Behind Your Bony Mask of Face

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We all know what this post is about.

Below I will present a few small screen grabs cut out of larger pics of skulls. Let's just say that the representative animals are quite diverse phylogenetically. But despite the disparate vertebrae groups there are some striking similarities - and differences - in the bony texture. Before I reveal what animals are represented I would like readers to simply focus on the textures and patterns at hand, without any phylogenetic prejudices in mind.

One of these things is not like the other…

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K.

Again, without any phylogenetic prejudices creeping into your decision making, what similarities and differences do you observe? Which examples are more textured, which are less? There is a story to tell in all of this I will suggest and after the players are revealed I am going to offer that it is more behavioral - more adaptationist (there is that dirty word again) - than strictly phylogenetic.
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A. Belongs to a stinkin' fur ball, the lowland Paca. Amazing is it not? These zygmotatic plates have something to do with the sound propagation of this cheeky little guy. Not only is this animal a bit of an outlier for its mammalian pedigree but it is also an outlier for its terrestrial inclinations.

Lowland Paca. Cuniculus Paca credit Paolo W. Viscari Specimen of the Week

B. One of several fishes I will be highlighting. This is the fanged blenny - Meicanthus grammisteus - which injects an opioid peptide into its prey to sedate them. Incidentally I came across a link to this fish on Facebook on the same day as the Carr paper was dominating my fb news feeds. Credit Brian Fry.

C. Some gnarly looking catfish I "borrowed" off of Flickr. Thanks credit and © to Lonmelo.

D. Another fish, the North American bowfin. Coincidentally another predatory ambush predator of murky aquatic haunts… are you sensing a pattern yet?

North American Bowfin credit

E. American Crocodile. Exceptionally large and rugose skull.

American Crocodile. Credit Daderot Public Domain

F. Another aquatic tactile predator - Suchomimus tenerensis !! credit James St. John.

G. Another nippy creature of murky habits: Amphiuma tridactylum found on Tet Zoo originally from flickr site Boneman 81.

H. Ho hum, another swamp monster Metoposaurus credit Jeyradan, public domain.

I. Daspletosaurus horneri. Does it really compare that favorably to the other examples? I'm not seeing it. There is a general rugose nature to it, but hardly as intricate bone texture as the other examples…

J. The alligator snapping turtle. Seriously what is up with highly textured facial bone among stealth aquatic predators? Credit, pic allows zoom in functions.

K. Yet another textured skulled aquatic predator, the phytosaur Pseudopalatus mccauleyi. Petrified National Park credit Park Ranger. uploaded FunkMonk

Did I hit you over the head enough with rugose skulled swamp monsters?

The recent paper on the Two Medicine Formation tyrannosaur and associated facial integument inferences (Carr et al., 2017) makes the case that crocodile facial integument is the best inference for tyrannosaur facial integument. Also worth mentioning is that this is the argument that Tracy L. Ford (Ford, 2015) has long been making for quite some time (though Carr et al. did not deem fit to mention him). While the media has certainly ran with the story, at least in the online paleo community reaction to this inference has been highly skeptical. I don't want to recount the variable criticisms to Carr et al's inferences as many have done that already. However what  should not be lost on our observations is that several of these textured skulls above come from animals that do not have tightly adhering skin texture such as the various amphibians. Or even have scales at all.

There is potentially a story to tell here...

Let me make another analogy to fantasy creatures. For me one of the most entertaining aspects of fantasy creature creation is to unpack the various - and often times disparate - elements from contemporary or extinct creatures that are spliced together to create a fictional animal. For me one of the most successful creature splices of recent years is the Bashee and Great leonopteryx from Avatar. Darren Naish does an excellent unpacking of the various inspirations and spliced bits of microraptor, bird, pterosaur, bat, fish, and sports car design that went into the creation of these animals.

It is the liberty in splicing all of these disparate animals together that creature creators do to make a strange but believable fictional animal  that paleontology needs to take more inspiration from.

