Giant hoatzins of doom

Inspired by the recent description of a new and exciting phorusrhacid – a giant predatory South American landbird – I have lately been producing various blog posts on members of this group, as you’ll know if you’re a regular reader. See Terror birds and More on phorusrhacids. There is lots more to say: I am planning also to blog about phorusrhacid skull and hand anatomy, and about their alleged survival into near-modern times. The horrible danger is that the distractions that constantly arise will cause me to veer off at a tangent, and already the paper on the British dinosaurs of the Kimmeridge Clay that Dave Martill, Sarah Fielding and I have finally had published has me wanting to move on to something else. I am also desperate to blog about the lions of Chobe National Park (Botswana), as their ability to kill adult elephants has finally been filmed and is due to be screened on TV next week. Furthermore, there is that AMAZING discovery of a bottlenose dolphin with a perfect pair of miniature hind-flippers (go here), and there is that Mantellisaurus thing. Stay tuned.

While I’m here I want to note some recent updates I’ve made to some articles. Firstly, thanks to comments that have been added by anonymous mammalogists (PLEASE leave your name when you leave a comment), I’ve done some minor updating to The first new European mammal in 100 years? We are now at 31 ‘100 year’ mammals. Secondly, inspired by some questions from Mark Abuys, I have also added some new comments to Graeme’s Pleistocene megafrog. Mark was interested in my mention of the carn-pnay, a crypto-frog from New Guinea with an alleged length of 30 cm.

To return to phorusrhacids...... in the previous posts I at least alluded to ideas about their affinities: it is universally agreed that their closest living relatives are the South American seriemas (or cariamids), and it is furthermore agreed that two fossil groups, the bathornithids of Eocene-Miocene North America and the idiornithids of Eocene-Oligocene Europe, are also close relatives of both seriemas and phorusrhacids. Several features unite these birds (Mayr 2002). They all have a strongly hooked bill, a simple, block-like hypotarsus (a site of ligament attachment on the posterior surface of the tarsometatarsus), distinctively proportioned toe bones, and a laterally compressed, strongly curved and sharp-tipped claw on the second toe (discussed previously here). All of these birds – idiornithids, bathornithids, cariamids and phorusrhacids – form a clade termed the Cariamae Fürbringer, 1888, and conventionally they’ve been regarded as part of Gruiformes, the group that includes rails, cranes, trumpeters and several other groups. A few other fossil groups have been suggested to be part of Cariamae, like the cunampaiids of Eocene Argentina.

Are cariamaens really close allies of rails and cranes? Storrs Olson (1985, p. 143) wrote that the classification of cariamaens within Gruiformes was ‘largely by default, as they do not clearly seem to belong in any other order’, and there is a long-running disagreement among ornithologists as to whether gruiforms are a natural group or not. We’ll look at this issue in the next post.

The South American landbird hypothesis

If you’re a regular reader of this blog you’ll know that I’m a big fan of ‘alternative’ theories on phylogenetic relationships (see, for example, We flightless primates). While I always find these ideas interesting, note the caveat that I do not necessarily endorse them: in fact they often turn out to be poorly supported or spurious. As it happens, one of my favourite ‘alternative’ theories involves cariamaens: it is the South American landbird hypothesis.

Based predominantly on the morphology of the carpometacarpus, some ornithologists have proposed that cariamaens are closely related to the Hoatzin* Opisthocomus hoazin**, that bizarre folivorous, arboreal bird that (uniquely among birds) practices foregut fermentation. In contrast to those of most other neornithines, the carpometacarpi of cariamaens and hoatzins possess a particularly broad, strongly bowed third metacarpal. This is also true of turacos and some cuckoos (indeed, many ornithologists have proposed that hoatzins might be close allies of turacos and/or cuckoos).

* Like ‘fossa’ and ‘sifaka’, ‘hoatzin’ is one of those words that is, apparently, not pronounced the way in which it is written. Some sources state that it is properly pronounced ‘watson’.

