shown (why the hell don't invertebrate palaeontologists at least use scale bars?) but I think it's a couple of millimetres long.

Update: The identity of this organism is now available here. Figure from Frýda & Blodgett (1998).

Name the Bug: Anomalurus pelii auzembergeri

Anomalurus pelii auzembergeri (photo from here)

The "scaly-tailed squirrels" of the family Anomaluridae are seven species in three genera of arboreal rodents found in western and central Africa. Like pretty much everything from western and central Africa, they're somewhat enigmatic. Relationships between anomalures and other rodents have long been debated; it seems likely that their closest relative is the springhaas Pedetes capensis, another African endemic (Blanga-Kanfi et al., 2009), but the relationship is not an overly close one, nor can we be really confident where the springhaas-anomalure clade sits in turn. One thing we can be reasonably sure of is that anomalures are not closely related to squirrels (despite the common name).

The names "scaly-tailed squirrel" and "Anomaluridae" (i.e. "strange tail") both refer to the double-row of keeled scales under the base of the tail, visible in the photo above. These scales are used to grip the tree on which the animal is climbing, and also as accessory landing gear in the two gliding genera, Anomalurus and Idiurus (Nowak, 1999). The monotypic third genus, Zenkerella insignis, lacks a gliding membrane (patagium). Idiurus and Zenkerella are currently regarded as more closely related than either is to Anomalurus, but this relationship does not seem to have been formally tested phylogenetically. In the two gliding genera, an elongate cartilaginous process extends from the elbow to support the patagium; similar processes have been evolved by other gliding mammals (Johnson-Murray, 1987), but anomalurids are remarkable in just how much it has been developed.

Anomalurus pelii is the largest of the anomalures, up to two kilograms in weight, and is found from Liberia to the Ivory Coast. The individual in the photo above is the Liberian subspecies A. pelii auzembergeri which differs from other subspecies in lacking bright white patches on the head and along the margins of the patagium, as seen in the photo below from here.



REFERENCES

Blanga-Kanfi, S., H. Miranda, O. Penn, T. Pupko, R. W. DeBry & D. Huchon. 2009. Rodent phylogeny revised: analysis of six nuclear genes from all major rodent clades. BMC Evolutionary Biology 9: 71.

Johnson-Murray, J. L. 1987. The comparative myology of the gliding membranes of Acrobates, Petauroides and Petaurus contrasted with the cutaneous myology of Hemibelideus and Pseudocheirus (Marsupialia, Phalangeridae) and with selected gliding Rodentia (Sciuridae and Anomaluridae). Australian Journal of Zoology 35 (2): 101-113.

Nowak, R. M. 1999. Walker's Mammals of the World 6th ed., vol. 1. Johns Hopkins University Press.

Name the Bug #5

(Attribution to follow)

Hint: it's not a bug.

Update: Identity now available here. Photo from here.

A Pathetic Plea for Recognition, and a Platypus-billed Duck

The closing date of submissions for this year's OpenLab, an annual collection of the year's best science-blog writting (as judged by the judges), is the 1st of December - a week from today. If there has been anything here at Catalogue of Organisms over the past year (since December the 1st last year), please (please!) submit it for consideration. Please! Go through the archive in the right sidebar, pick out your favourites, and make your contribution towards restoring my fragile sense of self-worth.

Otherwise, your humble host is still fairly knackered after getting back from the field yesterday (two weeks away = nearly three hundred e-mails [mostly spam], 1000+ entries on Google Reader, one pair crossed eyes). So just a brief finishing note:



