A story of cheeks, beaks, feathers, bizarre theropod dinosaurs, and truly, truly amazing fossils….
Yesterday I made a special visit to the University of Portsmouth’s School of Earth and Environmental Sciences in order to attend a talk by, and meet, Professor Altangerel Perle, the famous Mongolian palaeontologist and finder of awesome Cretaceous dinosaur fossils. From the 1970s onwards, Perle has personally excavated and described such incredible fossils as the fighting Velociraptor and Protoceratops, the alvarezsaurid Mononykus, the unusual giant dromaeosaurid Achillobator, and the therizinosauroids Erlikosaurus* and Segnosaurus. We spoke about Deinocheirus (no new fossils as of yet), I showed him a new taxon of Brazilian theropod and, I am ashamed to say, I even asked him about the Mongolian death worm. He had heard of it, but seemed pretty confident that it was mythical. Perle (this is not pronounced ‘pearl’ as I had imagined, but more like ‘per-lay’) can speak English (and Russian) but requires assistance these days, in part because of a stroke he suffered within recent years. He was accompanied by Batbayar, a Mongolian helper and translator, and by two representatives of the Scientific Exploration Society (SES), the group whom Perle has led into the Mongolian wilds in quest of fossils, flora and fauna. We were invited to come along with them on their next Mongolian expedition. I’d love to, of course, if only I had a spare few thousand pounds.
* Erlikosaurus was later spelt Erlicosaurus in several publications. The former spelling, however, is older (Barsbold & Perle 1980) and thus has priority.
All of this was memorable enough. However… Perle brought along some fossils with him. Real fossils. The ACTUAL HOLOTYPES of the therizinosauroids Erlikosaurus andrewsi and Segnosaurus galbinensis. Oh. My. God. I could not believe it. As you can see from the accompanying photos, we spent a lot of time examining, holding (carefully!!) and manipulating (very carefully!!) these stunning, three-dimensional remains (‘we’ = me, Richard Hing, Mark Witton and [much later in the day] Graeme Elliott). In the adjacent image, I’m holding the jaw of Segnosaurus, with Perle and Batbayar behind me (Perle is on the left). In the photo at the top of the page, Mark and Graeme are – carefully – getting the jaw of Erlikosaurus into articulation. To see and hold specimens that you ‘know’ from the literature is, for me, tremendously emotional, almost disturbingly so. It’s like meeting a famous person that you admire or idolize. From a more nerdy point of view, finally you have the chance to check the tiny details that you’ve always been curious about, or see for yourself the bits and pieces that people have argued about or interpreted differently.
Therizinosauroids are far better known nowadays that they were in the late 1970s when Perle first discovered them, of course. In recent years we’ve seen the description of the small basal form Beipiaosaurus from the Yixian Formation of Liaoning Province, of Alxasaurus from the Bayin-Gobi Formation, and of the North American taxa Nothronychus from the Moreno Hill Formation of New Mexico and Falcarius from the Cedar Mountain Formation of Utah*. Several new Asian taxa, including Erliansaurus and Neimongosaurus from the Iren Dabasu Formation, have also been described. These specimens and others have added considerably to our understanding of therizinosauroid anatomy and evolution. Therizinosauroids were of uncertain affinity during the 1970s and 80s but new work and new data has shown that they are not close relatives of ornithischians or plateosaurs as formerly proposed, but are in fact highly peculiar maniraptoran theropods close to oviraptorosaurs [adjacent image of Jim Kirkland and Falcarius skeleton from here].
* On the subject of American therizinosauroids, Perle told me that John Ostrom had once showed him a manual phalanx from the Morrison Formation that seemed to be from one of these dinosaurs. I can’t recall reading about such a record, but given that the Cedar Mountain Formation (source of Falcarius) overlies the Morrison Formation in places, I wonder if the provenance data was accurate.
Therizinosauroids were long-necked, and their elongate cervical vertebrae share a number of unusual features with the cervical vertebrae of oviraptorosaurs. They had long arms with powerfully muscled humeri and long, tridactyl hands sporting long, laterally compressed and weakly curved claws. In the biggest member of the group, Therizinosaurus, some of the hand claws were over 60 cm long, and don’t forget that this is the length of the bony claw; not of its attached keratin sheath. The pelvis is wide, with the anterior parts of the iliac blades flaring outwards to support a massive gut, and the pubic bones are rotated backwards, and not projecting forwards as is typical for saurischian dinosaurs. Their feet are remarkably short and stocky and highly unusual in that the first digit – the hallux – was enlarged and fully in contact with the ground, making the foot functionally tetradactyl (and thus unlike the bird-like functionally tridactyl foot of other theropods). Their hindlimb proportions (where the tibia is nearly equal in length to the femur) also indicate that they were poor runners. The tail was short (though read on), with the nearly complete tail of Neimongosaurus consisting of just 23 vertebrae.
