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.
Gunter Van Acker