This is the braincase of Talpanas lippa, a subfossil duck species, about the size of a mallard, described from Kauai by Iwaniuk et al. (2009) in Zootaxa today (and the article is freely available to all comers). As well as the braincase, Talpanas is also represented by pieces of jaw and leg bones and a partial pelvis. The name means "nearly blind mole-duck" - Talpanas would have had small, piggy little eyes, quite unusual in a bird, and would have almost certainly been nocturnal and flightless (flying blind is not usually recommended). Though the complete beak is still unknown, the available jaw pieces indicate that it would have been very broad. The leg bones indicate that Talpanas was a walker rather than a swimmer, so Talpanas was probably a forager for small invertebrates among forest litter; this is the lifestyle currently pursued by the kiwi, another nocturnal bird with relatively small eyes. Iwaniuk et al. suggest that Talpanas also resembled a platypus in using its broad bill to feel for invertebrates amongst the soil. The opening for the trigeminal nerve in the braincase is very large like that of a platypus (it's the opening labelled 'V' on the images above - take a look, it's freaking huge), indicating that Talpanas' bill would have been very sensitive to touch. Unfortunately, the skull of Talpanas is so unusual that its relationships with other anseriforms are obscure.

Thistle Be The One (Taxon of the Week: Carduoideae)

The cardoon Cynara cardunculus with humans to scale. Photo from here.

The composite-flowering plants of the Asteraceae are one of the largest (23,000 species, according to Wikipedia) and most distinctive plant groups out there - even a complete botanical dunce like yours truly can usually recognise an example of Asteraceae. Asteraceae include such plants as daisies and chrysanthemums in which the "flower" is in fact a large number of tiny flowers all pressed together, hence the old name for the family of "Compositae". Different authors have proposed different classifications within Asteraceae over the years, but twelve subfamilies were recognised by Panero & Funk (2008). The subfamily Carduoideae as recognised by these authors includes the three tribes Dicomeae, Tarchonantheae and Cardueae (earlier authors had used the name to cover a broader paraphyletic assemblage, or restricted it to include only Cardueae). The genus Oldenburgia may be included in Tarchonantheae or it may be placed in its own separate tribe (Funk et al., 2009). No unique morphological features characterise this subfamily (though most species have a ring of papillae on the style underneath the stigmatic branches), but it is well supported molecularly.

The tribes Dicomeae and Tarchonantheae are primarily found in Africa and Madagascar (two species of Dicomeae and one of Tarchonantheae are found in Asia). The seventeen species of Tarchonantheae (including Oldenburgia) are all shrubs or trees; the 75-100 species of Dicomeae include herbs, shrubs and trees. Tarchonantheae includes the genus Brachylaena, species of which predominate in southern African and Madagascan woodlands. Brachylaena species are noted for producing dense, high quality wood, and are also among the largest of the Asteraceae, reaching 40 m in height (Beentje, 2000).


Brachylaena discolor from southeastern Africa. Photo from here.

The largest by far of the three tribes is the Cardueae*, the thistles, with some 2500 species distributed through Eurasia from the Mediterranean to central Asia. The majority of Cardueae are herbs, though there are a few small shrubs or even small trees in the tribe. Most members of Cardueae have distinctive discoid flower heads** and, of course, many have spiny leaves.

*I have just been through the painful, arduous and not-entirely-productive process of trying to decide whether 'Cardueae' or 'Cynareae' is the correct name for this tribe; both names are used regularly. Lamarck & de Candolle published the name 'Cynarocephalae' in 1806 (Reveal, 1997); Cardueae was published by Cassini in 1819 (Solbrig, 1963). The question therefore hinges on whether the '-cephalae' in Cynarocephalae represents a suffix like '-idae' or '-aceae' or whether the name is descriptive of plants with 'heads like Cynara'; if the former, Cynareae has priority from 1806; if the latter, Cynareae was not published until 1830 (and illegitimately so at that) and Cardueae has priority. Botanists still seem to be in the process of duking out which interpretation is corrent, and I suspect that it may take the ICBN stepping in to settle the matter.

**Composite flower heads may contain both 'ray' and 'disk' florets (the little individual flowers). If you think of a daisy, the 'ray' florets are the ones around the edge that carry the large petals while the 'disk' florets are the central ones without petals. Discoid flower heads like those of Cardueae contain only disk florets and no ray florets.


Side view of flower head of Atractylis cancellata, a Mediterranean thistle species in which the rosette of (particularly evil-looking) leaves around the flower head curls upwards to surround it. Photo by Manuel Ramos.

