Monday, 10 March 2014

Episode 2: The Wealden River Masters, goniopholidid Crocodyliformes

Insert your own whoops, hollers,cheers, or discharging firearms here. 
Welcome to Episode 2 of the snappily-titled Wealden Crocodyliformes Trilogy!* We'll waste no time with introduction - read this if you haven't already - and dive straight into our second group, the goniopholidids. Much of the information herein is derived from Salisbury and Naish (2011) so, if in doubt, consult this tome for further details.

*Snappily titled? And it's about crocodiles...? Man, I'm so wasted on you guys.

Without question, the Wealden waterways were lorded over by a group of Crocodyliformes known as Goniopholididae. The namesake of this group, Goniopholis, is one of the more familiar Mesozoic crocodyliforms after famous taxa like Sarcosuchus and Deinosuchus, and is well known as a relatively ‘conventional’ crocodyliform compared to some of the other weirdo crocs doing the rounds in the Mesozoic. Goniopholidids are found throughout Jurassic and Cretaceous rocks in the Northern Hemisphere and are part of several famous fossil faunas, including being the best known crocodyliforms of the Wealden fauna. Miscellaneous goniopholidid teeth and scutes occur throughout the Wealden, and their existence has been known for a long time. Teeth ultimately attributed to indeterminate goniopholidids were found in Sussex during the 1820s by Gideon Mantell as part of the same collections of crocodile’ material which was later found to contain unappreciated early records of Wealden baryonychines.

Despite this long history, work on Wealden goniopoholids is still developing (Salisbury and Naish 2011). At one time, most Wealden goniopholid taxa were considered members of Goniopholis proper, the famous Owen-named genus of great historic significance. As with many 'classic' genera, Goniopholis is now appreciated to be a bit of a taxonomic mess and claims of 19 species are being scrutinised (e.g. Salisbury and Naish 2011; Andrade et al. 2012). Recent reviews have suggested that the Wealden goniopholidid assemblage contains a sole Goniopholis species from the Weald Sub-basin and two other genera from the Wessex Sub-basin, all known from good skull material and, in the latter instances, a series of articulated postcranial remains (Salisbury and Naish 2011). These named species include Goniopholis willetti from the Grinstead Clay Formation, Sussex; Anteophthalmosuchus hooleyi (below), from the Wessex and Vectis Formations of the Isle of Wight, and Vectisuchus leptognathus, also of the Wessex Formation (why Vectisuchus when it’s not found in the Vectis Formation? ‘Vectis’ is the Roman word for the Isle of Wight, so ‘Vectis’ frequently pops up in animal names from this part of the world). The latter was almost known from a complete skeleton, but a cliff fall during its collection rendered much of the hindlimb, pelvis and tail inaccessible. In spite of this incident, it’s still fair to say that this group has one of the better records among Wealden reptiles, and it might get even better. Fragmentary goniopholidid jaw fossils hint at further, unnamed species, but they are currently too poorly represented to warrant naming.

The Wealden goniopholidid Anteophthalmosuchus hooleyi takes advantage of a flooding river to hunt two stranded Hypsilophodon foxii. The big one is Using the Ballet to escape.

Goniopholidids vs. modern crocodilians, round 1: anatomy

What kind of Crocodyliformes were goniopholidids? Because these animals appear to resemble modern crocodiles in size, shape and probably lifestyle moreso than any other well-known Mesozoic Crocodyliformes, they are often reconstructed as ancient copies of large modern species like Nile or saltwater crocodiles (e.g. Karl et al. 2006 - see reconstruction here). This isn’t really the case, however: goniopholidids may look a little similar to modern crocodilians at first glance, but much of their anatomy is unconventional and their possible habits were likely rather different. If we compare these aspects directly, their differences will soon become apparent.

We’ll start with size. Here, it must be said, goniopholids are undoubtedly pretty similar to modern crocodilians. The largest Wealden goniopholidids – Anteophthalmosuchus and G. willetti - were large animals each attaining at least 3.5 m long. This is a pretty comparable size for many modern crocodiles, and may even seem a little on the small side compared to the 5 m+ lengths attained by some extant crocodilians. Don’t be fooled into thinking this makes Wealden goniopholids diminutive creatures, however: a 3.5 m long crocodyliform would somewhere around 200 kg in weight and stretch longer than your 3-seater sofa. These were undoubtedly big, bulky animals. Vectisuchus, by contrast, was a much smaller species, only attaining 1.2 m in length.

