Transylvanian Dinosaurs
Page 5
Although they never garnered much attention in the studies of paleo-ornithologists, nevertheless things were changing for Elopteryx, Bradycneme, and Heptasteornis. W. Pierce Brodkorb, of the University of Florida, was the first to cast doubt on the avian affinities of these forms in 1978, suggesting instead that they represented small nonavian theropods of uncertain affinity.40 Thereafter, many paleontologists followed Brodkorb’s lead in excluding these three Haţeg forms from Aves, suggesting instead that they were troodontids or dromaeosaurids, based on their size and the age of the beds from which they were recovered.41 We will question whether these assignments make sense, but first we will recount some exciting new theropod discoveries made in the 1990s.
The first time a portion of a theropod skull, a small one, was unearthed from a rock outcrop in the Sibişel Valley was in the early 1990s.42 The two elements of the skull roof (figure 2.13) suggested that we had something new among the Transylvanian dinosaurs. So we compared our new find with other theropods, proposing that the new Haţeg skull fragments came from a dromaeosaurid (we ended up calling it the “Romanian Raptor”), with its greatest similarity to Saurornitholestes lang-stoni, known from the Late Cretaceous of North America.43
Figure 2.12. The top end of the femur of Elopteryx nopcsai (top), the bottom end of the tibia of Bradycneme draculae (bottom left), and the bottom end of the tibia of Heptasteornis andrewsi (bottom right). Scale = 2 cm
A more recent analysis of all of this theropod material—as well as of abundant, newly collected specimens by Zoltán Csiki and Dan Grigorescu,44 and by Vlad Codrea, Thierry Smith, and coworkers45—has advanced Romanian dinosaur paleontology in two ways. First is an improved understanding of the fauna through the study of the new microfaunal collections—small teeth and bones—from several sites in the Haţeg Basin. From these, the Late Cretaceous fauna includes dromaeosaurids, a small troodontid theropod, and several curious small forms that are compared with the enigmatic Euronychodon, Richardoestesia, and Paronychodon, all known from elsewhere in Europe and North America at the end of the Cretaceous (figure 2.14). Second is a reevaluation of Elopteryx, Bradycneme, and Heptasteornis and their evolutionary relationships with other theropods. Although none of the Haţeg forms appears to be diagnostic at the species or generic level, Elopteryx nopcsai possesses features that suggest that its affinity lies within Maniraptora, the large theropod group that includes Deinonychus, Velociraptor, and birds. Elopteryx is clearly not a Late Cretaceous pelican, but a maniraptoran (possibly a dromaeosaurid,46 such as Velociraptor and Deinonychus, or a troodontid,47 like Saurornithoides). Nor are Bradycneme draculae and Heptasteornis andrewsi Late Cretaceous owls. Instead, B. draculae appears to be an indeterminate maniraptoran, and H. andrewsi is thought to be an indeterminate alvarezsaurid.
Figure 2.13. A reconstruction of the head of the Haţeg dromaeosaurid (top; restored after Dromaeosaurus), and a dromaeosaurid skull roof in dorsal view (bottom right), with a silhouette (bottom left) indicating its position in the skull. Scale = 3 cm
Figure 2.14. Theropod teeth from Transylvania: from a dromaeosaurid (left), a troodontid (middle), and Euronychodon (right). Scale = 3 mm. (After Csiki and Grigorescu 1998)
Thus far absent from the Haţeg assemblage is any evidence of a large dinosaurian predator, commonly at the top of the food chain in nearly all Mesozoic terrestrial fauna.48 The closest to this pinnacle is a single dorsal vertebra (5 cm in diameter) with pleurocoels, which represents no more than a medium-sized theropod.49 Instead, most theropod material from Transylvania consists of small teeth belonging to several kinds of small (2 m long), yet aggressive killers roaming the Haţeg region in the Late Cretaceous (Plate IV, top). Troodontids were also small predators, with long, slender, lightly built skulls that housed a large brain and very large eyes (positioned in such a way as to give these dinosaurs binocular vision) and was equipped with many small, but sharply recurved teeth. Alvarezsaurids, a group known principally from the Late Cretaceous of Mongolia, Argentina, and the United States, were small (about 1 m long), cursorial theropods with exceedingly diminutive, but robust fore-limbs and long, gracile hind limbs.50 The phylogenetic position of alva-rezsaurids within Theropoda is presently controversial: they have been placed within or just outside Avialae or as the sister group of Ornithomimosauria. Sauropods such as Magyarosaurus and other herbivorous inhabitants of the Transylvanian region surely would have feared an attack by any or all of these predators. The form and lifestyles of Euronychodon, Paronychodon, and Richardoestesia are much more problematic. We know almost nothing about any of these theropods beyond their small size and the details of their sharp teeth. Nor can we say much about their affinities. Euronychodon has been thought by some paleontologists to be a primitive ornithomimosaur, and by others to be an indeterminate coelurosaur or even an indeterminate theropod;51 Paronychodon may be a troodontid; and Richardoestesia, thus far, stands as a tetanuran of no known affinities. These are really only guesses, and there is little hope of understanding the evolutionary and ecological significance of these three small theropods without better material than we presently have.
