The pterodactyl that fell down the back of the sofa, part 2: Busted jaws, gutter casts and something quite unique

If you missed the first post, odds are you won’t grab what’s going on here: best head here to catch up before reading on.

We left our last post with something of a conundrum: a reappraisal of the skull pieces of Istiodactylus latidens specimen NHMUK R3877 – the best known of this historically important species - suggests that it had a jaw some 20 per cent shorter than assumed for an entire century (as detailed in Witton 2012). This revises the jaw length from 423 mm to 333 mm, which may not seem like much of a big deal, but has considerable implications for the taxonomy and functional anatomy of I. latidens, along with increasing the morphological disparity of pterosaurs generally: there are no other pterosaurs with skull proportions like this new, short-faced version of I. latidens. Faced with literally changing the face of this pterosaur as we know it, then, there is obvious good reason to question this finding. First and foremost, are there any other specimens of I. latidens that can shed light on its actual jaw length? Not really: the only other I. latidens skull remains also bear incomplete jaws, so they offer no assistance here. With this in mind, how about revisiting the previous length estimate for the I. latidens jaws, Walter Hooley’s calculations performed way back in 1913? Is there any good reason to continue using this estimate in light of our new assessment? (Image, above, shows Hooley's illustrations of the NHMUK R3877 skull pieces, including a long-skulled reconstruction and the otherwise ignored, 'third' skull piece [middle right]. From Hooley 1913.)

Hooley’s calculations
The most eye-catching fact of Hooley’s NHMUK R3877 skull length estimate is that it was not based on any skull anatomy at all but, instead, on the presumed depositional conditions and in situ positioning of the skull remains as preserved in the Vectis Formation cliffs. Note the use of the word ‘presumed’ here, too: NHMUK R3877 was recovered from an avalanche, so their original position within that cliff is not known.

Three siltstone boulders contained NHMUK R3877 (image, above, shows Hooley's sketches of the original boulders. Note how he assumes the bones are continuous: this is important later. From Hooley 1913). Each represents a bit of gutter cast, a high-energy sedimentary deposit that occupies long, scoured channels in previously deposited substrates. Such deposits are associated with large-scale, sediment-heavy currents and probably indicate that NHMUK R3877 was washed into the Vectis Formation sedimentary basin (presumed to represent a freshwater-brackish lagoon) by a storm. Along with the skull material, these boulders also held several incomplete limb bones in parallel alignment with the long axes of the cranial remains (see adjacent image). Two of these blocks fitted together perfectly (Hooley 1913), but a fourth – presumed to contain the missing mid-lengths of the limb bones and the majority of the missing skull pieces – was never recovered, so the third could not be reattached to the others. Hence, Hooley had to estimate just how big this missing block was. He presumed that the limb bones spanning the missing region were complete when deposited because their relative positions were identical across the gap, and thought the same must also apply to the skull, seeing the cranial extremities were preserved in the same approximate positions in each block. Hence, if Hooley could figure out the length of the limb bones, he would know both how large the missing block was and be able to deduce how much skull material was missing.

Happily, Hooley had some help in this endeavour: the complete humerus of the holotype of I. latidens (NHMUK R176). This specimen has very similar proportions to that of the NHMUK R3877 humerus, so was probably of similar length – 220 mm. Armed with this data, Hooley reasoned that the missing block was 89 mm long and, with this in mind, concluded that 283 mm of missing material length lay between the rostral and braincase pieces of NHMUK R3877. From this, Hooley suggested that NHMUK R3877 had a skull length of 560 mm, of which 423 mm represented jaw. If true, this would mean that the ‘third’ skull element of NHMUK R3877 discussed in our last post would only represent half the of the missing jaw length. This would not be entirely impossible given how parallel sided the maxillary region of I. latidens is, but…

The skull of NHMUK R3877 was clearly totalled when it was deposited
Hooley’s assumptions that the limb bones of NHMUK R3877 were continuous when deposited does not seem unreasonable as they show no signs of damage or breaking before deposition. The same cannot be said for the skull. The rostrum was clearly smashed at some point during transportation. Check it out for yourself: here’s the rostrum in right lateral and dorsal view (from Witton 2012).

See those big cracks there? They aren’t sutures between bones or unprepared matrix: they’re big fractures between broken regions of bone. Note how the posterior part of the rostrum has actually been displaced from the anterior (arrows indicate points of displacement): the damage is substantial enough that the rostrum no longer represents the in vivo appearance of this specimen. This may, of course, be expected from a specimen found in a gutter cast: high-energy deposits are unlikely to be kind to fragile cargo, and the long, relatively slender bones of istiodactylid skulls may have be prone to shattering in rough transportation. Indeed, the scours that gutter casts infill need large objects to scour out their grooves in the first place: perhaps the remains of this I. latidens were the tools used to make the scour in this instance*.