In fine anatomy, we start to see obvious differences between the ancient goniopholids and modern crocodilians. Goniopholidid backs were covered with two rows of rectangular osteoderms with interlocking pegs at their distal margins – they are much like the atoposaurids we met last time in this respect (Salisbury and Frey 2000). These are largely devoid of ornamentation with only slight keels along their dorsal surfaces. This configuration is rather different to the more complex and ornate osteoderm arrangements seen in modern crocodyliforms, and goniopholidid osteoderm shields would probably seem rather simple and inelegant by contrast. Also unlike modern crocs, goniopholid osteoderms do not extend far up the neck, perhaps because doing so would impair neck mobility (see below), and further osteoderms were found along their bellies. These were formed of hexagonal plates rather than long, rectangular ones however. Another key distinction between goniopholidids and modern crocodilians is found in their forelimbs. Most goniopholids have arms which are at least as long as their legs and many species - including Anteophthalmosuchus and Vectisuchus – have forelimbs which surpass the length of the hindlimb. This increased length is provided by relatively elongate humeri and wrist bones, and would give goniopholidids taller statures than those of all modern crocs, which are always shorter up front than behind. If we extending this comparison further, we’ll see that goniopholidid forelimb length is almost unique among all Crocdyliformes, being longer than virtually all of their relatives.

Like many modern crocodilians, goniopholidids possess the well-built, powerful skulls of formidable predators. There is also overlap in general skull shape with modern crocs too, with G. willetti and Vectisuchus having rather long, slender snouts which are narrower than the posterior regions of their jaws. By contrast, the skull of Anteophthalmosuchus belongs to a real bruiser; its jaws only gently converge from the enormous posterior region to form a chunky, roughly triangular skull with a rounded muzzle. Both skull types are equipped with goodly-sized, slightly recurved conical teeth which would not look out of place on modern crocodilians. Again however, there are differences in detailed anatomy. Of particular interest is the orbits of Anteophthalmosuchus, which only permitted forward vision rather than anterolateral as is usual for Crocodyliformes (its name, roughly meaning ‘forward-eye-crocodile’, reflects this - see Salisbury and Naish 2011). A similar condition is also seen in Vectisuchus, but it is not quite as well developed and seems to have arisen independently. Goniopholid skulls are also rather flatter than those of modern crocs, and have distinctly over-biting upper jaws. An unusual hollow in the cheek region, known as the maxillary depression, was also present, apparently representing an unusually large pressure-sensitive region of the goniopholidid face (Andrade 2009).
A house-proud Goniopholis willetti stands at the entrance to his burrow. Note his long arms, narrow jaws, and lack of a doormat.

Goniopholidids vs. modern crocodilians, round 2: habits

It may be expected that these similarities and differences between modern crocodilians and goniopholididis may translate to overlapping, but also slightly different lifestyles. Happily, because the anatomy of Wealden goniopholidids is well-documented, we can make some informed speculation as to how these animals may have lived and, indeed, this seems to be the case. The size and robust skeletons of G. willetti and Anteophthalmosuchus suggests that the ecological bucks of Wealden waterways stopped with them: occasional visits from spinosaurids aside, they were the largest predators in Wealden lakes and rivers and clearly well suited for tackling large prey items. We might imagine each as the apex predators of their respective waterways, taking small or medium-sized terrestrial animals, large fish and other aquatic reptiles as prey. This gives these animals a role much like those filled by several species of large crocodilians today. Smaller Vectisuchus, by contrast, probably ranked it in the mid-league of ancient Wealden ecosystems, probably capable of holding its own against most aquatic Wealden species but wanting to be wary of its larger cousins. Applying trends of snout shape and prey preference of modern crocodiles to Wealden goniopholidids suggests they likely differed in general prey preference: slender-snouted Vectisuchus and G. willietti probably took relatively smaller prey than the massively-jawed Anteophthalmosuchus. Through overall body size and jaw shape, these animals probably avoided stepping on each other’s ecological toes – at least Anteophthalmosuchus and Vectisuchus were contemporaries which probably practised niche partitioning (Salisbury and Naish 2011).

