My party and those marvellous metriorhynchids

[From left to right, the photo shows: Mike P. Taylor, Jeff Liston, Lorna Steel, Luis Rey, Carmen Naranjo, Anthony Butcher, Darren Naish, Will D. Naish, Toni Naish, Graeme Elliott, Mark Witton, Julian Hume, Will J. H. Naish. Look, I’d been drinking. A few people had left before I took the photo, and as usual many of those invited didn't turn up. You know who you are].

The party is over and everyone has gone home, and the last to leave was Jeff Liston of the University of Glasgow's Hunterian Museum. That’s not because he’s hard to get rid of, but because we drove him to the airport for his flight home. Jeff has just completed his phd on the immense suspension-feeding* Jurassic fish Leedsichthys, and while – heaven forbid – I’m not going to talk about fish today, it’s the neat insights I got into Oxford Clay marine reptiles that I’m going to cover here (the Oxford Clay is a globally important Middle Jurassic mudstone, stuffed full of fossil marine vertebrates and invertebrates). I have a thing about Mesozoic marine reptiles right now anyway, as I’m trying to finish a project on a pliosaur. More on that later.

* Not ‘filter-feeding’ as I said in a previous post. Jeff tells that filter-feeding is a specific form of suspension-feeding, and one that was likely not practised by Leedsichthys. Huh.

In a poster presented at the 1997 conference on secondarily aquatic vertebrates, Colin McHenry and I proposed that the flippered Jurassic marine crocodilian Metriorhynchus brachyrhynchus might have been a tough, aggressive little mother, a bit like the modern Pygmy killer whale Feresa attenuata. We partly came to this conclusion because a remarkable specimen – a Metriorhynchus tooth embedded in a skull roof bone of a Leedsichthys – indicates that Metriorhynchus was mean enough and nasty enough to take on anything. Metriorhynchus was about 3 m long whereas Leedsichthys was err, umm (checks to see if Dave Martill is around) at least 15 m long. In this case, healed bone around the tooth shows that the Leedsichthys survived the encounter, and swam around with the crocodilian’s tooth embedded in its head (Martill 1986). But, alas, this lovely story has had its day. I’ll say no more as Jeff hasn’t published yet. We move swiftly on.

Crocodilians today are more diverse than most people realise, but – forget this rubbish about crocodilians being evolutionary conservative – the diversity present among the fossil forms is seriously impressive (for a review see Naish 2001: free pdf here). While there are extant crocodilians that spend a lot of time at sea, the fossil record shows us that several groups became specialised for full-time marine life, and among them none were more specialised than the metriorhynchids, a Jurassic-Early Cretaceous group of tubular-skulled crocodilians that had limbs modified into paddles. A specialised down-curved tail tip formed the lower lobe to a tail fin, the upper part of which was formed entirely from soft tissues, though with internal support provided by tall, curved neural spines. We know that the tail fin was present because – like that of ichthyosaurs – it is preserved as a soft-tissue impression in some specimens (as shown in the adjacent image).

Unlike other crocodilians, metriorhynchids are remarkable in lacking the bony scutes that ordinarily cover the dorsal surface, and they are also remarkable for possessing massive laterally projecting prefrontal bones that stick out above and in front of the eye sockets. These expanded prefrontals appear to have gotten larger during the evolution of the group, and their function has always been mysterious: did they somehow protect the eyes during predation behaviour, or did they somehow enhance streamlining? While these possibilities remain untested (to my knowledge) it now seems that the primary function of the bones was that they housed enlarged salt glands, as exceptionally well preserved Argentinian metriorhynchids actually have their salt glands preserved within the prefrontals (Fernández & Gasparini 2000).

Reptiles haven’t evolved hyper-efficient kidneys as mammals have, and living marine reptiles void unwanted salt via specialised cranial glands. We’ve assumed that metriorhynchids (and other fossil marine crocodilians) had salt glands, but now we know exactly where those glands were. And the presence of the glands in this location might explain another unusual aspect of metriorhynchid cranial anatomy, namely the strange elongate, groove-like antorbital fossae present in these animals (the antorbital fossa is an accessory opening present on the side of the skull in archosaurs). Fernández & Gasparini (2000) suggested that the specialised fossae might have been used to drain unwanted saline secretions from the glands – a novel function for the structures not seen in other archosaurs.

Metriorhynchids are commonly depicted in artistic scenes showing Jurassic marine life, but it turns out that they looked rather different from conventional restorations. Most artists have depicted them as looking something like long-tailed pliosaurs, albeit of course with that subtriangular fin at the tail tip. Their fore- and hindlimbs are shown as proportionally large, similar in shape, and about equal in size. But good articulated skeletons show that metriorhynchids were different from this, with a fairly long body and limbs that differ substantially in size and shape. The forelimbs were surprisingly short, rounded paddles with only limited mobility. The hindlimbs were altogether different, being much longer and relatively slender. So Samuel Williston got all this right when he restored a metriorhynchid correctly in his 1914 book Water Reptiles of the Past and Present (see adjacent image), but hardly anyone seems to have taken any notice of him.