Overall then we have a picture of a remarkably odd group of long-necked, broad-bellied, short-tailed, stocky-legged dinosaurs with long hand claws, and short, tetradactyl feet. Combined, these features suggest that they were relatively slow moving, and probably herbivorous, relying on hindgut fermentation. This idea is supported by skull anatomy, as we’ll see in a moment. Falcarius was superficially different from later therizinosauroids in having a vertical pubis and in being long-tailed and with a more typical functionally tridactyl foot (Kirkland et al. 2005). Overall it would have looked more gracile than later, more specialized therizinosauroid taxa. Incidentally, Falcarius is of special relevance to me as it was initially claimed to be the same animal as Thecocoelurus, an enigmatic theropod from the Isle of Wight that Dave Martill and I interpreted as an oviraptorosaur (Naish & Martill 2002). The two are not the same by the way, nor is Thecocoelurus a therizinosauroid.
Therizinosauroids were generally known as segnosaurs in the older literature, and an unfortunate complication for the group’s taxonomy is that Clark et al.’s (2004) phylogenetic definition for the group defines it as ‘the least inclusive clade containing Therizinosaurus and Beipiaosaurus‘ (p. 153). If this definition is followed, then Falcarius is not a member of Therizinosauroidea, nor are any other taxa more basal than Beipiaosaurus (a Lower Jurassic lower jaw, named Eshanosaurus deguchiianus, has been regarded as the oldest known therizinosauroid but is more likely a misidentified plateosaur mandible).
The Erlikosaurus skull that I and Graeme are posing with in the adjacent photo is one of the best therizinosauroid specimens of them all (I don’t normally put my arm round Graeme, but it was a special occasion). Its preservation is just amazing and, while broken in places, even the thinnest-boned bits of the skull are complete or nearly so. First described, in Russian, by Perle (1981), it was later redescribed in full detail by Clark et al. (1994). The nostrils are huge and the sharp-edged toothless premaxillae form a broad, rounded beak that would almost certainly have been covered in beak tissue in life. Clark et al. (1994, p. 6) described bony denticulations along the beak margin near the midline, similar to those present in oviraptorosaurus. There are tiny projections in this region, but I couldn’t convince myself that they were real denticulations and not the result of breakage. Teeth line the maxillae; the teeth are lanceolate, transversely compressed and with serrated crown edges.
The lower jaws form a U-like shape when articulated and teeth extend much further rostrally than they do in the cranium, with only those parts of the dentaries on either side of the symphysis being edentulous. Atypically for a theropod, the teeth in both the upper and lower jaws are inset medially and low ridges run parallel to the tooth rows on the outside surfaces of the jaws. Paul (1984) argued that these inset tooth rows and lateral horizontal ridges demonstrated the presence of cheeks flanking the sides of the mouth. In recent years Larry Witmer and colleagues have argued that these features don’t necessarily indicate the presence of cheeks, but might instead show that dinosaurs equipped with these features had beak tissue lining the sides of their jaws, in which case (1) they looked tremendously freaky and (2) were unable to retain food in their mouths [adjacent image of cheekless Leptoceratops from here]. The notion of cheeked therizinosauroids hasn’t been discussed in the literature since Paul’s 1984 paper, in part because new data confirms that therizinosauroids are coelurosaurian theropods, a clade in which cheeks have otherwise been unsuspected. However, Greg Paul has pointed out (though not in print so far as I know) that cheeks are sometimes present in birds, specifically in Californian condors Gymnogyps californianus [image below from here]. Therizinosauroids may therefore have possessed both cheeks and a beak, though this remains speculative.
The toothless beak, lanceolate teeth and possible evidence for cheeks all agrees with evidence from the postcranial skeleton that therizinosauroids were predominantly herbivorous, perhaps supplementing their diet with fungi and animal material on occasion. It has been suggested that these dinosaurs caught fish, dug up ants and termites or harvested wasp nests, but none of these ideas are supported by morphological details.
Perle also brought along the holotype lower jaws of Segnosaurus galbinensis, part of GIN 100/80: a specimen that includes fore- and hindlimb bones, fragmentary vertebrae and a pelvis (Perle 1979). This animal is bigger and altogether different from Erlikosaurus in having a lower tooth count, differently shaped teeth, a far more strongly decurved lower jaw tip and other differences. Furthermore, a little bit more of Segnosaurus is known than usually reported, as Perle brought along a fragment that I’ve never seen reported in the literature: a Segnosaurus basioccipital. Essentially identical to the basioccipital of Erlikosaurus, it was nevertheless substantially larger.
