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Until recently, it seemed like the only controversy
surrounding the genus Psittacosaurus, was the question whether the 8 valid
species of this genus should be split into 2 or more genera. Now it looks like
psittacosaurids have a few more surprises in store.
About a week ago, I received a shipment of newly published
books that included "Dinosaurs of the Isle of Wight" (Martill & Naish) and
"Extreme Dinosaurs" (Luis Rey). Both books contains images of Psittacosaurus
with "hair-like" structures (mainly) on the dorsal side of the tail. Since
recent psittacosaur discoveries include skeletons associated with gastroliths,
it is also suggested that they may have been omnivorous. You can find the images
I'm referring to on; page 136 of "Dinosaurs of the Isle of Wight" (as part of a
cladogram) and on page 53 of "Extreme Dinosaurs" (as part of a painting that
contains both Psittacosaurus and Byronosaurus individuals).
On the interpretation of the gastroliths;
This is not the first time that a non-theropod genus is
thought to be not exclusively herbivorous;
1. pachycephalosaurs have been considered as omnivores, since
their teeth are almost identical to troodontid teeth (the latter are certainly
meat-eaters, and even possible omnivores).
2. heterodontosaurids, having a complex array of
teeth
3. some basal sauropodomorphs ("prosauropods") were probably
omnivorous.
Previous discussions on omnivorous diets in non-theropod
dinosaurs were in most cases based on tooth morphology, the inclusion of
gastroliths in this debate could be important.
On the other hand, there are several other dinosaur genera
that have remains associated with gastroliths;
1. Caudipteryx, an advanced maniraptoran
2. an unnamed ornithomimid (known from 12 skeletons,
China)
3. several neosauropods (Cedarosaurus, Dinheirosaurus,
Lourinhanosaurus, Sonorasaurus and possibly Seismosaurus)
4. the "prosauropods" Massospondylus and
Sellosaurus
5. there are even reports that the ankylosaurian Panoplosaurus
had gastroliths.
Caudipteryx and ornithomimids were almost certainly true
omnivores (feeding on small vertebrates, insects, but also leaves and fruit). It
is difficult to believe that the neosauropods and ankylosaurian listed above ate
anything other than plant material.
My conclusion is that gastroliths are not necessarily an
indication for an omnivorous diet. Gastroliths do, however, help to digest tough
material (thick branches and leaves, but also complex muscle tissue). So
Caudipteryx and ornithomimids (both had beaks and no slicing teeth) may have
used the gastroliths to digest the animal and plant material they ate. The
neosauropods, ankylosaurs and possibly psittacosaurs may have used the
gastroliths to digest tough plant material. It is not yet clear in which
category the prosauropods belonged.
I would like to see conclusive evidence for the reported
association of bones in psittacosaur stomach region (as mentioned in L. Rey's
book).
On the psittacosaur integument;
I presume that these structures were not closely comparable to
true feathers (images of Syntarsus and Eoraptor with feathers are already
difficult to comprehend, a feathered marginocephalian would be even more
controversial).
Let's assume that these integumentary structures were at least
keratinous, what could have been their function ?
1. unlikely for regulating body temperature, since the
integument only covered the tail.
2. possibly for display, but this is always a tempting
explanation for an indeterminate external feature
3. it's quite possible that psittacosaurs were highly
specialized mesozoic hedgehogs, if so, they could use their tail to defend
themselves against predators. BTW, all extant mammalian hedgehogs are
omnivorous, which brings us again to the first topic of this message, the
gastroliths.
On a related matter; why did psittacosaurs have such a short
temporal (Aptian to Albian) and geographic range (Mongolia & northern China)
? Were they outclassed as dominant omnivores by ornithomimids and troodontids
(fast runners and more intelligent).
Since this new image of psittacosaurs has now been published
in at least two books, I couldn't resist to share these thoughts with you. It
appears that several workers will soon publish on this discovery.
The fossils on which this discovery is based are very likely
from the Yixian Formation that yielded all those spectacular feathered theropod
specimens. The Yixian Fmt is the only currently quarried location that allowed
the preservation of these delicate integumentary structures. Psittacosaur
remains were already known from the early Cretaceous Liaoning area (P.
meileyingensis). If I recall correctly, another new non-theropod Yixian specimen
(?a basal ornithopod) with integumentary structures was reported a while ago on
the DML. The fact that only theropods were found during the initial years of
quarrying, gave us sufficient evidence to prove that at least several non-avian
theropods had feathers, but it also meant that our view of scaly non-theropods
continued to exist.
With both maniraptorans (neornitheans, deinonychosaurs,
compsognathids, therizinosaurs ...) and psittacosaurs having keratinous
integumentary structures, this could mean that these structures are basal for
Dinosauria and even Ornithodira. There are indeed few non-avian dinosaur clades
that are more distantly related to each other than psittacosaurs and
maniraptorans, in fact this compares closely to the definition of Dinosauria
"all descendants of the most recent common ancestor of Triceratops and
Neornithes", although I prefer the alternative (and in practice identical)
definition "all descendants of the most recent common ancestor of Iguanodon and
Megalosaurus", because the latter honours the historically important efforts of
the British dinosaur pioneers.
If keratinous integument was basal for Ornithodira, then the
following could be applicable;
1. the primitive condition of this integument became hair-like
and formed a kind of "fur" in pterosaurs (cf. Sordes)
2. the primitive condition of the integument was retained with
no or few modifications in very basal theropods and small-sized basal
ornithischians. The basal ceratopian psittacosaurs modified the integument into
the structures now known from their tails.
3. the primitive integument was quickly replaced by armour in
thyreophorans
4. the primitive integument was soon lost in Sauropodomorpha,
which had no need for such a heat-regulating structure, their size increased
steadily.
5. all large dinosaurs (sauropodomorphs, thyreophorans,
neoceratopians, euornithopodans and large non-coelurosaurian theropods) were
"secondarilly integument-less", and had reverted to a scaly skin
(non-overlapping scales, often with tubercles).
6. gradually evolved into feathery integument within
Neotheropoda, and eventually into true feathers (Maniraptora, Eumaniraptora).
The relatively small to medium-sized coelurosaurs continued to be covered by a
variant of the integument, but the larger tyrannosauroids also reverted to
a scaly skin, but with a more smooth, leather-like structure than other large
(more basal) theropods.
Conclusion : the possession of keratinous integument was truly
basal for Ornithodira and had initially evolved only for heat-regulating
purposes. Some smaller theropods were able to evolve the integument into complex
feathers that were eventually used for powered flight, most other dinosaurs
increased in size and soon lost the integument or only retained
remnants. If this basal nature of the integument proves to be correct,
this would mean that all dinosaurs were endothermic. The fully erect position of
the dinosaur hindlimbs was (as far as I am concerned) the first indication
of a possible endothermy of Dinosauria.
Regards,
Gunter Van Acker
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