Combine these weird limbs with the triangular tail-fin, and you can understand why there has been some uncertainty over how metriorhynchids swam. Hua (1997) showed that the metriorhynchid skull was remarkably porous and hence probably highly buoyant, and that they might therefore have floated at the water surface, ambushing prey with a swift burst. The tail-tip shape is most like that of carchariniform sharks and, according to Hua (1994), suggestive of sudden acceleration employed during slow, stalking predation.

What did metriorhynchids eat? Given that we’re talking about multiple species that differed in snout shape and tooth morphology, it seems that the species varied in their diet and behaviour. Massare (1987) and Vignaud (1997) looked at tooth morphology in Metriorhynchus. They showed that the long-snouted species M. superciliosus and M. leedsi had rather straight, robust, blunt-tipped teeth that belonged to the ‘crunch’ feeding guild, while broad-skulled species like M. cultridens had slender, slightly compressed, carinate teeth (carinae=cutting edges) that belonged to the ‘pierce’ guild.

Based on tooth style and feeding behaviour in living aquatic tetrapods, ‘crunch’ guild animals feed on prey with hard exteriors (such as shelled molluscs) while ‘pierce’ guild animals pierce the bodies of prey like fish. Accordingly, different metriorhynchid species were apparently doing different things, and contemporaneous taxa like the Metriorhynchus species that lived in the Oxford Clay fauna were probably exploiting different prey.

Stomach contents of a metriorhynchid were described by Dave Martill (1986) and included cephalopod hooklets, a belemnite guard and some long bones that Dave identified as those of the pterosaur Rhamphorhynchus. On the basis of these remains he proposed that metriorhynchids were probably opportunistic carnivores which, rather like most extant crocodilians, ate whatever came within reach. The ‘pterosaur’ bones at least proved misidentified however and turned out to be fish bones (Unwin, cited as pers. comm. in Forrest 2003), so the interesting idea of metriorhynchids feeding on pterosaurs presently lacks fossil support. A partial skeleton of the long-necked plesiosaur Cryptoclidus bears tooth marks, apparently made by a metriorhynchid, around the edges of its vertebrae, and Forrest (2003) interpreted this discovery as evidence for scavenging behaviour in members of the group. It is quite plausible then that they fed from submerged or floating carcasses, and given that they were contemporaneous with a diversity of large-bodied bony fishes, sharks, plesiosaurs and ichthyosaurs it’s likely that at least some of their diet was obtained this way.

At least some metriorhynchids look like they were big, scary macropredators whose diets extended beyond fish and molluscs. Dakosaurus, the geologically youngest member of the group, was in the news a lot in 2005 as D. andiniensis, an Argentinian species first named in 1985 (though initially misidentified as a species of Metriorhynchus), was described from new specimens (Gasparini et al. 2005). It even made the cover of National Geographic (Nat Geo image at left). These new fossils showed that D. andiniensis was a really fearsome looking beast, with a deep, broad rostrum and a remarkably low number of big, laterally compressed, serrated teeth. Teeth of this form are termed ziphodont. Metriorhynchids generally have 25-40 teeth in the maxilla, but D. andiniensis has just 10 or 11.

And while serrated teeth are no big deal among terrestrial predatory reptiles, they are in marine forms: mosasaurs have serrated teeth, but the serrations are ultra-fine. We don’t really know what D. andiniensis was doing, but terrestrial reptiles with deep snouts and ziphodont teeth – like theropods and sebecosuchian crocodilians – made a living by taking slashing, debilitating bites from large prey animals. Among marine reptiles and possibly marine tetrapods, D. andiniensis would have been unique.

That’ll do for now. Took Jeff to my trusty slow-worm location, but no sightings today. For the latest news on Tetrapod Zoology do go here.

Refs - -

Adams-Tresman, S. M. 1987. The Callovian (Middle Jurassic) marine crocodile Metriorhynchus from central England. Palaeontology 30, 179-194.

Fernández, M. S. & Gasparini, Z. 2000. Salt glands in a Tithonian metriorhynchid crocodyliform and their physiological significance. Lethaia 33, 269-276.

Forrest, R. 2003. Evidence for scavenging by the marine crocodile Metriorhynchus on the carcass of a plesiosaur. Proceedings of the Geologists’ Association 114, 363-366.

Gasparini, Z., Pol, D. & Spalletti, L. A. 2005. An unusual marine crocodyliform from the Jurassic-Cretaceous boundary of Patagonia. Sciencexpress 10.1126/science.1120803.

Hua, S. 1994. Hydrodynamique et modalités d'allègement chez Metriorhynchus superciliosus (Crocodylia, Thalattosuchia): implications paléoécologiques. Neues Jahrbuch fur Geologie und Paläontologie, Abhandlungen 193, 1, 1-19.

Martill, D. M. 1986. The diet of Metriorhynchus, a Mesozoic marine crocodile. Neues Jahrbuch fur Geologie und Paläontologie, Monatshefte 1986, 621-625.

Massare, J. A. 1987. Tooth morphology and prey preference of Mesozoic marine reptiles. Journal of Vertebrate Paleontology 7, 121-137.

Naish, D. 2001. Fossils explained 34: Crocodilians. Geology Today 17 (2), 71-77.

Vignaud, P. 1997. La morphologie dentaire des Thalattosuchia (Crocodylia, Mesosuchia). Palaeovertebrata 26, 35-59.