We might expect goniopholidids to exploit their large size in a similar way to modern crocodilians. Large modern crocodiles often focus their predation efforts to certain times of year when environmental conditions are favourable, such as times when rivers and lakes are in flood, when prey is particularly abundant, or at least the climate is more forgiving. Given how extreme the Wealden climate was - summer temperatures in some parts of the Wealden reached 36–40°C and experienced annual droughts (Sweetman and Insole 2010) – goniopholidids may have used similar strategies. As with big modern crocodilians, their large bodies hold ample reserves to wait out leaner or stressful times, and it’s possible that some goniopholidids waited out the long, hot Wealden summer in cooling pools or burrows (see image, above), while smaller crocs had fewer resources to fall back on and continued to exert themselves throughout hard times.

Beyond these generally favourable comparisons however, many aspects of goniopholidid anatomy hint at different habits to modern crocodilians. For instance, the development of goniopholidid maxillary depressions likely represent enlargements of sensory organs present in modern crocodilians used to detect prey at the water/air interface (Andrade 2009). All else being equal, does this indicate that goniopholids were more routinely grabbing prey at the water surface rather than diving for food or living generalist lifestyles? In other instances, it’s not clear what significance goniopholidid anatomical quirks may have. It’s difficult not to wonder why some Wealden goniopholidids possess entirely forward-facing eyes, for instance, and if this was related to feeding. Ordinarily, increased amounts of forward vision are associated with development of binocular vision and heightened abilities to judge distances. Might that mean predation techniques were unusual in some goniopholidids, involving chases, or carefully judged lunges and strikes at prey?

The preferred habitats and locomotory methods of goniopholidids are also worth pondering. There is some evidence that larger Wealden goniopholidids were mostly confined to a semi-aquatic existence, as their interlocking osteoderms likely strengthened their backs and improved terrestrial competency (as it does for atoposaurids and several other type of ancient crocodyliform), but their sheer weight likely impeded terrestrial locomotion over sustained periods (Salisbury and Frey 2000). The same is true of large modern crocodiles: here, reinforced vertebral joints perform a similar job to osteoderm bracing but still fail to facilitate effective, fast terrestrial locomotion for long periods. Larger crocodilians therefore spend much of their time in water, and certainly find most of their food there. If so, this makes the atypically long forelimbs of goniopholidids all the more interesting. Often, development of relatively equate limb lengths in quadrupeds is considered a sign of good terrestrial proficiency, betraying a well-balanced animal with effective carriage and equal gait efficiency in both limb sets. But how can this apply to large, heavy goniopholidids if they weren’t walking very much? Doubtless, an increased forelimb stride length was useful on occasions when large goniopholidids did leave the water, but why develop these features if much of their lives were spent in deep water? Did these animals ‘walk’ along river beds more than other Crocodyliformes? Was this trait even important for big adults? Perhaps smaller or juvenile goniopholidids took advantage of long forelimbs before they outgrew real terrestrial proficiency, spending more time on land before becoming more thoroughly aquatic at larger sizes. We could speculate all night about the intriguing possibilities here: Crocodyliformes are sophisticated creatures which do a lot more than eat, sleep and wander about: they also dig burrows, construct nests, climb onto trees and rocks, and are very sociable. Could their long forelimbs be related to these behaviours? Vertical size is seemingly more intimidating to modern Crocodyliformes than girth (Farlow and Dodson 1975) - might long arms and a tall stature have incurred social significance for goniopholidids? It’s not inconceivable that the long arms of goniopholidids were influenced by these activities rather than just locomotion, and I suspect an investigation into the evolution and functionality of their forelimbs would yield some very interesting results.