The best known of all of these small theropods are the dromaeosau-rids, fast-running, bipedal predators with a large, sharply curved claw on each hind foot. The best known of the Transylvanian dromaeosaurids is the newly discovered Balaur bondoc (“stocky dragon” in archaic Romanian). B. bondoc presently consists of a partially complete, articulated skeleton from the Sebeş Formation near Sebeş (a second specimen is known from the Densuş-Ciula Formation at Tuştea).52 Unlike many other dromaeosaurids, this theropod is peculiar in having short forelimbs, highly fused hands and distal hind limbs, a very retroverted pubis, and dual claws capable of extreme hyperextension on each foot, presumably for grasping or disemboweling prey (figure 2.15). Similar in size to contemporary Laurasian dromaeosaurids, B. bondoc is presently the best known theropod from the Late Cretaceous of Europe. It is thought that dromaeosaurids in general, and perhaps B. bondoc in particular, may have hunted in packs, dispatching their prey by leaping upon them, raking them open with their deadly toe claws, and using their rigid tail to maintain balance.
Figure 2.15. Left lateral view of what is known of the skeleton of Balaur bondoc. Scale = 10 cm. (After Csiki et al. 2010)
The Nodosaurid Ankylosaur Struthiosaurus transylvanicus
Ankylosaurs are one of the two major groups of ornithischians with bony plates on their back (the other being Stegosauria), and nature lavished a full suit of armor on them.53 Ankylosaurs were probably experts in hunkering down in self-defense.
The first-discovered ankylosaur was Hylaeosaurus. One of the original members of Owen’s Dinosauria, it was not clear, at the time of its discovery in 1832, what this bizarre animal truly looked like.54 Nor was it much clearer to Franz Nopcsa when the armored dinosaur—Struthiosaurus transylvanicus—was first discovered in the Haţeg Basin in 1912. At that time, few good ankylosaur specimens had been uncovered anywhere in the world. It took several more years and many discoveries for paleontologists to conclude that these were lumbering quadrupeds covered by a shell-like armor of bony plates and spines across the neck, back, and tail. With these discoveries, paleontologists also determined that ankylosaurs are principally formed into two groups: nodosaurids, with their great shoulder spines, and club-tailed ankylosaurids (box 2.3).
BOX 2.3 Evolutionary Relationships in Ankylosauria
Ankylosauria was first recognized as a group in 1923 by Henry Fairfield Osborn of the American Museum of Natural History in New York. These lumbering, armor-bedecked quadrupeds are now known to consist of two groups, more or less equal in diversity (14 or so species each): Nodosauridae and Ankylosauridae. Recent phylogenetic research by Matt Vickaryous and his coworkers, and by Atilla Ősi and László Makádi, has provided the basis for understanding their evolutionary history.
Note: See Ősi and Makádi 2009; Vickaryous et al. 2004.
The story of Struthiosaurus is as intricate and complex as that of the Transy
lvanian theropods. It begins in 1871, when Emmanuel Bunzel, an Austrian physician and avid fossil collector, described new vertebrate material recovered from the Gosau Beds of Muthmannsdorf, near Vienna.55 One specimen consisted of a small braincase fragment that he thought was rather birdlike; Bunzel named the specimen Struthiosaurus austriacus (“Austria’s ostrich-reptile”). However, Harry Govier Seeley first identified the ankylosaurian nature of Struthiosaurus in 1881.56 Thereafter, when Nopcsa studied the ankylosaur material from the Haţeg Basin, he compared it closely with S. austriacus and the other ankylosaurs from Europe. On the basis of these studies, Nopcsa considered the Haţeg ankylosaur to be generally the same as Bunzel’s Struthiosaurus, yet sufficiently different to merit a new species designation, Struthiosaurus transylvanicus.57
S. transylvanicus is currently known from skull elements (the brain-case and portions of the skull roof and cheek region), as well as vertebrae, the shoulder girdle, and dermal armor belonging to at least two individuals collected from the Sânpetru Formation in the Haţeg Basin (figure 2.16). There is new material of S. transylvanicus, collected at Oarda de Jos and Vurpăr by researchers from Universitatea Babeş-Bolyai Cluj Napoca, which should provide additional information on the anatomy of this nodosaurid.58 All of these specimens are relatively small, but they are thought to be from adult individuals, suggesting that members of this species grew to not much more than 2 m in length. What we know about Struthiosaurus is due in large part to studies in the 1990s of S. austriacus by Xabier Pereda-Suberbiola, a Basque dinosaur paleontologist also working in Paris, and Peter M. Galton, a paleontologist and anatomist at the University of Bridgeport in Connecticut.59
The skull of S. transylvanicus is robustly built: most of the holes at the back of the head, through which the jaw muscles could bulge, are reduced or closed, the back surface of the skull is fused into a single unit, and the top of the head is covered with armor. Although their placement and appearance are yet unknown, armor plates also covered the back of the body, and a large spine projected from its shoulder. In addition, the outer surface of the shoulder girdle bears a prominent ridge (pseudo-acromion), where strong muscles were attached to move the large and muscular forelimb.