*This is a good example, by the way, of why palaeontologists need to know something of sedimentology and taphonomy, even if their primary interest is in the biology of extinct animals. Take note if you’re a palaeontology undergraduate who’s bored silly by sedimentology lectures: you genuinely need to know this stuff!

All considered, we can conclude at least two things:

1. We cannot assume – as Hooley did – that the proportions of the posterior skull will neatly taper to the preserved rostrum. The rostrum was clearly taller in life than it is in this specimen.
2. We have good evidence that the skull was badly damaged when deposited, which may mean it wasn’t articulated and continuous when preserved. The association of the upper and lower jaw tips indicates that the damage occurred close to, or during, deposition, or else these elements may have separated when tumbled around during further transportation.

Thus, there’s good taphonomic evidence that Hooley’s assumptions of skull continuity being flawed, which undermines his elongate I. latidens skull reconstruction. Frustratingly, we cannot directly test Hooley’s ideas anymore: we have no record of the specimen in situ, and the original composition of the NHMUK R3877 blocks is no longer clear they’ve been entirely prepped away. With no other hypotheses on the table, it seems that the only reliable indication of I. latidens jaw length can now be achieved with the ‘forgotten’ skull piece, which can be directly measured to give us an idea of the jaw metrics. Happily, as mentioned in the last post, there seems to be good morphological congruence between all three pieces of the NHMUK R3877 skull, so this doesn’t seem an unreasonable move at all, and I think we can be confident that the significant proportions of the I. latidens skull can now be predicted. And, if so, it turns out that they’re quite unusual.

A makeover for Istiodactylus latidens
We mentioned at the top of the post that the jaw length of I. latidens can now be measured at 333 mm (as composited and reconstructed, above. From Witton 2012). As such, we can now estimate the entire skull length at around 450 mm (the posteriormost part of the braincase is poorly known, so this remains an estimate until more remains are found), which is considerably shorter than the 560 mm assumed by Hooley and others. In this configuration, the posterior skull is very large with a height nearly 40 per cent of the jaw length, and a width at the jaw joint around a third of the same dimension. For a ‘long snouted’ pterosaur (that is to say, a species that is not considered characteristically short-faced like anurognathids or tapejarids), these values are huge (well, if we ignore taxa that increase their skull heights with crests. That’s cheating.). The tooth row, already considered to be very short, is now even shorter in occupying only the first 27 per cent of the jaw. In addition, this reconstruction brings the aforementioned slenderness of the maxilla to our attention (to my knowledge, the first time this has really been acknowledged): even with the shorter skull configuration offered here, it’s extremely thin with a height not even 2 per cent of the jaw length. Wow. In short, it seems that familiar-old Istiodactylus latidens, an animal we thought we knew well for many years, may be even more unusual than we've given it credit for.

If this assessment is correct, I. latidens is now one of the most easily characterised pterosaurs out there. This isn’t such a big deal, though: there has never been any doubt over the validity or diagnosis of this species. Far more interest stems from what this new skull reconstruction may mean for the workings of its skull during feeding (seriously, what can you eat with a cheek bone thinner than a pencil in a skull over 400 mm long?) and what all this may mean for istiodactylid taxonomy. We’ll discuss these in the final post in this little series.

References

• Hooley, R. W. 1913. On the skeleton of Ornithodesmus latidens; an Ornithosaur from the Wealden Shales of Atherfield (Isle of Wight). Quarterly Journal of the Geological Society, 96, 372-422.
• Witton, M. P. 2012. New insights into the skull of Istiodactylus latidens (Ornithocheiroidea, Pterodactyloidea). PLoS ONE, 7, e33170.

The pterodactyl that fell down the back of the sofa, part 1: another ‘unexpected discovery’

It’s no secret that many palaeontological ‘discoveries’ aren’t made in the field, but are actually stumbled into by researchers working in museum collections. This is not surprising in the slightest: the flagship museums of many nations are rammed with - literally - millions of specimens. Some of these are virtually undocumented and unknown, even to experts in relevant fields, and require visiting researchers to be in the right frame of mind, to recognise and appreciate as something worthy of putting on record. It’s also well known that the preparation of many discoveries can take such a long time that the treasures brought back from exhibitions to exciting localities can sit, unknown, decades after decade. It is not inconceivable to imagine a whole career being forged by picking your way through the archives of big museums, looking at fossils that have been missed, unopened or in need of reappraisal.

