One of the central
tenets of palaeobiology is that similar looking skeletal structures in
different taxa convey similar functions in life. Hence, the presence of
serrated teeth, like those of extant carnivorous varanid lizards, imply
carnivory in theropods, and the convergent acquisition of long, graceful lower
legs in gazelles and ornithopods suggests cursoriality in the latter. While
some of these form/function relationships have proved relatively robust to
quantitative, experimental testing, the generality of several classical
form/function comparisons has been questioned by recent work. For example,
experimental studies on living teleost fish have shown that skeletal morphology
alone does not predict jaw movements: predictions made on bones alone fail and
real jaw movements could only be deduced when soft tissues and nervous control
mechanisms were factored in (e.g. Lauder 1995). These, and other similar
studies, have shown that we should no longer rely uncritically on simple
form/function correlations, but should test these assumptions through
experiment or modelling. This will allow us to avoid erroneous functional
predications that would otherwise resonate through ecological reconstructions
and discussions of homology, as well as influencing other functional work.
Thanks to the development of new and
refined experimental methods, as well as sophisticated computer modelling
techniques, we are now in a position where we can test at least some of the
mechanical properties of fossil skeletons (and of living tissues) in ways far
more rigorous than the early comparative anatomists could have imagined. With
this in mind, my colleagues Stephan Lautenschlager, Charlotte Brassey, David
Button and I decided to look at the skull function of three different
herbivorous dinosaurs to investigate some aspects of the form/function
question.
We selected skulls of the Late
Cretaceous therizinosaurian theropod Erlikosaurus
(the subject of Stephan’s PhD), the Late Triassic sauropodomorph Plateosaurus (from David’s PhD thesis),
and the Late Jurassic thyreophoran Stegosaurus
(based on the complete, but disarticulated skull of ‘Sophie’ the NHM’s new
specimen, which was CT scanned and reconstructed virtually by Charlotte).
Although these taxa are widely separated in time and space, and are
phylogenetically distant from each other, we chose them as their skulls are
superficially similar in several respects, due to many of the features
classically associated with a herbivorous diet. Many of these features were
acquired convergently, though some are due to their shared deep phylogenetic
heritage. All three taxa have skulls that are relatively elongate and narrow,
with low snouts, and the snouts are relatively long in comparison to overall
skull length. The external openings are large, the mandibles are slender with
slightly depressed jaw joints, there is no evidence for substantial kinesis
within the skull, and the teeth are coarsely denticulate, relatively small,
numerous and did not occlude. Traditionally these features have been associated
with ‘weak’, fast bites, a lack of sophisticated chewing mechanisms, or indeed
of any real specialisation (e.g. Norman & Weishampel 1991). As a result,
it’s generally been thought that these skulls would have functioned similarly
in life, with corresponding ideas about probable food plants and ecological
roles (e.g. reliance on ‘soft’ vegetation, lack of oral processing).
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| From left to right, skulls of Erlikosaurus, Stegosaurus and Plateosaurus (Image courtesy of Stephan Lautenschlager/University of Bristol) |
However, when we subjected models of
these skulls to multibody dynamic and finite element analyses, what we found
surprised us (Lautenschlager et al. 2016). Instead of behaving similarly, each
of the skulls has its own unique function. Stegosaurus
had a higher than expected bite force, in the range of 166–321 N, which
overlaps with that of some living mammalian herbivores. By contrast, those of Erlikosaurus and Plateosaurus were much lower and similar to each other (50–121 N
and 46–123 N, respectively). These differences in bite force were accompanied
by differences in stress patterns within the skulls. Plateosaurus seems have experienced the lowest and most evenly
distributed stress patterns (implying a skull adapted to deal with a variety of
different forces), whereas overall peak stresses were much higher in Erlikosaurus and Stegosaurus. In Stegosaurus,
stresses were concentrated in the snout, whereas in Erlikosaurus they seem to have been highest in the posterior part
of the skull. In addition, the skull of Erlikosaurus
experienced the greatest amount of deformation during biting, but those of both
Stegosaurus and Plateosaurus experienced very little shape change.
 |
| Finite element models of 'Sophie' the NHM Stegosaurus, the image at the rear grossly exaggerated to look at possible deformation patterns (image courtesy of Stephan Lautenschlager/University of Bristol) |
These results imply that each taxon
had quite different feeding strategies, a conclusion that differs from previous
ideas about these ‘unspecialised’ herbivores. For example, the differences in
maximum bite force suggest that these taxa might have been feeding on diverse
sorts of vegetation, with the higher bite force of Stegosaurus implying that it was able to feed on a broader, or
tougher, range of plant parts/types than either the ‘prosauropod’ or
therizinosaur. This higher bite force was enabled by a larger jaw muscle mass
in Stegosaurus and/or an arrangement
of the jaw muscles that allowed more efficient conversion of muscle force into
bite force. The lower bite forces of Plateosaurus
in combination with its high cranial robustness are consistent with low fibre
herbivory, dealing with soft vegetation that required little chewing, and/or
omnivory (the skull could have withstood dealing with struggling small prey,
for example). Erlikosaurus appears to
have been specialised to use the tip of its snout in plucking vegetation, as
the skull performs exceptionally badly when biting food at the back of the
mouth. Nipping soft vegetation with the tips of the jaw is also consistent with
its low bite forces.
Previously, these three taxa were
all thought to be relatively ‘boring’ herbivores that simply nipped and
swallowed soft plants. It now seems that one was eating much tougher
vegetation, another was a generalist that could exploit different food sources,
and the third was a specialist with a rather delicate way of feeding itself.
This work shows that first appearances based on simple application of the
form/function paradigm can be misleading. Novel functions have now been
revealed that would have gone unnoticed if it were not for detailed biomechanical
modelling of each skull. This leads me to wonder what other functional
surprises might be lurking in dinosaur skulls, especially as so few have been
really thoroughly studied in this way.
References
Lauder, G.V. 1995. On the inference of function
from structure. In Functional Morphology
in Vertebrate Paleontology (ed. J.J. Thomason), pp. 1–9. Cambridge: Cambridge
University Press.
Lautenschlager, S., Brassey, C., Button, D. J. & Barrett, P.M. 2016. Decoupled form and function in
disparate herbivorous dinosaur clades. Scientific
Reports 6: 26495. doi:10.1038/srep26495
Norman, D.B. & Weishampel, D.B. 1991.
Feeding mechanisms in some small herbivorous dinosaurs: processes and patterns.
In Biomechanics in Evolution (eds J.M.V.
Rayner & R.J. Wootton, pp. 161–81. Cambridge: Cambridge University Press.
