Monday, 11 November 2019

Aquilarhinus palimentus: A new species of Hadrosaurid Dinosaur from the Late Cretaceous of Big Bend National Park, south-western Texas.

Hadrosaurs, or Duck-Billed Dinosaurs, were large herbivorous Ornithischian Dinosaurs widespread across Laurasia (Eurasia plus North America; Laurasia split away from the southern continents in the Triassic, the split into North America and Eurasia during the Cretaceous) as well as South America and Antarctica during the Late Cretaceous. They were descended from the earlier Iguanadontids, but with more sophisticated jaws and teeth, which allowed them to chew their food, not by side-to-side motion as in a modern mammal, but by a unique flexion of the upper jaw parts, which moved apart as the lower jaw moved upwards. The term 'Hadrosauroid' refers to the widest possible grouping of Hadrosaurs, including all animals in the group after their evolutionary split with the Iguanadontids in the Early Cretaceous, whereas the term 'Hadrosaurid' refers to the most derived members of the group, which appeared in the Late Cretaceous and are split into two subgroups, the Lambeosaurs, which had hollow bony crests, thought to have been used for making sounds, and the Saurolophides, or Hadrosaurines, which either lacked crests or had solid ones.
In a paper published in the Journal of Systematic Palaeontology on 12 July 2019, Albert Prieto-Marquez of the Institut Catal a de Paleontologia Miquel Crusafont at the Universitat Aut onoma de Barcelona, and the Museu de la Conca Dell a-Parc Cretaci, and Jonathan Wagner and Thomas Lehman of the Departament of Geosciences at Texas Tech University, describe a new species of Hadrosaurid Dinosaur from the Late Cretaceous Aguja Formation of Big Bend National Park, south-western Texas.

The Aguja Formation of western Texas preserves one of the southernmost Campanian ( 83.6  to 72.1 million years old) terrestrial vertebrate faunas in North America. Nearly the entire fauna, however, is known from the uppermost part of the formation (the upper shale member). Until recently, very little was known about the vertebrate fauna found in the lower part of the formation (the lower shale member). S diverse assemblage of small Theropod, Lizard and Mammal teeth (the ‘Lowerverse local fauna’) has been described from a single locality in the lower shale member, as has a more general fauna and stratigraphy, but otherwise the vertebrate fossils of the lower Aguja Formation remain undocumented.
Terrestrial and paralic strata of the Aguja Formation are widely exposed in and around Big Bend National Park in south-western Texas. These strata are underlain by and intertongue with marine deposits of the Pen Formation. The Aguja consists of two eastward-thinning intervals of terrestrial strata (the lower and upper shale members) separated by a westward-thinning wedge of interposed marine strata – the McKinney Springs tongue of the Pen Formation. The upper shale member is widely exposed throughout Big Bend National Park and surrounding areas, but the lower shale member is mostly exposed on private ranches west of the park; it thins and pinches out in the south-western part of the park and does not extend into the eastern portion.
The lower shale member of the Aguja Formation consists primarily of thick beds of lignitic clay-shale with several prominent sandstone and coal beds, particularly near the base. In the middle of the lower shale member there is a thin zone of interbedded very fine sandstone and carbonaceous mudstone with conspicuous iron-manganese concretions. This concretionary interval is the only part of the lower shale member that yields significant vertebrate fossils, and the holotype specimen of the new Hadrosaurid documented here was collected from these strata. 
The lower shale member is interpreted as having accumulated primarily in a coastal swamp or marsh environment. The specimen was recovered from within a bed of mudstone having irregular bedding planes covered with abundant large carbonised leaf and wood fragments, many of which are riddled with Teredolites borings. The bones are partially enclosed in large concretions of iron-manganese oxides. Some of the bones were broken prior to burial, but articulation surfaces are intact and the cortical surfaces exhibit little cracking, indicating that the bones had been subject to minimal transport or weathering prior to burial. Immediately overlying the bone-bearing bed is a thin layer with abundant small Ostreid Bivalve shells, but no other remains are preserved at the site.
The vertebrate fauna from the lower shale member dates back to between 81 and 80 million years ago, corresponding to the early Campanian. This fauna may be slightly older than that of the Wahweap Formation in Utah and lower Two Medicine Formation in Montana. Only a few Hadrosaurids have been described from these strata, such as Gryposaurus latidens and Acristavus gagslarsoni (both from the Two Medicine Formation of Montana) and these are among the oldest known Hadrosaurids. The new species from the lower shale member is therefore significant in adding to our understanding of the early evolution and diversity of the group. In particular, this animal is uniquely positioned, both phylogenetically and temporally, to expand our understanding of the early evolution of Hadrosaurid supracranial ornamentation.