The outcome

In sum, then, it seems that we need to be cautious when thinking of goniopholidids as 'conventional' Crocodyliformes or simply forebears of modern crocodiles. Many aspects of their anatomy are not only different from those of modern crocodiles, but actually downright odd, and likely impacted on their habits and lifestyles significantly. Palaeoartists - bear all this in mind the next time you set out to draw your goniopholidids skulking in the background of your dinosaur artwork.

For the concluding post in the Wealden Crocodyliformes Trilogy, we're going out with a bang and an exciting new discovery - and it's not very far off now. Stay tuned!

References

  • Andrade, M. B. (2009). Solving a century-old mystery: the structure and function of the maxillary depressions of Goniopholis (Crocodylomorpha, Neosuchia). In Journal of Vertebrate Paleontology (Vol. 29, pp. 54A-55A). 
  • Andrade, M. B. de, Edmonds, R., Benton, M. J., & Schouten, R. (2011). A new Berriasian species of Goniopholis (Mesoeucrocodylia, Neosuchia) from England, and a review of the genus. Zoological Journal of the Linnean Society, 163(s1), S66-S108.
  • Farlow, J. O., & Dodson, P. (1975). The behavioral significance of frill and horn morphology in ceratopsian dinosaurs. Evolution, 353-361.
  • Karl, H. V., Gröning, E., Brauckmann, C., Schwarz, D, & Knötschke, N. (2006). The Late Jurassic crocodiles of the Langenberg near Oker, Lower Saxony (Germany), and description of related materials (with remarks on the history of quarrying the “Langenberg Limestone” and “Obernkirchen Sandstone”). Clausthaler Geowissenschaften, 5, 59-77.
  • Salisbury, S. W. & Frey, E. 2000. A biomechanical transformation model for the evolution of semi-spheroidal articulations between adjoining vertebral bodies in crocodilians. In Grigg, G. C., Seebacher, F. & Franklin, C. E. (eds) Crocodilian Biology and Evolution. Surry Beatty & Sons (Chipping Norton, Aus.), pp. 85-134.
  • Salisbury, S. W. & Naish, D. (2011). Crocodilians. In Batten, D. J. (ed.) English Wealden Fossils. The Palaeontological Association (London), pp. 305-369.
  • Sweetman, S. C., & Insole, A. N. (2010). The plant debris beds of the Early Cretaceous (Barremian) Wessex Formation of the Isle of Wight, southern England: their genesis and palaeontological significance. Palaeogeography, Palaeoclimatology, Palaeoecology, 292(3), 409-424.

7 comments:

  1. Very interesting. Goniopholidids are some fascinating crocs, especially due to their large forelimbs. Keep up the good work. Can't wait for the final episode in this awesome trilogy!

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  2. Where do they come out phylogenetically ? ; depending on how basal they where, could they still have had lips, and been partially endothermic? Like somewhere in-between endo- and ecto-?

    --Sean

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    1. How silly of me, forgot to mention this in the article. They're fairly fairly derived neosuchians, on the branch leading to Eusuchia. My assumption is, therefore, that they'd have fairly eusuchian-esque facial characteristics and physiology, but someone who knows Crocodyliformes more thoroughly may be able to tell you different.

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  3. Well over a tonne for a 3.5m crocodyliform seems excessive, for example, in table 1 of Erickson et al. (2012) we find that captive crocodilians around 3.5m long can weight from 180kg to 270kg and since captive individuals can be 25% fatter than wild animals of the same length (Erikson et al. 2004) a more realistic range could be from 145 to 215kg... unless of course you said that figuratively or as hyperbole.

    Anyway, excellent post! I learned quite a few things about this guys, I really was one of those that thought they were rather generic. I'm going to read the final episode now.

    -Miguel

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  4. Thanks for the comment, Miguel, and you're right - that mass doesn't seem right! I've corrected it with figures based on Erickson's data. My initial comment was based on an article I read about similarly sized saltwater crocs, which contained that (probably erroneous) factoid.

    Glad you enjoyed the croc series. They've been a lot of fun to write. More things on obscure Wealden animals soon, no doubt!

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  5. Very interesting, I have just a seminar tomorrow where I talk about Goniopholidids and its lifestyle. There were a couple of things in your text that I did not know, you have given me some good ideas.

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