Species of Cardueae most often bring themselves to humanity's attention through the fact that a number of them are significant weed species, and very few Cardueae are regarded with any sort of affection. The Scotch thistle Onopordum acanthium is of course popular in Scotland where it is the national flower; according to legend, a Scottish encampment was saved from a sneak attack by Vikings when one of the invaders yelled out after stepping on a thistle, alerting the sentries to their presence. Also granted a certain regard is Cynara cardunculus, the cardoon/globe artichoke. Earlier classifications recognised two species, the cardoon C. cardunculus grown for its edible stalks and the artichoke C. scolymus grown for its similarly edible flower heads, but there is no doubt that the latter is a horticulturally derived variety of the former. Perhaps the best demonstration of this is that escaped seeds from artichoke fields in California and Australia have given rise to wild populations of 'cardoons' (Sonnante et al., 2007). I will also note that artichokes would also be a feature of my ideal garden - not because I'm a fan of eating artichokes (I think they're pretty tasteless) but because these two-metre tall thistles are such spectacular plants.

And that's all you'll be hearing from me for a little while - five-thirty tomorrow morning, I leave for two weeks in the field. Feel free to talk among yourselves until I get back.

REFERENCES

Beentje, H. J. 2000. The genus Brachylaena (Compositae: Mutisieae). Kew Bulletin 55 (1): 1-41.

Funk, V. A., A. Susanna, T. F. Steussy, & H. E. Robinson. 2009. Classification of Compositae. In Systematics, Evolution, and Biogeography of Compositae (V. A. Funk, A. Susanna, T. F. Stuessy & R. J. Bayer, eds) pp. 171-189. International Association for Plant Taxonomy (IAPT): Vienna.

Panero, J. L., & V. A. Funk. 2008. The value of sampling anomalous taxa in phylogenetic studies: major clades of the Asteraceae revealed. Molecular Phylogenetics and Evolution 47 (2): 757-782.

Reveal, J. L. 1997. Early suprageneric names in Asteraceae. Compositae Newsletter 30: 29-45.

Solbrig, O. T. 1963. Subfamilial nomenclature of Compositae. Taxon 12 (6): 229-235.

Sonnante, G., A. V. Carluccio, R. Vilatersana & D. Pignone. 2007. On the origin of artichoke and cardoon from the Cynara gene pool as revealed by rDNA sequence variation. Genetic Resources and Crop Evolution 54 (3): 483-495.

"Electronic Publication of Nomenclatural Acts is Inevitable"

The Jurassic mecopteran Lichnomesopsyche gloriae, one of six new fossil species not published today. The black line is highlighting the long proboscis; the scale bar represents 10 mm. Image from Ren et al. (2009).

So sayeth Mike Taylor (for my own confused ramblings through the quagmire of electronic publication, read my earlier posts on the subject). And this day presents us with a spectacular demonstration of that point.

In a paper in today's issue of Science, Ren et al. (2009) have presented an analysis of Jurassic to early Cretaceous long-proboscid scorpionflies and their role as probable pollinators of nectar-producing gymnosperms (as has also been suggested for kalligrammatid lacewings). As part of this study, Ren et al. present descriptions of six new species and two new genera of fossil scorpionflies. Nothing out of the ordinary here, except that (Science being Science, with its notorious restrictions on article length) the species descriptions are published in the Supporting Online Material.

From the point of view of the ICZN, Science is a perfectly valid forum for publication - thousands of copies are printed every week. But these printed editions don't include the online supplements, so the online-only component of the journal is currently not a valid publication. Technically speaking, the new species of Ren et al. (which are referred to and illustrated but not described in the print version) are nomina nuda. They are not valid names. But these online-only names have not appeared in some far-flung unfrequented corner of the internet, they have appeared in one of the world's most prominent science journals (like it says on the label). Their validity is going to be pretty much taken for granted.

REFERENCES

Ren, D., C. C. Labandeira, J. A. Santiago-Blay, A. Rasnitsyn, C.-K. Shih, A. Bashkuev, M. A. V. Logan, C. L. Hotton & D. Dilcher. 2009. A probable pollination mode before angiosperms: Eurasian, long-proboscid scorpionflies. Science 326: 840-847.

Name That Bug: Ponopterix axelrodi

Ponopterix axelrodi (from Bechly, 2007).