** That’s not a typo. Furthermore, the name Opisthocomus cristatus, used by some authors relatively recently (e.g., Chatterjee in The Rise of Birds) was coined (so far as I can tell) by Johann Illiger in 1811 and thus post-dates Statius Muller’s 1776 publication of the name Phasianus hoazin (that’s right - the hoatzin was originally described as a type of pheasant).

Hoatzin and seriema skeletons are also somewhat similar overall: I suppose you could believe that seriemas (and hence other cariamaens) are just big, long-legged hoatzins, modified for a cursorial, raptorial lifestyle whereas the various modifications possessed by hoatzins reflect their folivorous lifestyle (these features include a bizarre sternum where the keel is virtually absent anteriorly, thereby allowing room for the huge crop, and a notably deep, short pelvis) [the adjacent photos show, from top to bottom, a hoatzin skeleton, and a Cariama cristata skull and partial postcranium. Sorry the hoatzin photo is so bad]. Hoatzins and seriemas also possess a few bony and soft-tissue characters that are shared only by these birds, turacos and cuckoos: these include details of the hip musculature and the presence of distinctive bony recesses on the top of the pelvis.

Olson (1985) supported the possible monophyly of the South American landbird group, writing ‘the seriemas and hoatzins appear to be part of an early radiation of primitive land birds, members of which have persisted in South America, perhaps as a result of its isolation’ (pp. 143-144). He further suggested that falcons ‘probably represent a raptorial branch of this radiation’ (p. 144), a suggestion presumably based on the anatomy of caracaras. Note that falconids are essentially a South American group (only a few, recently evolved genera have left the continent). A few fossil taxa might also be interpreted as providing support for the monophyly of this group. Mourer-Chauviré (1983) regarded idiornithids as similar to both seriemas and hoatzins and her conclusions are similar overall to those of Olson (1985). Incidentally, both Olson and Mourer-Chauviré came up with the same idea independently. A long delay in publication meant that Olson (1985) came out after Mourer-Chauviré (1983), by which time Olson decided not to rewrite his text: ‘partly out of laziness and frustration but more to show that we arrived independently at the same basic conclusions’ (p. 151).

Among the most enigmatic of Cenozoic fossil birds is Foro panarium (image at left) from the Eocene Green River Formation of Wyoming: it seems to combine features of hoatzins, cuckoos and turacos but, interestingly, is also superficially raptor-like. This could be used to provide tentative support for Olson’s idea that falcons might be linked to hoatzins and other South American landbirds.

We saw above that hoatzins have often been linked with turacos, and that turacos share with hoatzins and seriemas that unusual and distinctive robust, bowed third metacarpal, as well as other characters. Might turacos also, then, be members of the South American landbird group? True, they aren’t South American but African (though with fossil representatives in Europe). In what might be regarded as a deviant version of the South American landbird group theory, Chandler (1997) proposed that turacos were the sister-taxon to Cariamae. In a novel twist, he further announced that New World vultures (vulturids, aka cathartids) were the next closest relatives of the turaco-cariamaen clade. He noted that fossils, osteology and preliminary biochemical data all provided supportive evidence for this novel hypothesis, though unfortunately he only ever published an abstract on it, and a full paper has yet to appear.

Based – it has to be said – on just a handful of detailed morphological characters, combined with some inference based on biogeography and superficial similarity, the South American landbird group theory suggests the following: that there might be a hoatzin-cariamaen clade, probably persisting as relicts in South America but more widespread during the early Cenozoic. It may perhaps involve turacos, and perhaps also falcons. Finally, New World vultures (which have all their earliest fossil occurrences in the Old World) might be allies of the South American landbird group. I am deliberately avoiding bringing in cuckoos and accipitrids (the latter noted by Olson as being possible allies of the turacos) in order to keep things simple-ish. Furthermore, the South American trumpeters (psophiids) share a number of characters with cariamaens and have also been regarded as part of this story by some authors.