Paleontologists have relied on these and other features of Struthiosaurus to provide clues to its kinship and lifestyle (Plate IV, bottom). It is clearly a nodosaurid, based on the scapula having a pseudoacromion that is displaced downward and backward toward the shoulder joint and ends in a knoblike projection.60 Nopcsa thought that there may have been teeth in the front of the upper jaws (known as premaxillary teeth), unlike more derived nodosaurids such as Edmontonia and Panoplosaurus, whose premaxillary teeth are absent. If this is true, Struthiosaurus would assume a relatively primitive position in nodosaurid phylogeny.
Figure 2.16. The Transylvanian nodosaurid ankylosaur, Struthiosaurus transylvanicus (above), and a reconstruction of its head (below). (Struthiosaurus from an original plate from Nopcsa 1929a; restored head after Pawpawsaurus and Edmontonia)
Given its size and posture, we assume Struthiosaurus was a very low-browsing feeder, foraging no more than a meter above the ground. The relatively narrow and scoop-shaped front of the upper and lower jaws of other nodosaurids (but not yet known in Struthiosaurus) was probably overlaid with a horny covering (called a rhamphotheca), also seen in living turtles and birds, and suspected in many ornithischian dinosaurs. The shape of this region of the head suggests that these animals were somewhat selective feeders, plucking or biting at particular kinds of leaves and fruits with the sharp edge of the rhamphotheca.61
Food, once cropped, was apparently chewed through a combination of simple up-and-down puncturing and fore-and-aft grinding. Beyond this suggestion, which we deduce from the pattern of wear on the teeth, ankylosaur mastication is puzzling. For example, the triangular teeth of both nodosaurids and ankylosaurids do not appear particularly well suited to a diet of plants; they are small, not very elaborate, and less tightly packed together in the jaws than the teeth of other ornithischian dinosaurs. On the basis of these simple teeth, Nopcsa thought that Struthiosaurus instead fed on insects,62 although no one today doubts that its diet was dominated by plants. An extensive bony palate closed off the oral cavity from the nasal cavity, allowing these ankylosaurs to chew and breathe at the same time. Moreover, deeply inset tooth rows suggest the presence of deep cheek pouches to keep food from falling out of the mouth. The jaw bones themselves were relatively large and strong, although lacking enlarged areas for muscle attachment. Every jaw feature except the teeth suggests that ankylosaurs were adept chewers.
Perhaps the paradox of unsophisticated teeth set in strong, cheek-bound jaws can be understood by looking not at how ankylosaurs chewed food before swallowing, but at the other end of the animal, where much of the plant digestion must have been accomplished by gut fermentation. What the teeth couldn’t accomplish mechanically, the gut could, by breaking down the roughly chopped leaves by chemical means. A deep, broadly rounded rib cage circumscribing an enormously expanded abdominal region indicates that the digestive tract was huge.
Nopcsa’s interest in Struthiosaurus extended beyond how and what it ate to how it moved and how its small brain controlled the movement of its great mass. We know the brain size of this Transylvanian nodosaurid from a latex cast of the brain cavity that Nopcsa had made for his detailed study of the beast. Measuring less than 50 ml, the brain of Struthiosaurus was very small, even by dinosaur standards—only sauropods had smaller brains for their body size. There were no intellectual giants here, nor was athleticism their strong point. In addition, Struthiosaurus, like other ankylosaurs, were among the slowest moving of all dinosaurs. According to Australian paleontologist Tony Thulborn’s calculations,63 these armored dinosaurs walked at a leisurely pace of about 3 km/hour and probably ran no faster than 10 km/hour, about the average running speed of an elephant among living animals. Struthiosaurus was plodding, to be sure, but certainly not without defenses. It probably was able to stab at predators and competitors alike by firmly planting its hind limbs, ducking its head, and rolling its strong shoulders, with their formidably long spines, forward. Otherwise, Struthiosaurus hunkered down, relying on a suit of bony armor to protect it from packs of troodontid or dromaeosaurid theropods.