Obviously I'm going to have to refrain from using fossil insects as ID challenges in future, or at least confiscate Adam Yates' copy of Grimaldi & Engel (2005) before I do so to stop him from identifying them so quickly*.

*Unless, of course, I cruelly exploit Grimaldi & Engel's neglect of Palaeozoic polyneopterans.

Ponopterix axelrodi is a member of the Jurassic to Cretaceous insect family Umenocoleidae from the Lower Cretaceous Crato Formation of Brazil. Umenocoleids were originally described in 1973 as beetles, which they resemble in having the front pair of wings hardened into a pair of elytra (wing covers). However, while elytra are only found in two orders among the Recent insect fauna (beetles and earwigs), umenocoleids represent a third independent origin of elytra and are in fact related to dictyopterans (the clade that includes cockroaches, mantids and termites). The retention of a short ovipositor in Umenocoleidae (visible in the specimen above at the very end of the abdomen) places them just outside crown Dictyoptera, though a position closer to polyphagoid cockroaches has also been suggested (which would imply more than one loss of the ovipositor among dictyopterans).

As Adam pointed out, umenocoleids differ from beetles in that wing venation is still marginally visible on the elytra (among crown-group beetles, the original venation has been completely obliterated) and in the presence of cerci (two tail-like appendages at the end of the abdomen, one on either side of the ovipositor in females; cerci are absent in paraneopteran and holometabolous insects). The anterior light patch at the base of the elytron in the specimen above is also present in another specimen of the same species illustrated in Grimaldi & Engel (2005), so this was the original colour pattern of the animal when it was alive*.

*Don't let the poor reputation of cockroaches put you off - many roaches are very attractive insects, boldly patterned in contrasting colours**.

**Just be careful of the desert cockroaches that walk around with their backsides pointed into the air. If they feel that a potential threat is approaching too close, they can fire a stream of foul-smelling liquid towards it from a pair of abdominal glands. Not pleasant.

Umenocoleids also inspire the one detail in Grimaldi & Engel (2005) that causes me to scream with frustration. In the caption to their photo of Ponopterix axelrodi, G & E make the remark, "Umenocoleid roaches are known from the Late Jurassic to Cretaceous, though a putative living species exist". A living umenocoleid? Tell me more! Unfortunately, Grimaldi and Engel provide no citation for this statement, and I have been unable to find any reference to a living umenocoleid anywhere else. I'm still holding out hope, though.

REFERENCES

Bechly, G. 2007. 'Blattaria': cockroaches and roachoids. In The Crato Fossil Beds of Brazil: window into an ancient world (D. M. Martill, G. Bechly & R. F. Loveridge, eds). Cambridge University Press.

Grimaldi, D., & M. S. Engel. 2005. Evolution of the Insects. Cambridge University Press. 755 pp.

Name That Bug # 4

Another fossil for you all. Scale bar is five millimetres; attribution, as always, to follow.

Update: Identity now available here. Photo from Bechly (2007).

Name That Bug: Meioneurites spectabilis

Meioneurites (Parameioneurites) spectabilis (from Engel, 2005).

This image wasn't up ten minutes before being identified by Adam Yates. Meioneurites spectabilis is a member of the Jurassic butterfly-like lacewing family Kalligrammatidae. Kalligrammatids have been featured on Catalogue of Organisms previously.

In the fossil above, you can see that kalligrammatids lacked the coiled proboscis of a butterfly (though their mouthparts are much more elongate than other lacewings, and kalligrammatids were probably nectar feeders like butterflies). Also, if you look very closely at the wings (you'd probably have to zoom in), you may be able to make out that there is a higher density of veins in the wings than in butterflies.



For comparison, here's another kalligrammatid, Sophogramma lii, in a figure from Yang et al. (2009). Pretty.

REFERENCES

Engel, M. S. 2005. A remarkable kalligrammatid lacewing from the Upper Jurassic of Kazakhstan (Neuroptera: Kalligrammatidae). Transactions of the Kansas Academy of Science 108 (1-2): 59-62.

