Introduction
At the end of the 19 th century, the famous paleontologist
Ferenc (Franz) Nopcsa von Felső-Szilvás found a proximal
fragment of a femur belonging to a small theropod dinosaur,
nearby Sânpetru locality in the Hațeg Basin (Andrews 1913).
This fossil, along with a multitude of other Cretaceous
vertebrate remains discovered from this basin, have been
purchased by the British Museum of Natural History. The
British paleontologist Charles William Andrews received
some limb bone fragments from Nopcsa, which the latter did
not consider to be of “reptilian but rather
avian in origin”
(Andrews 1913). Andrews (1913) assigned the femur
fragment to a new species named
Elopteryx nopcsai. Since the
initial description, more fossil material, especially femur
fragments, has been discovered and assigned to this genus
(Lambrecht 1929, 1933, Grigorescu & Kessler 1980, Kessler et
al. 2005).
A fossil related to this genus has been discovered near the
Nălaț-Vad locality (Smith et al. 2002, 2022) by paleontologists
at Babeș-Bolyai University of Cluj-Napoca. It is the proximal
fragment of a right femur, which shares many characteristics
with the
Elopteryx nopcsai holotype.
Systematic paleontology
Dinosauria Owen, 1842
Saurischia Seeley, 1888
Theropoda Marsh, 1881
Maniraptora Gauthier, 1986
Paraves Sereno, 1997
Avialae Gauthier, 1986
Elopteryx Andrews, 1913
Elopteryx nopcsai Andrews, 1913
Comparative Description
BBU-PSM V1009 is not as well preserved as the holotype (Fig. 2). The great trochanter is fragmentary (Fig. 2. A, D), and the circular fossa present on the femoral head of the holotype (Andrews 1913) is razed, as a consequence of its taphonomy and post-taphonomy events (Fig. 2. C, F). In both BBU-PSM V1009 and the holotype, the femoral head is well-developed and stands higher than the great trochanter (Fig. 2. A, D). In the proximal view, both specimens share a great trochanter of a slightly triangular outline, the anterior angle being the most acute (Fig. 2. B, E). The posterior angle is defined by the insertion of the obturator muscles (Andrews, 1913), taking the shape of a circular fossa in both the holotype and the new specimen (Fig. 2. B, C, E, F). In the medial view, a concave surface is present under the great trochanter and bears a small crest on the posterior side, indicating the emergence of the linea aspera, a feature present in both specimens (Figs. 2. C, F, 3. B). In UBB-PSM V1009, a second crest, which is slightly more prominent, is present on the very bottom of the medial surface, but it is likely to represent an artifact of preservation (Fig. 3. B). Ultimately, this fossil shares the general outline of the holotype, possessing diagnostic features described in Andrews (1913), enough to relate it to the Elopteryx nopcsai species. Furthermore, in cross-section, the reduction of the medullary cavity and the thickening of the cortical bone is obvious (Fig. 3. F). A thin, intermediate layer, marked by small holes surrounding the medullary cavity, is present, which could be indicative of the vascular canals of the medullary bone (Fig. 3. F). On the cranial portion, this intermediate layer has been completely replaced by a thickened cortical bone. The medullary cavity consists of two hemispheres that vary greatly in size and definition (Fig. 3. F). A similar configuration of the medullary cavity is also found in Pengornithidae (the cross-section of the femur IVPP-V15576 in O’Connor et al. 2018). In this specimen, the medullary cavity, whose hemispheres are similar in size and shape, is wider in proportion to the rest of the bone. It is also worth noting that the cortical bone in IVPP-V15576 is proportionally thinner than in BBU-PSM V1009. Acknowledging the fact that the bones of flightless birds generally possess