Predictably, I cannot help but find the idea that condors, caracaras, turacos and hoatzins are all close kin of phorusrhacids highly appealing, but it does all seem very vague and sadly lacking in good character support. So, do larger studies – those incorporating molecular and/or morphological information from lots of taxa – support a possible link between these birds? See the next post (Goodbye my giant predatory, cursorial, flightless hoatzin).

For the latest news on Tetrapod Zoology do go here.

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Chandler, R. M. 1997. New discoveries of Titanis walleri (Aves: Phorusrhacidae) and a new phylogenetic hypothesis for the phorusrhacids. Journal of Vertebrate Paleontology 17 (supplement to 3), 36-37.

Mayr, G. 2002. A new specimen of Salmila robusta (Aves: Gruiformes: Salmilidae n. fam.) from the Middle Eocene of Messel. Paläontologische Zeitschrift 76, 305-316.

Mourer-Chauviré, C. 1993. Les Gruiformes (Aves) des Phosphorites du Quercy (France). 1. Sous-ordre Cariamae (Cariamidae et Phorusrhacidae). Systématique et biostratigraphie. Palaeovertebrata 13, 83-143.

Olson, S. L. 1985. The fossil record of birds. In Avian Biology, Volume III, pp. 79-238.

More on phorusrhacids: the biggest, the fastest, the mostest out-of-placest

In the previous post we looked briefly at phorusrhacid diversity, stopping on the way to look at the discovery and naming of that ‘well known’ species Phorusrhacos longissimus from the Miocene of Argentina. All of this has been inspired by Chiappe & Bertelli’s (2006) description of the immense new specimen BAR 3877-11, an unnamed Miocene phorusrhacine phorusrhacid that represents one of the biggest members of the group: its skull is 71 cm long and the live animal probably stood 3 m tall (life restoration at left).

By comparing BAR 3877-11 with phorusrhacids known from fairly complete skeletons, we can estimate that it was about 10% bigger than the previously largest known phorusrhacids. But its markedly slender tarsometatarsus indicates that it was gracile, and thus almost certainly not as heavy as the far more robust giant aepyornithids (aka elephant birds, restricted to Madagascar bar a few dubious reports from continental Africa and elsewhere) and dromornithids (aka mihirungs, an Australian group argued to be giant waterfowl). These slender legs suggest that BAR 3877-11 was a fast-moving, cursorial predator, and Chiappe & Bertelli state that ‘the long-established correlation between their corpulence and reduced cursorial agility needs to be re-evaluated’. In other words, they imply that previous studies have associated giant size with ponderous locomotion.

This is somewhat misleading however, in that the only reason that some giant phorusrhacids have been thought of as relatively slow-moving is that they belong to that particular robust-limbed subgroup, the Brontornithinae. The biggest brontornithine, Brontornis burmeisteri from the Miocene of Argentina, was also arguably the biggest phorusrhacid prior to the discovery of BAR 3877-11, but its leg bones are immensely wide and stocky for their length, and its tarsometatarsi are between 50 and 60% the length of its tibiotarsi. These features suggest that it was a walking bird, not a runner, and it is on the basis of this that some authors have interpreted brontornithines as scavengers. You’ll know from discussions about tyrannosaurs that a pure scavenging lifestyle is highly unlikely for any flightless tetrapod (for energetic reasons). Sure, they probably did scavenge (you can imagine them trying to scare teratorns, or a group of hyaena-like borhyeanids, away from a carcass), but they probably foraged for live prey of various kinds as well.