The new species is named Aquilarhinus palimentus, where 'Aquilarhinus' means 'Eagle-nosed' and 'palimentus' means 'shovel-chin'. The specimen from which the species is described, TMM 42452-1, comprises a sphenoid fragment, both nasals, the right maxilla, the right jugal, the right quadratojugal, the partial left and right palatines, a partial right dentary, a partial first ceratobranchial, a partial neural arch of atlas, fragments of two cervical centra, two cervical ribs, a partial sacral rib, a left carpal, a nearly complete left manus, the postacetabular process of the right ilium, a fragment of right ischium, a partial astragali, pedal phalanxes III-1, and four pedal unguals. Most of the material was collected in 1983 by Thomas Lehman, Neal LaFon, and Kyle Davies, with the remainder being excavated in 1999 by Jonathan Wagoner and Thomas Lehman. The preserved parts of the skull and jaws were disarticulated but closely associated. All of these bones were found within 4 m² and clearly represent a single individual. This specimen was briefly described by Wagner in 2001 as Kritosaurus sp.  Subsequent study of the specimen, however, indicates that the new species cannot be included within Kritosaurus and differs sufficiently from other Hadrosaurids to warrant recognition a new genus and species.
 Geographical and straigraphical location of the type and referred material of Aquilarhinus palimentus. Abbreviations: cg, conglomerate; cl, clay; si, silt; ss, sandstone. Prieto-M arquez et al. (2019).
Aquilarhinus palimentus is a Hadrosaurid Dinosaur possessing nasals transversely broad across the skull table; a premaxillary shelf of maxilla at anterior apex flat and as broad as the proximal segment of the palatal process; a palatine extending nearly horizontally to contact the maxilla; and a dorsally reflected symphysial process of the dentary. It differs from all other Hadrosaurids in possessing maxilla combining jugal process and ectopterygoid ridge continuous with ventral margin of jugal facet, and differs from all other Hadrosaurids except Latirhinus uitstlani in having a nasal enclosing the extremely broad and sub-circular lateral profile of the external bony naris.

In general aspect, the skull as reconstructed was almost certainly tall and would have had a steeply sloping facial profile, as in Gryposaurus. There is circumstantial evidence to indicate that the skull roof sloped anteroventrally, and the external bony naris was exceedingly large. A remarkable departure from the generally narrow saurolophine aspect of this species is a general mediolateral expansion of the skull. This is evidenced by the broad exposure of the nasal on the skull roof, the shallow angle of inflection of the palatine vault, and the mediolateral extent of the symphysial process of the dentary. This expansion is here interpreted as dilation of the skull mediolaterally about the midline, resulting in greater separation of paired cranial elements. Dilation of the cranium appears to be a morphogenetic accommodation to the modification of the anterior dentary.

Additional Hadrosaurid elements were recovered at Rattlesnake Mountain from the same stratigraphical interval as the holotype of Aquilarhinus palimentus, some bones within a short distance of the collection site of the holotype. Although this material might pertain to Aquilarhinus palimentus, none of these isolated bones exhibit diagnostic features that would allow certain attribution. This material includes a braincase wall, a parietal bone, a frontal, a postorbital, a scapula, a humerus, a pubis, an ilium, a femur, and a tibia.

With reconstructed quadrate and skull lengths of 26 cm and 57 cm, respectively, TMM 42452-1 is a relatively small Hadrosaurid individual. For example, compared to other Hadrosaurids from the same time period, the skull of TMM 42452-1 is estimated to be only 50% the length of the skull of Gryposaurus latidens, and 65% of the length of the skulls of Acristavus gagslarsoni and Probrachylophosaurus bergei. By analogy with Gryposaurus spp. it could be assumed that the nasal arch of Aquilarhinus palimentus would become more prominent with growth. However, it is also possible that the nasal arch of Aquilarhinus palimentus experienced a different allometric growth trajectory than that of Gryposaurus spp. Therefore, Prieto-M arquez et al. are uncertain at this juncture regarding the degree of maturity of TMM 42452-1, pending the discovery of additional specimens.

Reconstruction of the skull and mandible of Aquilarhinus palimentus. Areas coloured in brown indicate bones belonging to specimen TMM 42452-1. Prieto-M arquez et al. (2019).

