The Flexibility of Plesiosaur Necks

I can't believe that I've let this poor blog collect dust for over a month. Well, I sorta can. Unlike many other blogs, even some alleged Science Blogs, I don't (OK, no longer) have any interest in posting fluff on politics, religion, soon-to-be-stupid memes, my social life (sic) and blah blah blah. I'm hardly qualified to write about the subjects this blog frequents and I don't think the Internet needs to be any more polluted with preachin' to the choir commentary and knee-jerk rants. When I do get the urge to fluff, I'll make sure to put it at the top of posts so people will instinctively know to skip it.

Sauropterygians are derived aquatic diapsids characterized by features such as an enlarged upper fenestra, the loss of a lower temporal arch, a rigid quadrate and others. I'm using "plesiosaur" as all of the members of the clade Plesiosauria, which is defined by the presence of limbs modified into hydrofoils, limb girdles forming large ventral plates, absent nasals, short tails, rigid trunks and others (O'Keefe 2002). This clade includes plesiosauroids such as cryptoclidids, elasmosaurids, plesiosaurids, polycotylids as well as pliosauroids such as brachaucheniids, pliosaurids, leptocleidids and rhomaleosaurids. Traditionally it was thought that "plesiosaurs"/plesiosauroids were long necked and "pliosaurs"/pliosauroids were short necked but this is not always the case (Smith 2008). The pliosaur morphotype has evolved on three separate* occasions (in polycotylids, pliosaurids and rhomaleosaurids) and variations in body type within clades appears to be very complicated (O'Keefe 2002).

*Interestingly, Williston suggested that short necks may have evolved on more than one occasion - in 1907 (O'Keefe 2002). His comments on the functions of long plesiosaur necks (below) also sound like they could be taken from a recent abstract.

From: David Thomas Ansted’s The Great Stone Book of Nature (George W. Childs, Philadelphia, 1863). The pterosaur is priceless.

As the headache-inducing illustration above shows, plesiosaurs were commonly depicted as using their long necks in a snake-like, swan-like or harpoon-like manner. Plesiosaur necks had astoundingly high counts of vertebrae (up to 72 in Elasmosaurus) but the lengths of the vertebrae were also elongated and the articulations were close and rigid (Smith 2008). Is it me, or does there seem to be a pattern of elongated reptile necks not being particularly flexible (e.g. azhdarchids, Tanystropheus)? Anyways, Samuel W. Williston noted the lack of flexibility in 1914 and suggested that plesiosaurs generally held their necks out strait and captured prey by moving the anterior portion of their necks with downwards and lateral movements. Zammit et al. 2008 claims that as recently as 1993 a paper reconstructed elasmosaurs as having an "S-shaped" neck in the vertical plane, but the author of that paper describes such reconstructions as "fanciful" (Storrs 1993). While the idea of plesiosaurs with very flexible necks fell out of scientific favor long ago, popular culture still depicts them in illustrations such as these.

There is at least one plesiosaur with what appears to be an adaptation for vertical movement in its neck. A juvenile specimen (only 700 mm, 28 inches in length) of the lower Cretaceous Leptocleidus had steeply angled zygapophyses in its cervical vertebrae which suggest the capacity for vertical movement (Kear 2007). The degree of flexibility was not mentioned, but it was apparently different enough from other taxa to suggest different prey types (Kear 2007). I feel that I am obliged to mention that Leptocleidus is a rhomaleosaurid* with the pliosaur-type morphology, apparently unlike many of its relatives (O'Keefe 2002). Leptocleidus doesn't seem to be too terrible short necked (as depicted here), it still had >20 cervicals and Fig. 6 of Kear 2007 shows the neck to be about twice the length of the head. Steeply angled cervical zygapophyses are unique as far as I can tell (which isn't very far), but I think it would be interesting to see if other attempts at "pliosaurs" and/or other juvenile Leptocleidus had this feature. Australian fossils of freshwater plesiosaurs are not strongly informative of taxonomy for the most part, but interestingly some suggest Leptocleidus (Kear 2006). Maybe having an unusually flexible neck in the vertical plane is useful for living in shallow near shore marine, brackish and freshwater environments - the juvenile in question was from marine deposits. Also problematic is that freshwater plesiosaurs in Australia were apparently subjected to cold to near-freezing conditions according to Kear (2006) - I couldn't imagine a 28 inch juvenile managing that. Freshwater plesiosaurs are potentially very interesting, they've been found worldwide from the early mid-Jurassic to the Late Cretaceous by the way, and I'd be curious about any morphological adaptations.

* Edit: Or not. A more recent phylogeny showed that species assigned to Leptocleidus belong to the sister clades Polycotylidae and Leptocleididae. This clade, Leptocleidoidea is a sister group to Pliosauridae. A clade of those two groups is a sister clade to Rhomaleosauridae (Smith and Dyke 2008). Thanks to Darren Naish for pointing this out.