Having mentioned teratorns, while they are now known from the Upper Oligocene/Lower Miocene of South America (Olson & Alvarenga 2002), and thus were contemporaneous with brontornithines, the immense teratorn Argentavis is only known from the Late Miocene, and brontornithines are unknown from this time. So, sorry, you shouldn’t imagine Brontornis scrapping with Argentavis. The oldest teratorn, Taubatornis campbelli, is actually from the same unit – the Tremambé Formation – as the brontornithine Physornis brasiliensis. Both lived alongside New World vultures, flamingos, screamers and caviomorph rodents [the adjacent photo, of the brontornithine Paraphysornis, is borrowed from Cais de Gaia's phorusrhacid blog post].

Getting back to Chiappe & Bertelli’s claims about phorusrhacid running speed, highly relevant is a recent study specifically devoted to this issue. Based on limb proportions and limb bone strength, Blanco & Jones (2005) estimated the running speed of the mesembriornithine Mesembriornis, the patagornithine Patagornis, and a giant phorusrhacine specimen from the Pliocene or Pleistocene of Uruguay. This latter bird (significant in being the youngest phorusrhacid from South America) was one of the largest members of the group, with an estimated height of c. 2.5 m. It might be a species of Devincenzia. Patagornis and the giant phorusrhacine were predicted to have running speeds of 14 metres per second (about 50 km/h) while Mesembriornis had a ridiculous predicted running speed of 27 metres per second (about 97 km/h). For comparison, emus run at 14 metres per second, an ostrich reaches perhaps 17 metres per second (about 60 km/h) and cheetahs are reported to reach or exceed 27 metres per second.

Unsurprisingly, Blanco & Jones (2005) doubted if their predictions were accurate and they wondered if the unusual bone strength of some phorusrhacids – Mesembriornis in particular – might be related to something other than running speed. Could it be something to do with kicking? Based on the forces needed to break bones, they showed that Mesembriornis would be able to produce a force of over 2000 Newtons with its kick: strong enough to fracture bones. Thus it’s possible that some phorusrhacids used kicking as a way of breaking open bones to feed on marrow (how do the brontornithines, with their super-robust limb bones and inferred scavenging habits, fit into this?). It’s also possible that the birds used this kicking power to stun or kill prey, and here you will of course be thinking of the Secretary bird Sagittarius serpentarius, a cursorial raptor (superficially similar to a seriema) that kills or stuns snakes and other terrestrial prey with repeated kicks.

The foot claws of some phorusrhacids also support the idea that they used their feet in maiming or killing as the claws are laterally compressed, curved and sharp-tipped. That’s not necessarily the normal morphology for predatory birds, as many cursorial birds (and non-avian theropods) actually have rather blunt, stout foot claws. Tonni & Tambussi (1988) described the foot morphology of the Miocene psilopterine Psilopterus and showed that its foot claws were nearly identical to those of the living seriema Cariama cristata.

What’s interesting about this is that the second toe claw in Cariama is slightly enlarged relative to those of digits III and IV (as you can see from the very poor accompanying photos). Seriemas reportedly use the claw to aid in tree-climbing, and I’d like to know if they use it in attacking or killing prey. A great many other birds, including raptors and many passerines, have similarly enlarged claws on digit II however. I’ve previously been guilty of comparing this ‘enlarged’ claw with the raised sickle-claw seen in dromaeosaurids and other Cretaceous theropods, but what we have in phorusrhacids and seriemas clearly isn’t as elaborate, so there’s no indication that they used it to slash open the bellies of prey or anything like that (and here I’ll avoid the debate about the function of sickle-claws*). I doubt if even small phorusrhacids climbed trees, so presumably they used the claw in dispatching or manipulating prey.

* A recent study has claimed that sickle-claws could not function as slashing or stabbing weapons, but were perhaps used instead as climbing crampons, enabling dromaeosaurids to climb the bodies of their prey. I feel that there are major flaws in this study and that its conclusions are erroneous (see comments in Naish 2006).