Yang Q., Zhao Y.-Y. & Ren D. 2009. An exceptionally well-preserved fossil kalligrammatid from the Jehol Biota. Chinese Science Bulletin 54 (10): 1732-1737.

Name That Bug # 3

Mr Greenslade, tell the masses what's the challenge:



Perhaps a little easy, but I'm feeling charitable. Size of image about ten centimetres across. Attribution, of course, to follow identification.

Update: The identification post for this image (from Engel, 2005) is now available here.

Cranes Off the Rails (Taxon of the Week: Grues)

The 'Messel rail' Messelornis cristata - a specimen with preserved plumage. Photo from here.

Despite its presentation in years of fieldguides and other popular books, the bird order 'Gruiformes' has in recent times been scattered to the four winds, with analyses both morphological and molecular proclaiming its polyphyly. Nevertheless, molecular analyses such as Hackett et al. (2008) continue to support a clade roughly corresponding to the suborder Grues as recognised by Cracraft (1973)* containing the cranes and the rails. The morphological analysis of Livezey & Zusi (2007) on the other hand, does not support this clade, but it does support monophyly for each of the two primary divisions within Grues, the ralloid and gruoid lineages.

*Just to confuse matters, the name "Grues" has been used by different authors for clades of differing inclusivity. Mayr (2009), for instance, uses "Grues" for the Aramus + Gruidae clade, and refers to the larger clade as "core Gruiformes".

The ralloid line contains the living families Rallidae*, the rails, and Heliornithidae, the finfoots (or should that be finfeet?) Cracraft (1973) regarded the Cretaceous Laornis edvardsianus as a stem ralloid, but no-one else seems to have taken him up on this suggestion. More reliably on the ralloid stem are the Palaeocene to Oligocene Messelornithidae (Mayr, 2009). Messelornithids were medium-sized birds (about the size of a small chicken) best known from Messelornis cristata for which over 500 specimens are available, some even with preserved feathering. Messelornis was highly terrestrialised with limited flight capabilities and almost ludicrously long legs (loss or reduction of flight has been a common occurrence among the Grues). Its beak was relatively short and the overall appearance of Messelornis would probably have not been dissimilar to the modern cariamas.

*Hackett et al. (2008) resolved the Rallidae as paraphyletic to Heliornithidae, with Sarothrura (the flufftails) closer to Heliornis than to the other two included rails Himantornis and Rallus. A few places, at least online, have suggested recognising Sarothrura as a separate family from the Rallidae as a result, but I'd recommend waiting for a more detailed analysis with greater coverage of the Rallidae. Increased taxonomic coverage may return the flufftails to the other Rallidae, or it may make it more appropriate to treat the finfoots as derived rallids.


The sungrebe Heliornis fulica of tropical South America (I tried to find a picture of one carrying chicks, but no luck). Photo by Jerry Oldenettel.

The finfoots of the Heliornithidae are three species (one in Asia, one in Africa, one in South America) of tropical grebe-like birds, renowned for their reclusiveness. The South American sungrebe Heliornis fulica is the most distinctive in appearance of the three species (though mitochondrial analysis indicates that it and the Asian Heliopais personata form a clade to the exclusion of the African Podica senegalensis - Fain et al., 2007) and is also very distinct in its nesting behaviour. Heliopais and Podica, like most aquatic birds, have chicks that hatch out reasonably well-developed and immediately able to swim after their parents. Heliornis, in contrast, has altricial chicks that hatch out after only ten to eleven days of incubation. The really amazing bit, though, is what happens after the chicks hatch. The male sungrebe has a shallow pouch under each wing and he is able to transport the chicks inside this pouch, even flying with them. Whether the chicks remain in the pouches permanently or whether they are only placed in them while the male is travelling remains unknown. Funnily enough, while this chick-carrying behaviour was described by Alvarez del Toro in 1971, it had originally been recorded almost 140 years earlier by Prince Maximilian of Wied. It seems that everyone else had assumed the prince was smoking something.


Grey-winged trumpeters, Psophia crepitans. Photo by A. Vinot.