I think the Carr paper is valuable because it draws attention to crocodile skull texture. Yes it is true that tyrannosaurid skulls show some gross similarity to crocodile skulls in sharing a rugose texture and if you had to draw a rough comparison as to what animal best matches large tyrannosaur skull texture crocs are a good analogy. But crocs evince this rugosity all over the skull, tyrannosaurids have many smooth parts with the rugose sections being localized across more of the rostral sections. Of the animals presented above I would posit the Daspletosaurus skull as being the most different texturally from the others. People are naturally drawn to to comparing tyrannosaurids to crocodiles because they are related and predatory. However most theropods (except spinosaurids) are quite distinct from crocodiles ecologically. And that is the gist of what I am suggesting, we are potentially witnessing bone texture as an ecological signal - not as a phylogenetic/anatomical one. Instead of asking; what other archosaur skull looks most like a tyrannosaur skull? we should be asking; why does a crocodile skull look so similar texturally to a temnospondyl, catfish, snapping turle etc etc. skull? 

When we see a diverse array of animals exhibit a remarkable similarity in facial bone texture as well as a remarkable congruence of ecological niche - ambush predator of murky, aquatic haunts - we have to seriously question if this bone texture is really a phylogenetic-anatomical message or an ecological one. There is more than a reasonable and persistent trend of highly pitted, rugose, and textured skulls among aquatic and amphibious stealth/ambush predators. The question is why?

My hypothesis is that such skulls in aquatic predators - highly textured with increased surface area but still maintaining streamlining - work as enhanced sound/vibrational interceptors. Like an old, well used catching mit these rugose skulls are better able to intercept, transmit, and "grab" acoustical/vibrational frequencies in a visually limiting aquatic environment. Vibration may travel through tissues in different ways and bone might offer an added layer of frequency interception that - when combined with other tactile organs  (nerve endings, pressure domes, "whiskers" etc. etc. ) allows for a more comprehensive reading of the environment.

So why do tyrannosaurs - and many theropods - have such rugose skulls? Well in the case of spinosaurids (and maybe other theropods that exploited aquatic environments predominantly) it is possible they converged on a highly textured design for the same reason that other aquatic predators potentially did - it enhanced sensitivity. For most other other theropod skulls - including tyrannosaurids -  I would like to advance an argument from material science: that there is a relationship between bonding strength and surface roughness. In this case the two materials are skin and bone and a rugose bone texture allows for skin to better anchor on the skull - growing into all of the nooks and crannies with increased surface area - in light of a particularly traumatic bite prone existence. That this bone texture is most prominent on the parts of the "snout" most devoid of overlaying musculature and exposed to bites we should expect this rugose nature to be most prominent there. Which it is. Unlike a crocodile skull which displays rugose formations across almost all of the skull - in line with the potential use of such rugose formations to discern water borne vibrations.

Crocodylus porosus credit 

There is potential analogy to the textured skull of theropods acting as a structural adhesive to skin argument: hippo skulls. Animals which have tremendous lips and a very battle prone, bite weary existence. They don't have smooth bone where the lips and skin anchor - they have textured, rugose bone .

credit stock vault author Bjorgvin Gudmundsson

And for the record I do agree with Carr et al. (and Tracy L. Ford btw) that tyrannosaurids (& other theropods) did have exquisitely sensitive, tactile faces. But it was through large lips that grew out from the neurovascular foramina that these nerve endings felt and sensed their world - both the real time struggles of their prey and the touch of a mate or hatchling. It is patently obvious that the pattern of foramina on the dentary (bottom jaw) are arranged in such a way that the upper teeth will not cut into the labial tissue that grows out from them. Note that this is not the pattern we see the foramina take in the crocodile dentary - where they emerge right up next to the teeth.

I also agree that they were probably more romantic and tender than we might typically imagine.

Sources

Carr, T. D. et al. A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system. Sci. Rep. 7, 44942; doi: 10.1038/srep44942 (2017).online

Ford, T. L., 2015, Tactile faced Theropods: Journal of 
Vertebrate Paleontology, SVP 75th annual meeting, Meeting Program & Abstracts, 
p. 125.


"A Long habit of not thinking a thing wrong, gives it a superficial appearance of being right, and raises at first a formidable outcry in defense of custom". Thomas Paine

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