While conventionally regarded as South American birds, there have been occasional reports of phorusrhacids from elsewhere. Two supposed European members of the group, Ameghinornis minor from Eocene-Oligocene France and Aenigmavis sapea from Eocene Germany, were identified in the 1980s (actually, Ameghinornis minor was first described [as Strigogyps minor] in 1839, but its new name and proposed affinity to phorusrhacids weren’t published until 1987). Both were weakly flighted or flightless birds about the size of a partridge.

Given that a few other Eocene European tetrapods have been suggested to be particularly closely related to South American taxa (namely the ratite Palaeotis, the peradectine opossums and the supposed anteater Eurotamandua), Ameghinornis and Aenigmavis were thought to perhaps indicate that phorusrhacids had originated in Europe and later spread (via Africa) to South America (Peters & Storch 1993). However, reanalysis has shown that both names are best regarded as junior synonyms of Strigogyps, and furthermore that Strigogyps differs significantly from phorusrhacids in lacking the derived characters that unite the members of this group (Mayr 2005). We’re not actually sure what Stigogyps is (though its tarsometatarsus is similar in some details to that of a trumpeter), but its re-evaluation strikes phorusrhacids off the list of European fossil taxa. Incidentally, there are unpublished Palaeocene and/or Eocene fragments from England and North America that, inspired by the 1987 identification of Aenigmavis, have also been suggested to be phorusrhacids. They await evaluation but, like Strigogyps, it is doubtful if they really have anything to do with Phorusrhacidae.

I should point out that the other European Eocene forms previously regarded as being of South American affinity have also been reinterpreted. Palaeotis, a small ratite argued by some to be a stem rhea, has more recently been found to be outside of the clade that includes rheas, ostriches, cassowaries and emus. Peradectine opossums may or may not be of South American origin: however, by the Eocene they occurred in North America and Europe and they later occurred in Asia and Africa. They do not seem to provide special evidence for a faunal link between South America and Europe. Finally, the supposed anteater Eurotamandua seems not to be an anteater, nor even a xenarthran, and as such there is nothing South American about it [adjacent image shows, at top, skeleton and life restoration of Eurotamandua, with the Eocene pangolin Eomanis at bottom. Taken from here].

What is almost certainly a non-American phorusrhacid was reported in 1987… from the Eocene of Antarctica (Case et al. 1987). Known only from the anterior part of the premaxillae, the specimen must have belonged to a reasonably large bird, but not much more than that is known about it. Older phorusrhacids are known from the Palaeocene of South America, so the specimen does not demonstrate that phorusrhacids originated in Antarctica: rather, it probably shows that they were common to both continents prior to their separation in the Oligocene.

An interesting parallel is provided by the fossil record of sloths, as while long regarded as of South American origin, the oldest sloth is a Middle Eocene fossil from Seymour Island (Antarctica). From time to time people make the point that some really interesting, major events in tetrapod history must have occurred in ancient Antarctica – if only it wasn’t for that damned ice sheet. Luckily, we’re doing all we can to get rid of it (that’s meant to be ironic). Anyway, we might speculate that Antarctica was home to numerous phorusrhacid lineages prior to its glaciation, but we’ll likely never know about them.

More to come…

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Blanco, R. E. & Jones, W. W. 2005. Terror birds on the run: a mechanical model to estimate its maximum running speed. Proceedings of the Royal Society of London B 272, 1769-1773.

Case, J. A., Woodburne, M. O. & Chaney, D. S. 1987. A gigantic phororhacoid(?) [sic] bird from Antarctica. Journal of Paleontology 61, 1280-1284.

Chiappe, L. M. & Bertelli, S. 2006. Skull morphology of giant terror birds. Nature 443, 929.

Mayr, G. 2005. “Old World phorusrhacids” (Aves, Phorusrhacidae): a new look at Strigogyps (“Aenigmavis”) sapea (Peters 1987). PaleoBios 25, 11-16.

Naish, D. 2006. The Carnivorous Dinosaurs [review]. The Palaeontology Newsletter 62, 122-126 [free pdf available here].