Back to plesiosaur necks, how flexible are they anyways? The genesis of this post was a paper by Zammit et al. (2008) which rigorously examined just that in the elasmosaur Aphrosaurus. The authors created life-sized 2D models of the vertebrae in dorsal and lateral view and used the minimum and maximum amount of intervertebral cartilage to create a possible range (Zammit et al. 2008). Models were also made of a boid, snake-necked turtle and sea lion for comparison - these tended to produce slight underestimates (Zammit et al. 2008). It turns out that Aphrosaurus could bend its neck 87–155° in the dorsal plane - far from the 360°+ needed for a swan-like posture - and motion in the ventral plane (75–177°) and lateral plane (94–176°) appears to have been greater (Zammit et al. 2008). The authors mention an unpublished master's thesis which showed a similar pattern from Cryptoclidus and Muraenosaurus (both cryptoclidids) and noted that the vertebral centra in those genera had concave articular faces and rounded lateral margins, imply more vertebral movement (Zammit et al. 2008). Exact figures were not given, but the vertebral count (~40) was lower so the cryptoclidid necks are not necessarily more flexible overall.

Zammit et al. mention that cervical zygapophyses are inclined more posteriorly so the back of the neck has increased vertical flexibility at the expense of lateral flexibility; the amount of flexibility also decreases going towards the posterior end of the neck. Previous papers (which I can't access) mention a "tongue in groove" structure also in the posterior part of the neck may be analagous to zygantrum–zygosphene articulations in snakes, which reduce torsion (Zammit et al. 2008, Moon 1999). Elasmosaurs seem to lack a mid-neck increase in flexibility that appears to have been present in cryptoclidids (Zammit et al. 2008). As far as function, Zammit et al. conclude that a strait held neck combined with lateral and/or ventral movement to capture prey is plausible but arching and slight s-curves appear possible as well; these are consistent with models of elasmosaurs as benthic grazers, ambush predators, and active predators using snake/turtle-like strikes.

It is worth mentioning that plesiosaurs typically* had dorsolaterally oriented orbits (e.g. the Lower Jurassic elasmosaurid Occitanosaurus - Bardet et al. 1999), indicating that they could not see prey below them. It would seem very odd then that elasmosaurids have been known to consume benthic prey, but a recent reconstruction (McHenry et al. 2005) shows the plesiosaur with its head and neck at an angle where it could probably see what it was eating. Plesiosaurs are also known to be capable of underwater olfaction, but I'm guessing that it wasn't acute enough to make up for a considerable blind zone. I'd hate to be terribly bold here, but perhaps plesiosaurs captured prey with lateral and/or dorsal movement of the neck. Considerable ventral movement would still be important for the aforementioned grazing on benthic prey and perhaps for sneaking up on some animals. Just... throwing it out there.

* I haven't come across any exceptions



Well, I lost most of a night's sleep thanks to this post, but it was worth it.


References:

Bardet, Nathalie et al. 1999. A new Elasmosaurid Plesiosaur from the Lower Jurassic of Southern France. Palaeontology 42, 927–952

Kear, Benjamin 2006. Plesiosaur remains from Cretaceous high-latitude non-marine deposits in Southeastern Australia. Journal of Vertebrate Paleontology 26, 196–199

Kear, Benjamin. 2007. A Juvenile Pliosauroid Plesiosaur Reptilia: Sauropterygia) from the Lower Cretaceous of South Australia. J. Paleont. 81, 154–162

McHenry, C. R. et al. 2005. Bottom-Feeding Plesiosaurs. Science 310, 75

Moon, Brad R. 1999. Testing an Inference of Function From Structure: Snake Vertebrae Do the Twist. Journal of Morphology 241, 217–225

O'Keefe, F. Robin. 2002. The evolution of plesiosaur and pliosaur morphotypes in the
Plesiosauria (Reptilia: Sauropterygia). Paleobiology 28, 101–112

Smith, Adam Stuart. 2008. Fossils Explained 54: Plesiosaurs. Geology Today 24, 71-75

Smith, Adam S. and Dyke, Gareth J. 200 The skull of the giant predatory pliosaur Rhomaleosaurus cramptoni: implications for plesiosaur phylogenetics. Naturwissenschaften. Available

Storrs, Glen W. 1993. Function and Phylogeny in Sauropterygian Evolution. American Journal of Science, 293-A, 63-90

Zammit, Maria et al. 2008. Elasmosaur (Reptilia: Sauropterygia) neck flexibility: Implications for feeding strategies. Comparative Biochemistry and Physiology, Part A 150, 124-130