Based on their position within coelurosaur phylogeny – they are surrounded by oviraptorosaurs, birds and deinonychosaurs (all of which are known to possess indisputable vaned feathers on their limbs and tails) – we would predict that therizinosauroids were feathered. Little Beipiaosaurus preserves filamentous integumentary structures on its body and limbs but isn’t well-preserved enough to confirm that remiges and rectrices were present, though – again, based on related taxa – they probably were. The presence of a pygostyle-like structure in Beipiaosaurus (Xu et al. 2003) strongly suggests that it sported a rectricial fan of feathers, as did members of various other maniraptoran clades. Does this mean that we should imagine all therizinosauroids as feathered, even the giants like Therizinosaurus? In the absence of evidence to the contrary I would say yes, no matter how outlandish the concept of a feathered behemoth weighing several tons might seem to be. It is possible though that such big forms retained, say, remiges and rectrices (plausibly used for visual display), but lost the integumentary covering on the body [in the adjacent image, Mark holds court while a throng of enthralled students, and palaeobotanist Bob Loveridge, look on. Yes, that's Mark Witton, the pterosaur worker].
But would there really have been selective pressures encouraging the loss of a feathery integument in therizinosauroids? While people generally imagine the Mesozoic world to have been a thermally stable hothouse where big insulated animals would have faced meltdown, we’re actually not sure that this was true and in fact some studies indicate otherwise. Particularly intriguing is that Cretaceous continental interiors such as those of Mongolia may not have been that warm, and apparently subjected to cold winters with strong winds (Barron & Washington 1982, Sloan & Barron 1990, Sellwood et al. 1994).
Clearly, these perpetually interesting bizarre dinosaurs leave us with a lot of unanswered questions. It was a great day, a great honour to meet Prof. Perle, and we had a great time in getting to see such truly extraordinary fossils.
Refs – –
Barron, E. J. & Washington, W. M. 1982. Cretaceous climate: a comparison of atmospheric simulations with the geological record. Palaeogeography, Palaeoclimatology, Palaeoecology 40, 103-133.
Barsbold, R. & Perle, A. 1980. Segnosauria, a new infraorder of carnivorous dinosaurs. Acta Palaeontologica Polonica 25, 187-195.
Clark, J. M., Perle, A. & Norell, M. A. 1994. The skull of Erlicosaurus andrewsi, a Late Cretaceous “segnosaur” (Theropoda: Therizinosauridae) from Mongolia. American Museum Novitates 3115, 1-39.
– ., Maryanska, T. & Barsbold, R. 2004. Therizinosauroidea. In Weishampel, D. B., Dodson, P. & Osmolska, H. (eds) The Dinosauria, Second Edition. University of California Press (Berkeley), pp. 151-164.
Kirkland, J. I., Zanno, L. E., Sampson, S. D., Clark, J. M. & DeBlieux, D. D. 2005. A primitive therizinosauroid dinosaur from the Early Cretaceous of Utah. Nature 435, 84-87.
Naish, D. & Martill, D. M. 2002. A reappraisal of Thecocoelurus daviesi (Dinosauria: Theropoda) from the Early Cretaceous of the Isle of Wight. Proceedings of the Geologists’ Association 113, 23-30.
Paul, G. S. 1984. The segnosaurian dinosaurs: relics of the prosauropod-ornithischian transition? Journal of Vertebrate Paleontology 4, 507-515.
Perle, A. 1979. Segnosauridae – a new family of theropods from the Late Cretaceous of Mongolia. Sovmestnaya Sovetsko-Mongol’skaya Paleontologicheskaya Ekspiditsiya, Trudy 8, 45-55 [in Russian].
– . 1981. A new segnosaurid from the Upper Cretaceous of Mongolia. Sovmestnaya Sovetsko-Mongol’skaya Paleontologicheskaya Ekspiditsiya, Trudy 8, 50-59 [in Russian].
Sellwood, B. W., Price, G. D. & Valdes, P. J. 1994. Cooler estimates of Cretaceous temperatures. Nature 370, 453-455.
Sloan, L. C. & Barron, E. J. 1990. Equable climates during Earth history. Geology 18, 489-492.
Xu, X., Cheng, C., Wang, X. & Chang, C. 2003. Pygostyle-like structure from Beipiaosaurus (Theropoda, Therizinosauroidea) from the Lower Cretaceous Yixian Formation of Liaoning, China. Acta Geologica Sinica 77, 294-298.