Olson, S. L. & Alvarenga, H. M. F. 2002. A new genus of small teratorn from the Middle Tertiary of the Taubaté Basin, Brazil (Aves: Teratornithidae). Proceedings of the Biological Society of Washington 115, 701-705.

Peters, D. S. & Storch, G. 1993. South American relationships of Messel birds and mammals. Kaupia 3, 263-269.

Tonni, E. P. & Tambussi, C. P. 1988. Un nuevo Psilopterinae (Aves: Ralliformes) del Mioceno tardio de la Provincia de Buenos Aires, Republica Argentina. Ameghiniana 25, 155-160.

Swan-necked seals

I am asked, quite frequently, about Acrophoca longirostris. What is it? It’s a fossil seal from the Miocene and Pliocene Pacific coast of South America, described from good fossils by Christian de Muizon in 1981. Within the seal, or phocid, family, it belongs to a group called the monachines: the monk seals (monachins), elephant seals (miroungins), and Antarctic seals (lobodontins). As fossil tetrapods go, seals aren’t regarded as particularly charismatic, nor do they attract much attention away from a handful of specialists. So what makes Acrophoca so interesting?

It’s the contention that this animal should be regarded as a ‘swan-necked seal’: a pinniped that was sort of something like a mammalian version of a plesiosaur. Life restorations of Acrophoca are few are far between, but those that that have been published (on the cover of Muizon 1981, and in Naish 2001: reproduced above, at the correct orientation!), or exist as museum murals (that at Karlsruhe’s Staalische Museum für Naturkunde), do indeed make this seal look particularly long-necked compared to living phocids. Barnes et al. (1985) even went so far as to describe Acrophoca as ‘serpentiform’ (p. 41). All in all, it sounds like Acrophoca would have looked particularly odd. So here’s the low-down.

In 2002, new Acrophoca material from Chile was described by Stig Walsh and myself. While, based on various detailed differences, it seemed likely that we had a species distinct from A. longirostris, we elected to leave the Chilean material as Acrophoca sp. This was sensible, because Stig’s observations revealed that there was evidence for multiple Acrophoca species known from various horizons within the Miocene-Pliocene coastal sediments of Peru. Accordingly, we wrote ‘We have elected not to name [this] possible new species at this time because new Peruvian material of … Acrophoca … is under description and new species, represented by substantial remains, have been recovered (Muizon, pers. comm. 2000)’ (p. 840).

Exactly this has turned out to the case for some of the other marine mammals known from these sediments. The bizarre ‘swimming sloth’ Thalassocnus, for example, was originally described for a single species (T. natans) assumed to be a one off. We now know that it was just one of several species within this genus, some of which were far more specialised for marine life than was T. natans. The youngest species, T. carolomartini and T. yaucensis, are poorly known, but what is known suggests that they were seal-like in some of their features, rather than sloth-like (Muizon et al. 2004) [for more on these sloths go here]. Given that one of the undescribed Acrophoca species is much longer-skulled than A. longirostris (Walsh & Naish 2002) – already notable for its long skull (hence the name) – it’s conceivable that, as in Thalassocnus, some of the as-yet-unnamed species were altogether weirder looking. But we won’t know this for sure until they get described. Watch this space.