Last week, I played this very game myself, by bringing attention to an overlooked pterosaur specimen held in the bowels of the Natural History Museum, London (Witton 2012 – free to download from PLoS ONE. Image, above, shows the title slide from a talk I've given on this research). The twist to my own version of this tale is that the element I described does not only belong to a very familiar pterosaur species, but a very familiar specimen. The short version of this story is that reappraising a long-forgotten component of a well-known British pterosaur suggests that its 100-year standard skull reconstruction is incorrect, which has obvious knock-on effects for its taxonomy and functional morphology. Those of you with lives to lead may as well log of here, but, if you have a lot of time to kill, read on over this series of posts for more back story and details.

The who

The animal in question is Istiodactylus latidens, a 4.2 m span ornithocheiroid from the Lower Cretaceous Vectis Formation, of the Isle of Wight (image, above, shows I. latidens launching, from my upcoming book. Some people may be happy to hear that the first draft has been submitted!). I. latidens is the largest species of Istiodactylidae and the only member of this group known to occur outside of Cretaceous deposits of China’s Liaoning region*. Istiodactylids are characterised by their muzzles of interlocking, razor-edged teeth and have been called the ‘duck-billed pterosaurs’ by some but, as we’ll see later, this analogy is plain daft: there is nothing at all duck-like about istiodactylid jaws). Istiodactylus has been known for a long time, though much of its early research history is murky. The documentation of its discovery, and early inventories of material referred to this species, are particularly vague. It’s not even known how pterosaur grandpapa Harry Seeley knew of I. latidens unusual teeth when he named the species in 1901, as the holotype specimen appears to lack jaw elements (‘latidens’ means ‘broad tooth’). Howse et al. (2001) suggested that an un-described skull in the University Museum of Zoology, Cambridge may belong to the holotype material, but this has not been confirmed. Seeley’s naming of the material was also of borderline validity, and some nomenclatural wrangling was needed to straighten out the taxonomy of the specimens we now know as Istiodactylus (Howse and Milner 1993; Howse et al. 2001).

*There have been several claims to the contrary, however. Bakker [1998] reported an alleged istiodactylid jaw from the Upper Jurassic Morrison Formation of Colorado, but this has not been accepted by the pterosaur community and seems to represent something more akin to Darwinopterus or a basal ctenochasmatoid. A reappraisal of that specimen is needed to say anything definite, though. Arbour and Currie (2010) named an Upper Cretaceous istiodactylid from British Columbia, Gwawinapterus beardi but, for reasons discussed in Witton (2012), I have considerable doubt that this material is pterosaurian, let alone an istiodactylid. There are reports, however, of a Cretaceous istiodactylid from Lebanon: I think a description is underway.

The what
I. latidens is amongst the best known istiodactylids of all and was, for much of the 20th century, one of the only pterosaurs represented by substantial, uncrushed three-dimensional material. Of the various specimens referred to I. latidens, one is preferentially discussed far more than the others: NHMUK R3877. Represented by an almost complete skull (see below) and a good portion of postcranial material, this specimen is the material that most associate with the name I. latidens. NHMUK R3877 was collected in 1904 from Atherfield Point on the Isle of Wight, and subsequently described and illustrated in detail by Reginald Walter Hooley (1913), a dedicated ‘amateur’ palaeontologist who collected and described many important specimens of Cretaceous reptiles from the Isle of Wight. Pterosaur workers have added little detail to the picture of I. latidens since Hooley’s day and, aside from a little nomenclatural juggling in the 90s and 00s, the picture of I. latidens has remained unchanged since the 1913 description. Hooley’s picture of NHMUK R3877 has become very familiar thanks to its continual discussion in pterosaurian technical literature, be it for anatomical comparisons or for use in functional morphology. We also still use Hooley’s original bone dimension estimates for NHMUK R3877, of which there are many: though well-preserved, few bones are complete. This latter issue is the reason for the waters around I. latidens and NHMUK R3877 finally being unsettled after a century of stillness. (Image above is Stafford Howse's life reconstruction of I. latidens, primarily based on NHMUK R3877. From Howse et al. 2001)

Ripples in the pond
Last June I travelled up to the NHM with neonate palaeontologists Kirsty Morgan and Georgia Maclean-Henry with a goal of photographing NHMUK R3877 for my book. The skull was my main priority, as the two skull pieces of NHMUK R3877 show details of istiodactylid skulls fantastically. One block shows the elongate, delicately-built posterior region, and the other contains both jaw tips, complete with smiling, interlocking teeth. These remains do not articulate, as the middle region has long been considered missing or, perhaps, only represented by useless scraps of bone. Generally, around 300 mm has been thought missing from the mid-jaw region, giving I. latidens a long skull length of 560 mm (Hooley 1913). On our trip, I ended up riffling through the many drawers containing NHMUK 3877 more thoroughly than usual and, in the odds-and-sods drawer, containing some of the less impressive bits and pieces of the specimen, I stumbled across this:

You’re looking at the c. 140 mm length of maxilla and portion of mandible from NHMUK R3877, a rather unimpressive collection of bones representing the mid-jaw length of the skull and lower jaw. There are only a few features worthy of mention, being the groove extending along the medial surface of the maxilla and the very shallow depth of the same bone, which measures 6 – 7 mm along its length. I must admit to having been rather ignorant of this third skull piece in the past, but Hooley knew of it, identified it and even figured it (Hooley 1913, Pl. XXXVII, Fig. 4). Until perhaps fairly recently**, however, it was in a rather unprepared state and of considerably less interest than the other, sexier bits of the fossil. Interestingly, I’m not alone in my ignorance: this portion of the skull that has never been incorporated into a reconstruction of I. latidens skull (despite at least four efforts that I know of) and has not been mentioned, to my knowledge, since Hooley’s brief description of it in 1913. Because I’m basically a child with an irrepressible urge to articulate broken fossils bones where possible, it didn’t take long for me to start wondering if this broken bit of jaw would fit with either of the other skull pieces. The answer was almost certainly yes, to both of them: it seems that a bridging element to the two NHMUK R3877 skull pieces was there all along, but had simply been forgotten or ignored.

**I’m unsure of the exact date of preparation, but I recently noticed that a photograph in Wellnhofer’s (1991) pterosaur encyclopaedia shows the specimen in the unprepared state. There is no date provided for the photo, but it does suggest that the specimen was left not prepped for several decades following Hooley’s description.

It must be said that I didn’t take this idea lightly: I not only asked my colleagues and NHM curator (and Pterosaur.Net contributor) Lorna Steel for corroboration of the fit, but sought further reassurance from David Martill and three pterosaur-studying PhD students before I believed my own eyes. After all, there is 100 years of intellectual inertia around the length of the NHMUK R3877’s jaw, and I figured that others would have found some flaw with their close association. Turns out that there wasn’t: for whatever reason, people had simply not put the material back together. The fit, it must be said, is not pin-point perfect, but the dimensions of the broken regions, the position of the maxillary medial grove, and the displacement of the dentary from the upper jaw are very close matches. This suggests that we’re actually only missing millimetres of the NHMUK R3877 jaw, not almost 300 mm. If this is the case, then the length of jaw between the two large skull pieces is only half that supposed for the preceding century, so we may seriously need to seriously overhaul our impression of the I. latidens skull. But should we be so hasty? After all, Hooley was no fool, so perhaps we need to consider his ideas in greater depth before rejecting his proposed jaw length. Perhaps there are other interpretations about the fit of this third element we could also consider. For that, you'll have to come back for part 2...

References

• Arbour, V. M. and Currie, P. J. 2010. An istiodactylid pterosaur from the Upper Cretaceous Nanaimo Group, Hornby Island, British Columbia, Canada. Canadian Journal of Earth Sciences, 48, 63-69.
• Bakker, R. T. 1998. Dinosaur mid-life crisis: the Jurassic-Cretaceous transition in Wyoming and Colorado. S. G. Lucas, J. I. Kirkland, & J. W. Estep. (eds.) Lower and Middle Cretaceous Terrestrial Ecosystems, New Mexico Museum of Natural History and Science Bulletin, 14, 67-77.
• Howse, S. C. B. and Milner, A. R. 1993. Ornithodesmus – a maniraptroan theropod dinosaur from the Lower Cretaceous of the Isle of Wight, England. Palaeontology, 36, 425-437.
• Hooley, R. W. 1913. On the skeleton of Ornithodesmus latidens; an Ornithosaur from the Wealden Shales of Atherfield (Isle of Wight). Quarterly Journal of the Geological Society, 96, 372-422.
• Howse, S. C. B., Milner, A. R. and Martill, D. M. 2001. Pterosaurs. In: Martill, D. M. and Naish, D. (eds.), Dinosaurs of the Isle of Wight, Palaeontological Association, Field Guide to Fossils 10, pp. 324-335.
• Seeley, H. G. 1901. Dragons of the air. Meuthuen and Co., London, United Kingdom, 239 pp.
• Wellnhofer, P. 1991. The Illustrated Encyclopaedia of Pterosaurs. Salamander Books Ltd., London. 192 pp.
• Witton, M. P. 2012. New insights into the skull of Istiodactylus latidens (Ornithocheiroidea, Pterodactyloidea). PLoS ONE, 7, e33170.