The presence of a gentle arch on the dorsal surface of the nasals of Aquilarhinus palimentus is reminiscent of the nasal crest in species of the Saurolophine Gryposaurus. Gryposaurus ranges from the early Campanian to possibly the late Maastrichtian and is distributed from southern Canada to south-western Texas Notably, Gryposaurus latidens, from lower Campanian strata of the Two Medicine Formation of Montana, represents one of the oldest known Hadrosaurids, with an estimated age of 80 million years. A nasal arch is also present in Rhinorex condrupus, a sister taxon to Gryposaurus spp. from the Neslen Formation of eastern Utah, in late Campanian strata dated to 75–74.5 million years ago. Despite sharing an arcuate dorsal profile of the nasal, Aquilarhinus palimentus is clearly different from Rhinorex condrupus and Gryposaurus spp. The external bony naris (at least the dorsal extent) of Gryposaurus spp. and Rhinorex condrupus is significantly narrower anteroposteriorly than that of Aquilarhinus palimentus. The maxilla of Gryposaurus shows a jugal tubercle instead of a jugal process, and its premaxillary shelf is steeply inclined at least 40° relative to the tooth row compared to the less inclined shelf of quilarhinus palimentus which is inclined at 30 °. Finally, the anterior process of the jugal of Gryposaurus and Rhinorex condrupus is substantially deeper with a relatively short anterior apex, compared to the shallow process and long apex of Aquilarhinus palimentus.

Right nasal of the holotype specimen (TMM 42452-1) of Aquilarhinus palimentus. (A) Lateral view; (B) interpretive line drawing of the lateral view; (C), medial view; (D), interpretive line drawing of the medial view. Dark grey indicates reconstruction, cross hatching indicates broken surfaces, open circles represent concretionary matrix. Prieto-M arquez et al. (2019).

Naashoibitosaurus ostromi is a Kritosaurin Saurolophine originally described on the basis of a skull and fragmentary postcranium. The nasals display a low but acute arch. Unlike Aquilarhinus palimentus, however, the nasal crest of Naashoibitosaurus ostromi lies farther posterodorsally relative to the posterior margin of the external bony naris. Most notably, the posterodorsal margin of the external bony naris is very narrow in Naashoibitosaurus ostromi, less than half the width of the broad sub-circular margin enclosed by the nasal of Aquilarhinus palimentus. In addition, a deep circumnarial fossa excavates the lateral surface of the nasal surrounding the external bony naris of Naashoibitosaurus ostromi, whereas the fossa is distinctly shallower in Aquilarhinus palimentus. As in other Hadrosaurids, but unlike Aquilarhinus palimentus, the maxilla of Naashoibitosaurus ostromi shows a dorsal jugal tubercle rather than a jugal process and the occlusal plane displays a maximum of two functional teeth. The dorsal surface of the premaxillary shelf is concave and substantially narrower, about half the width of that in Aquilarhinus palimentus. Similarly, the palatal process of Naashoibitosaurus ostromi is much shallower, about half the depth seen in Aquilarhinus palimentus.

Although the premaxillae of Aquilarhinus palimentus were not recovered, there is no indication that the bizarre mandibular symphysial configuration in this species was related to the circumnarial structure, nor is it likely to be related to any part of the vocal, thermoregulatory, defensive or other physiological behavioural systems functionally linked to the rostrum. Prieto-M arquez et al. consider it more likely that the autapomorphic dentary symphysis is linked to restructuring of the rostrum for a unique feeding strategy. The projection and curvature of the symphysial processes of the dentary, nearly horizontal palatines and wide nasals all suggest that this animal was broad snouted, if not simply broad headed, also suggesting trophic adaptation. The dentaries and their symphysial processes would have met in a ‘W’-shaped section anteriorly. Assuming the predentary retained its ancestral relationships to the features of the anterior dentary, it would have been shaped like two trowels laid side to side. The raised symphysis might have led to a raised ridge that passed between the concave predentary fossae along the midline, much like the strengthening ridge leading from the handle onto the blade of a spade or shovel. The anterior extension of the symphysial processes, and the general proportions of the dentaries, suggests that the predentary may have been relatively long.

The hypothesized shovel-shaped ‘bill’ and widening of the skull in Aquilarhinus palimentus may have been adapted for shovelling out and scooping up vegetation. The central reinforcing ridge developed from the dentary symphysis would produce two strong arches in cross section to resist the strain of pushing through sediment or vegetation. The dentary was straightened relative to that of ancestral Hadrosaurids, likely in order to bring the predentary into alignment with the axis of the mandibles, reducing bending stress along the length of the bone and keeping material from falling out of the predentary. The widening of the mandible, and corresponding widening of the cranium, produced a wider ‘scoop’.

