The following blog post was really scheduled for a couple of
months ago but I just never got round to publishing it. Having said all that,
the science is still cutting edge and just as fascinating as it ever was so I
hope you find it of interest.
Forelimb reduction
in theropod dinosaurs has long since fascinated. It is a constant source of
interest that some of the largest carnivorous dinosaurs reduced the size of
their forelimbs to such a degree that it would appear that they were almost
becoming vestigial or, at the very least, were only capable of speculative
rudimentary function. Chief amongst palaeontologists looking at this enduring
mystery is Sara Burch of Ohio University.
To tackle these enduring questions, Burch approached the
issue in quite a unique way – firstly by constructing a model of the ancestral
forelimb muscle arrangement highlighting any associated plesiomorphies. Then,
by looking at the phylogenetic signals generated by examining the evolutionary
processes in various theropod lineages, Burch has come up with some useful
data.
Both allometric and evolutionary trends suggest that there
is no evidence for a general reduction in forelimb size throughout Theropoda. A
clade by clade study also revealed interesting myological trends – particularly
in Abelisauridae and Tyrannosauridae. Abelisaurids display a quite unique
morphology and yet, despite their forelimbs appearing to be useless, the study
reveals them to be still functional. It is unlikely that they were capable of
much, however, and it may be that the forelimbs were used for sexual
stimulation – speculative, of course, but not the first time this has been
suggested for abelisaurids or, indeed, tyrannosaurids.
Tyrannosaurids themselves have robust and muscular arms and
one suggestion to try and describe a particular use for these forelimbs was
that it enabled the animal to push itself up from the ground when required.
This particular hypothesis was modelled accordingly with the outcome that there
is no justifiable support for the theory. But what about possibly using their
arms for grasping or holding prey? Muscle correlation concludes that this was
indeed possible supporting the evolutionary trend that reduction in forelimb
size is not necessarily about vestigiality but rather to satisfy an
evolutionary demand. As with abelisaurids, the theory that tyrannosaurids may
have used their forelimbs intraspecifically remains untestable.
Pterosaurs currently remain the biggest flying animals of
all time and yet was there a size limit that dictated how big a pterosaur could
be and still manage to take off, fly and land? Colin Palmer, of the University
of Bristol, and Mike Habib of the University of Southern California, have been
addressing these very issues.
Azhdarchids were the
giants of the pterosaur world but estimates of their weight and mass vary considerably.
For example, Chatterjee & Templin (2004) suggested a body weight in the
region of 70Kg whilst Mark Witton (2008) and Witton and Habib (2014), more
realistically for an animal approaching the size of a giraffe, estimated 260Kg.
The general consensus is that the latter is probably correct.
As for wingspan it appears that the most common sizes vary
from 5 metres to 9 metres with an upper limit, perhaps, of twelve metres – true
giants. Computer models were generated for pterosaurs with 6, 9 and 12 metre
wingspans which were as structurally and aerodynamically as accurate as the
fossil record allows bearing in mind most of the data is derived primarily from
ornithocheirids.
For an azhdarchid to remain in the air depends, essentially,
on the available power that is generated by the muscles. The model indicates
that these pterosaurs were capable of generating sufficient power to maintain
station once airborne and, interestingly, that the ability to stay up does not
limit the size of the pterosaur. In other words, pterosaur maximum size is
limited by the amount of tensile stress generated but is not limited by size
alone. Therefore, it is quite feasible for a pterosaur with a 12 metre wingspan
to remain airborne.
What about landing? A large extant bird requires a stopping
distance of around 4 metres per second and the models predict that large
pterosaurs also fall within this range and the authors point out that the
robust hind limbs of azhdarchids make for a pretty sturdy undercarriage so it
seems possible that a pterosaur with a 12 metre wingspan could also land
safely.
But could a pterosaur of these proportions generate the
sufficient power, technique and thrust to launch itself into the air? It is safe to assume that pterosaurs utilised
both forelimb and hind limb musculature to achieve take off and the computer
models generated for this research reflect this although, interestingly, 80 to
90% of the required take-off thrust for both birds and pterosaurs is developed
from the hind limbs.