The gruoid lineage includes Psophia, the trumpeters, Aramus guarauna, the limpkin, and Gruidae, the cranes, as well as the fossil taxa Parvigrus pohli, Geranoididae and Eogruidae. Most recent authors agree that Aramus and Gruidae form a clade to the exclusion of Psophia. The chicken-sized Oligocene Parvigrus was originally described by Mayr (2005) as sister to Aramus + Gruidae, but he later (Mayr, 2009) revised its position to stem gruoid. Parvigrus lacked the long beak of limpkins and cranes, as do the Recent trumpeters, three species of similarly chicken-sized birds found in northern South America.

Whether Geranoididae and Eogruidae possessed crane-like long beaks is an unknown factor as skull material for both has not been found. Cracraft (1973) placed both outside the crown gruoids, but Clarke et al. (2005) placed Eogruidae inside the gruoid crown as sister to Aramus + Gruidae. The Eocene Geranoididae have been described only from leg bones (Wetmore, 1933, assigned some wing bones to Geranoides jepseni in his original description of this species but did not describe them) so little can be said about them except that they were large and long-legged. Wetmore (1933) commented on the unusually wide spacing of the trochleae (the 'knuckles') at the end of the tarsometatarsus suggesting that Geranoides had very widely splayed toes, but Cracraft (1969) later attributed to wide spacing to post-mortem distortion. Cracraft (1969, 1973) included a number of Eocene birds in the Geranoididae but admitted a lack of derived characters uniting them; Geranoididae may represent a paraphyletic assemblage of basal gruoids.


Distal ends of tarsometatarsi of the eogruids Proergilornis and Ergilornis, showing reduction of the inner trochlea in Proergilornis and its loss in Ergilornis. Figure from Cracraft (1973).

The Eocene to Pliocene Eogruidae were also decent-sized long-legged birds from central Asia and (in later times) Europe. Earlier authors recognised two families, Eogruidae and Ergilornithidae, but 'ergilornithids' are now recognised as derived eogruids. Eogruids were highly cursorial birds and a humerus attributed to Ergilornis suggests that it was flightless, though the earlier Eogrus aeola shows no sign of being so (Clarke et al., 2005). Originally three-toed, eogruids showed a reduction in the size of the inner toe, and Ergilornis and Amphipelargus (the latest of the eogruids) lost it entirely (it is easy to present a progression from flying and three-toed to flightless and two-toed, but be warned that three-toed species survived into the Miocene, well after the appearance of the two-toed forms). The only other birds to reduce the number of toes to two are the ostriches, and a relationship between ostriches and eogruids has been suggested in the past (generally in association with the idea that the ratites do not form a monophyletic group). However, Cracraft (1973) confirmed that eogruids were more similar in their fine morphology to gruoids than ostriches, and modern phylogenetic analyses do not support a close relationship of ostriches and gruoids.

Many people carry the impression that flightlessness in birds is associated with lack of predators. However, eogruids evolved flightlessness in an environment in which predators were no rarity (amongst others, they shared their world with such horrors as hyaenodonts and entelodonts*). Similarly, while the exact circumstances in which they became flightless is unknown, modern ostriches (Africa), emus (Australia) and rheas (South America) all live alongside significant predators or at least did so until recently. Obviously, something other than lack of predators is at play here.

*I always imagine Roald Dahl's hornswogglers to be something like an entelodont.

REFERENCES

Clarke, J. A., M. Norell & D. Dashzeveg. 2005. New avian remains from the Eocene of Mongolia and the phylogenetic position of the Eogruidae (Aves, Gruoidea). American Museum Novitates 3494: 1-17.

Cracraft, J. 1969. Systematics and evolution of the Gruiformes (class, Aves). 1, The Eocene family Geranoididae and the early history of the Gruiformes. American Museum Novitates 2388: 1-41.

Cracraft, J. 1973. Systematics and evolution of the Gruiformes (class Aves). 3, Phylogeny of the suborder Grues. Bulletin of the American Museum of Natural History 151: 1-127.

Fain, M. G., C. Krajewski & P. Houde. 2007. Phylogeny of "core Gruiformes" (Aves: Grues) and resolution of the limpkin–sungrebe problem. Molecular Phylogenetics and Evolution 43: 515-529.

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