thicker, more reinforced walls and less developed medullary cavities than those of flighted birds (Currey & Alexander 1985, Habib & Ruff 2008) and that Pengornithids were adapted for flight, the atrophied state of the medullary cavity and reinforcement of the cortical bone in E. nopcsai suggests that it was a secondarily flightless taxon while adapting to an exclusively terrestrial lifestyle. Throughout the evolution of non-avian theropods, stability during locomotion was mainly maintained by a long, counter-balancing tail (Pittman et al. 2013). In the absence of a counter-balancing tail, avialans, especially the members of the Pygostylia clade, possess a very pronounced great trochanter, which serves as an attachment point for powerful iliotrocantericus muscles, which keep the femur sloped in a cranial position and bring the birds center of gravity forward, thus aiding in maintaining the balance of the organism (Gatesy 1999, Clifton et al. 2017). Pygostylians also have a very underdeveloped femoral head, breaking the trend seen in non-avian theropods, which have a very pronounced femoral head (Egawa et al. 2022) that stands taller than a poorly developed or absent great trochanter since these animals mostly relied on their tails for stability. Having a great trochanter, which is relatively comparable in height to the femoral head, the configuration seen in Elopteryx stands between what is seen in pygostylians and most non-avian theropods. A similar configuration to that seen in Elopteryx is also found in the Oviraptorosauria clade. Members of this clade, such as Anomalipes zhaoi (ZCDM V0020, Yu et al. 2018) and Nankangia sp. (GMNH F1000, Lü et al. 2013), have a very pronounced great trochanter, which is comparable to the femoral head in height (standing slightly taller). In proximal view, however, the great trochanter in Oviraptorosauria (Anomalipes zhaoi specimen ZCDM V0020, Yu et al. 2018) does not expose the same triangular outline as in Elopteryx, the posterior angle being especially underdeveloped, which indicates the likelihood of these similarities being convergent traits. Acknowledging the fact that oviraptorosaurs were also known for having unusually short tails, an intermediary between pygostylians and other non-avian theropods (Persons et al. 2013), this hints at a possibility that Elopteryx had a tail of similar length as oviraptorosaurs, an assumption which can only be confirmed following the discovery of more complete fossil material.
History and Classification of Elopteryx
Andrews (1913) erected the new genus
Elopteryx, based on a femur fragment and a tibiotarsus fragment discovered near the Sânpetru locality. These fragments have been initially compared by Andrews to the bones of Pelecaniformes. Showing very few similarities with the specified bones, Andrews suggests that
Elopteryx had a more terrestrial lifestyle, and, despite these discrepancies, the British paleontologist still classified
Elopteryx in the Pelecaniformes clade. The initial interpretation of Elopteryx as a terrestrial Pelecaniform remained undisputed until the discovery of a better-preserved femur fragment (Lambrecht 1933) that sported multiple similarities with paravian theropods (Csiki & Grigorescu 1998). In more recent papers, the systematics of Elopteryx has become more unclear. Osmólska & Barsbold (1990) placed
Elopteryx, alongside multiple other Romanian genera known from fragmentary remains (i.e.,
Heptasteornis and
Bradycneme), in the Troodontidae clade. This genus
Elopteryx becomes a great subject of debate. Kessler et al. (2005) redescribed the fossil material attributed to
Elopteryx and spotted some features seen in characteristics of Alvarezsauridae, but no conclusion is taken due to the fragmentary nature of the remains.