![]() |
Anhanguera forelimb musculature - from Witton & Habib 2010 |
Taking other factors into account such as oscillation and
both launch speed and height indicates that it is problematic for pterosaurs to
achieve take off as they get bigger since they could not evolve a sufficient
muscular array that would be structurally efficient for take-off. The results
suggest that we can predict with confidence that pterosaurs with a 9 metre
wingspan could successfully take off but with a lesser degree of confidence
that those with wingspans between 9.5 and 11.5 metres could achieve launch.
Thus this research is indicative that 12 metres is currently
the absolute wingspan limit for pterosaurs – which we know of. This is really
interesting research and it would be fascinating now to see the response if
another pterosaur is found with perhaps a 14 or 15 metre wingspan. Unlikely of
course but that would really put the fat into the fire because who would not
want to know how that would have been achievable and what other factors would
need to be considered.
Theropods come in all shapes and sizes and we are familiar
with the various sizes and morphological differences in their teeth but what
about the actual mandible itself? Is theropod jaw form representative of
function or, indeed, is the taxanomic, morphological and functional diversity
of the lower jaw a predictor of functional and biomechanical diversity? Emily
Rayfield, of the University of Bristol, and her colleagues have been asking
these very questions.
A sample size of 103 specimens was analysed using a
combination of geometric morphometry and biomechanical metrics. The authors
point out that the sample size, although relatively broad, is still nowhere
near large enough but included a diverse group sampling which included, amongst
others, non-tetanuran theropods, non-maniraptoran tetanurans and
maniraptorformes themselves.
So do theropods that feed on different things have different
shaped jaws? Apparently not since the authors discerned no particular signal
from this analysis. What about functional traits in theropod jaws? The authors recognised 19 traits in 68 of the
taxa that are related to jaw robustibility and the enlargement of the coronoid
process which contribute to the overall shape and biomechanical variation. As a
side note it is worth pointing out the oviraptorosaurs, although included in
the analysis, are quite distinct from this overall grouping and were obviously
doing something very different.
Theropods that are herbivorous or exhibit omnivorous
tendencies display different shaped jaws to traditional carnivorous types which
is to be expected. Theropod mandibular evolution throughout the Mesozoic
suggests that there is likely to be a link between form and function and this
is supported by phylogenetics which does indeed exert a strong signal. However,
this is probably exaggerated by the fact that Maniraptora filled many different
ecological niches from the Late Jurassic onward which would have demanded more
morphological variation. The overall link, therefore, between morphological
change and functional diversity is
tenuous at best and suggests that perhaps the shape of the jaw does not always
necessarily reflect the evolved and/or derived state of different theropod jaw
mechanics.
References
Burch, S. 2014 Osteological, myological and phylogenetic
trends of forelimb reduction in nonavian theropods. Journal of Vertebrate
Paleontology, SVP Program and Abstracts Book, 2014, pp100.
Chatterjee, S. and Templin, R. 2004. Posture, locomotion,
and paleoecology of pterosaurs. Geological
Society of America Special
Publication 376: 1-64.
Palmer, C. & Habib, MB. 2014. All time giants of the
air: new approaches to calculating the limits to the size of pterosaurs. Journal
of Vertebrate Paleontology, SVP Program and Abstracts Book, 2014, pp200-201.
Rayfield, E., Conium, R., Benson, R. & Anderson, P. 2014.
Ecomorphological and functional variation in the theropod dinosaur mandible. Journal
of Vertebrate Paleontology, SVP Program and Abstracts Book, 2014, pp212.
Witton, M. P. 2008. A new approach to determining pterosaur
body mass and its implications for pterosaur flight. Zitteliana Reihe B 28:
143-158.
Witton MP, Habib MB (2010) On the Size and Flight Diversity
of Giant Pterosaurs, the Use of Birds as Pterosaur Analogues and Comments on
Pterosaur Flightlessness. PLoS ONE
5(11): e13982. doi:10.1371/journal.pone.0013982
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