The Transylvanian Ornithopods: Zalmoxes
Zalmoxes, named for the Dacian god of the underworld and immortality who was famous in Romanian lore for living, teaching, and healing from his subterranean crypt, was one of two sorts of Transylvanian ornithopods. Our account of Zalmoxes and how it fits into the ornithopod scheme of things begins in 1897, when Nopcsa first recognized some distinctive bones and teeth recovered from localities along the banks of the Sibişel River.64 These he called Mochlodon, because of their similarity to ornithopod specimens from the Gosau Beds of Austria (the same locality that yielded the bones of Struthiosaurus austriacus) that Seeley had named Mochlodon in 1883.65 Rhabdodon priscus, another Late Cretaceous dinosaur from southern France originally described in 1869 by Philippe Matheron, a geologist from Marseille,66 was also similar to the Haţeg Mochlodon material, but Nopcsa initially regarded them as different kinds of ornithopods. By 1915, however, he was considering the possibility that the two were from the same species, perhaps being male and female. Since Rhabdodon was the earlier find, Nopcsa conceded Matheron’s claim of priority and renamed the Mochlodon material from Transylvania Rhabdodon, for Matheron’s animal.67
Once Rhabdodon was properly named to Nopcsa’s satisfaction, where did it belong on the ornithopod family tree? Nopcsa was the first to advocate particular affinities for this Haţeg ornithopod in 1902. Comparing it with other ornithopods, he noted that Rhabdodon seemed to be more primitive than Iguanodon (from the Early Cretaceous of Europe), but more closely related to Camptosaurus (from the Late Jurassic of North America). Thereafter, Rhabdodon was either included in a group that contained Iguanodon (either Iguanodontidae or the more encompassing Iguanodontia) or shifted to the group of small
ornithopods called Hypsilophodontidae.68
Figure 2.17. Skull elements of Zalmoxes, one of the Transylvanian ornithopods (left) and a reconstruction of its head (right). (Skull elements from an original plate from Nopcsa 1904)
Although Nopcsa regarded the proper parentage of Rhabdodon as solved, from the perspectives of both taxonomy and evolutionary relationship, we were far from convinced when we started examining the remains of this beast. So, working closely with Zoltán Csiki (Universitatea din Bucureşti) and David Norman (Sedgwick Museum at Cambridge University), we tried to bring the animal Nopcsa ended up calling Rhabdodon (figure 2.17) into its present-day context. We combed through dusty museum drawers across Europe, trying to compare our Romanian specimens with material of Rhabdodon from France and Spain, and Mochlodon from Austria, as well as with other specimens from England and North America (Camptosaurus, Hypsilophodon, and Tenontosaurus, among others), integrating this trove of data using cladistic analyses (box 2.4).
Here’s what we discovered during the course of our research. The specimens from France and Spain that have been referred to as Rhabdodon probably all belong to the same genus and species, a conclusion that was confirmed by material and analyses by Marie Pincemaille and her colleagues at Université Montpellier in the late 1990s (figure 2.18).69 This creature naturally retains the name proposed in 1869 by Matheron, Rhabdodon priscus. However, our work also identified two species from Transylvania, both of which are now known from various localities in the Haţeg Basin and several other locales—Vurpăr, Oarda de Jos, Jibou, Bărăbanţ, Lancrăm, Sebeş—in the Transylvanian Depression.70 In fact, the two Transylvanian species appear more closely related to each other than either is to Rhabdodon from France and Spain. We considered this a prime reason for coining a new name, and in 2003 we settled on Zalmoxes. The two species were called Zalmoxes robustus and Zalmoxes shqiperorum (“shqiperorum” honoring Nopcsa’s love of the northern Albanian tribes).71 Of the two, Z. robustus is quite well known, based on upwards of 300 specimens, whereas Z. shqiperorum is known from a nearly complete skeleton from Nălaţ-Vad, as well as a few dozen bones and teeth from elsewhere in the Haţeg Basin. It is further known from the Şard Formation at Vinţu de Jos on the southern margin of the Transylvanian Depression,72 and from the Jibou Formation near Jibou along its northern margin.73