Csiki et al. (2010) have published a thorough description of one relatively well-preserved specimen collected near the Sebeș-Glod locality from Early Maastrichtian aged rocks, introducing the new taxon name
Balaur bondoc. The thoracic and caudal vertebrae, one shoulder girdle, one pelvis, and multiple leg bones (paradoxically, both femurs - left and right - are missing) have been discovered. By far, the most striking feature of this animal are the very large first and second toe on each foot, adorned with hypertrophied, hook-like talons, the main reason
Balaur bondoc was initially classified as a dromaeosaurid. In a more recent paper by Cau et al. (2015), multiple features uncharacteristic of the Dromaeosauridae, such as the atrophied third finger, the fused metatarsus, and scapulocoracoid and the co-ossification of the pelvic bones, were identified in Balaur. These features are very unusual for most theropods but prove to be common in Avialae, especially in the basal members. Thus, Balaur bondoc is reclassified as a basal member of Avialae, being placed outside of Pygostylia. Under this new interpretation, even the distinct foot morphology makes more sense, as the hypertrophied first toe is characteristic of Avialae. More features identified by Cau et al. (2015), such as the atrophic condition of the hand articulation, a very wide and spacious pelvic canal, and a robust metatarsus, clash with the initial image of Balaur as an active predator, as these features are only found in herbivorous animals. Apart from being used in restraining their prey (Fowler et al. 2011), it is likely that the hypertrophied recurved talons characteristic of dromaeosaurids would also aid these animals in climbing (Manning et al. 2006), which is why Cau et al. (2015) suggested that the unusual foot morphology characteristic of Balaur bondoc, especially its very dromaeosaur-like sickle claws, were likely an adaptation to a partially arboreal lifestyle, assisting the relatively robust animal while climbing. It is also worth mentioning that Cau et al. (2015) point out that the articular joints of the hallux would have made this digit highly mobile, an adaptation that could have further aided
Balaur in climbing and perhaps a rudimentary form of perching.
Buffetaut & Le Loeuff (1998) reported a large pelvic bone and a proximal femur fragment from some Maastrichtian rocks of southern France and coined a new genus,
Gargantuavis. The pelvis has a very pronounced antitrochanter, and the acetabulum is placed next to the fourth sacral vertebra. Moreover, the pelvis is notably wide and unlike what is seen in semi-aquatic flightless Mesozoic birds such as
Hesperornis, which suggests
Gargantuavis had a terrestrial lifestyle. The proximal femur fragment, which is poorly preserved, has a rounded trochanteric crest, similar to what is seen in modern Palaeognathae (Buffetaut & Le Loeuff 1998). The clear absence of the fourth trochanter from the femur, precisely the attachment of the caudofemoralis muscles, suggests this animal lacked a bony tail, the main reason for its initial placement within the Pygostylia clade. Following these observations, Buffetaut & Le Loeuff (1998) suggest that
Gargantuavis is likely the largest Mesozoic avialan for which we have concrete fossil evidence. The subsequent unearthing of a second, more well-preserved femur (Buffetaut et al. 2019) only backs the initial classification of
Gargantuavis as a Pygostylian.
Mayr et al. (2020a) have described a pelvis bone from Nălaț-Vad locality. The specimen, UBB-V649, bears many similarities with the pelvis reported from southern France. The small discrepancies between the two bones, such as the smaller size of UBB-V649 and its placement of the acetabulum next to the fifth sacral vertebra, can be explained by the fact that these two specimens were discovered in relatively distant geological formations and from rocks of different ages. An intriguing aspect covered in Mayr et al. (2020a) is the fact the femur fragments that were attributed to Elopteryx fit very nicely in the UBB-V649 acetabulum, suggesting the Nălaț-Vad pelvis could also belong to this genus. Mayr et al. (2020a) also point out that the femur fragments discovered in southern France were erroneously attributed to the genus Gargantuavis due to the isolated and unarticulated nature of the fragments.
Initially, mostly due to the lack of corresponding material, the synonymy between
Elopteryx and
Balaur was revoked (Brusatte et al. 2013). However, Mayr et al. (2020a) highlight multiple similarities between UBB-V649 and the pelvis attributed to the genus
Balaur (specimen EME.VP313), such as the pronounced antitrochanter and the same number of sacral vertebrae. Although the discovery of UBB-V649 did not end this debate, it is undoubtedly a crucial piece of evidence that highlights the very possible synonymy between
Elopteryx,
Balaur and this fossil, but a conclusion could be based only on more better-preserved fossils.
Fred
Figure 1. Comparison between the specimen described by Andrews (1913) (A, B, C; flipped illustrations) and specimen BBU-PSM V1009 (D, E, F). tr.- great trochanter, l.t.- ligamentum teres insertion hole, l.a.- linea aspera, o.m.- attachment point for the obturator muscles, h.- femoral head.
Figure 2. Specimen BBU-PSM V1009 in: A - proximal, B - medial, C anterior, D - posterior, E - lateral, and F – distal views.