Was Acrophoca really ‘swan necked’? Well, judging from the specimens that Muizon described, it wouldn’t have had a neck anywhere as near as long (proportionally) as that of a plesiosaur, but it still would have looked longer in the neck than any extant seal: Muizon’s (1981) data shows that Acrophoca far exceeds other monachines in the length of its cervical vertebrae, and in total cervical column length. Based on similar-sized individuals, extant monachines have cervical column lengths of 218-249 mm, while Acrophoca is at 329 mm. King (1983) showed that in most seals, cervical column length is between 17 and 19% of the total length of the vertebral column – not different from terrestrial carnivorans like dogs (also 17%). In Muizon’s skeletal reconstruction, the neck length of Acrophoca is about 21% of total vertebral column length. So, yes, a bit longer-necked than living seals, but ‘swan-necked’? Well, even living pinnipeds that we don’t regard as particularly long-necked have necks flexible enough to allow a startling lengthening effect when they lunge, stretch or spy-hop. Bonner (1994) noted that a seal can ‘extend its neck suddenly when it strikes prey. Seals can do the same thing on land, as many a seal researcher has discovered to his or her cost’ (p. 18). This applies particularly for some otariids and for lobodontins like Hydrurga. What I’m getting at is that, when alive, Acrophoca would have been capable of looking even longer in the neck than we might think just from its fossils. But clearly it’s a stretch to imagine this animal as having a long long long neck like a swan, or a plesiosaur, so, sadly, ‘swan-necked seal’ really is a bit of an exaggeration.

The phylogenetic affinities of Acrophoca might also tell us something about its appearance in life. The problem is that this is a particularly contentious topic among seal experts. Muizon (1981) regarded Acrophoca as a lobodontin: that is, a member of the group that includes Hydrurga (the leopard seal), Ommatophoca (Ross’ seal) and Lobodon (the crabeater seal), and specifically as the sister-taxon to Hydrurga. Though it’s been suggested that lobodontins might be paraphyletic with respect to Monachus (a highly problematic genus within seal phylogeny), most recent studies have supported lobodontin monophyly (Bininda-Emonds et al. 1999). If Acrophoca is a member of this group, as Muizon (and Walsh & Naish) concluded, then we should be thinking of living lobodontins, and specifically Hydrurga, as the closest extant models. And I find that particularly interesting, because it’s my opinion that Hydrurga is one of the most ‘disturbing’ looking of all mammals: it’s a chunky mother of a seal – thickset and quasi-reptilian, with a theropod-like jaw line and robust skull demarcated from its neck. I would imagine Acrophoca as a paradoxical combination of these traits: quasi-reptilian and Hydrurga-like on one hand, but also longer-skulled, longer-necked and overall more gracile. It would have been odd, and don’t forget that other species in the genus would have looked even odder.

Right, back to work. I have Crato Formation turtles and British big cats to be dealt with in the next few days. More to come on those topics here soon.

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Barnes, L. G., Domning, D. & Ray, C. E. 1985. Status of studies on fossil marine mammals. Marine Mammal Science 1, 15-53.

Bininda-Emonds, O. R. P., Gittleman, J. L. & Purvis, A. 1999. Building large trees by combining phylogenetic information: a complete phylogeny of the extant Carnivora (Mammalia). Biological Reviews 74, 143-175.

Bonner, N. 1994. Seals and Sea Lions of the World. Blandford (London), pp. 224.

King, J. E. 1983. Seals of the World. British Museum (Natural History) (London), pp. 240.

Muizon, C. de 1981. Les vertébrés fossiles de la formation Pisco (Pérou). Première partie: deux nouveaux Monachinae (Phocidae, Mammalia) du Pliocene de Sud-Sacaco. Travaux de l’Insitut Français d’Études Andines 22, 1-161.

- ., McDonald, H. G., Salas, R. & Urbina, M. 2004. The youngest species of the aquatic sloth Thalassocnus and a reassessment of the relationships of the nothrothere sloths (Mammalia: Xenarthra). Journal of Vertebrate Paleontology 24, 387-397.

Naish, D. 2001. Sea serpents, seals and coelacanths: an attempt at a holistic approach to the identity of large aquatic cryptids. In Simmons, I. & Quin, M. (eds) Fortean Studies Volume 7. John Brown Publishing (London), pp. 75-94.

Walsh, S. A. & Naish, D. 2002. Fossil seals from late Neogene deposits in South America: a new pinniped (Carnivora, Mammalia) assemblage from Chile. Palaeontology 45, 821-842.