The adaptations of this animal, as interpreted by Prieto-M arquez et al., bring to mind those of Gomphotherid Proboscideans, Sirenians, Hippopotamus, the South American Pyrotheres and, especially, Desmostylians. These groups are or were relatively large-bodied herbivorous Mammals, many either semiaquatic or closely related to aquatic forms. Moreover, these taxa share narial specializations, as also occur in Hadrosaurids. These analogies suggest that Aquilarhinus palimentus may have been a paralic, possibly semiaquatic species specialised for digging in loose wet sediment. This habitus is consistent with the facies interpretation of the strata in which the type specimen was found.

Trophic specialization does not seem exceptionally common among Hadrosaurids; some non-Hadrosaurids (e.g. Protohadros byrdi) and the Lambeosaurine Angulomastacator daviesi have strongly ventrally deflected rostra, but a direct connection to feeding habits has not been proposed. Some palaeontologists have have proposed trophic specializations based on the diameter of the premaxillary ‘beak’. While this is understandable, Prieto-M arquez et al. feel that until the morphogenetic relationship between the beak and the circumnarial fossa can be explored in detail, it is wisest to consider this tentative. The conformation of the dentary symphysis in Aquilarhinus palimentus is so far the most likely candidate for a clear trophic specialisation among Hadrosaurids.

Ancestral state reconstructions for the presence of an osseous cranial crest (in its simplest form, an elevation of the skull roof above the sub-horizontal ancestral cranial profile) indicate that the most recent common ancestor of the clade including Saurolophidae and the Aquilarhinus-Latirhinus lineage probably sported a crest. The presence of a cranial crest was also likely ancestral for Saurolophids as a whole, including both Lambeosaurines and Saurolophines. Therefore, the ‘solid crests’ of Saurolophines and the ‘hollow crests’ of Lambeosaurines are most likely homologous. It would, then, be less appropriate to say that the crest has evolved several times among Saurolophids, and more reasonable to suggest that it is the conformation of the crest, not the presence or absence of a crest, which has been most labile among Hadrosaurids.

Both parsimony and maximum likelihood optimization methods support the hypothesis that the arched nasals of Aquilarhinus palimentus did not evolve independently from those of derived Hadrosaurids such as Gryposaurus and other Kritosaurins that are deeply nested within Saurolophinae but are in fact homologous. This may indicate that the arched nasal crest is the ancestral conformation of the crest, and all other Hadrosaurid crests are ultimately derived from the arched nasal crest.

The lack of sufficient cranial material in Hadrosaurus foulkii prevents ascertaining whether crests were ancestral for Hadrosauridae. The unadorned skull of another non-Saurolophid Hadrosaurid, Eotrachodon orientalis suggests tentatively that the earliest Hadrosaurids lacked supracranial crests. On the other hand, osseous crests must have been lost at least twice within Saurolophinae (Acristavus gagslarsoni and Edmontosaurini) and might have been lost in Eotrachodon orientalis as well. So far, there is only evidence that enclosure of the nasal passages to form a ‘hollow’ crest occurred once, in Lambeosaurines. However, this must have happened quite early in their evolution.

Aquilarhinus palimentus represents a new genus and species of Hadrosaurid from the early Campanian of south-western Texas. This taxon is characterised by several autapomorphies of the facial skeleton, the most remarkable of which consists of a dentary symphysis that is reflected dorsally as well as anteromedially projected. The latter condition, along with evidence of a relatively broad skull, suggest that this Hadrosaurid may have fed by shovelling material, possibly soft water plants.

This new species is one of the oldest Hadrosaurids and one of the few non-Saurolophid Hadrosaurids known to date, forming a group of ‘broad-nosed’ forms with Latirhinus uitstlani that branched off before the major clades Saurolophinae (‘solid-crested’ and unadorned taxa) and Lambeosaurinae (‘hollow-crested’ taxa). This adds to the diversity of pre-Saurolophid taxa known to date, previously restricted to Hadrosaurus foulkii and Eotrachodon orientalis, and suggests the existence of a previously unrealized diversity of Hadrosaurid lineages that evolved prior to the main radiation of the clade.

Supracranial crests were possibly present ancestrally in non-Saurolophid Hadrosaurids, as well as in the common ancestor of Saurolophidae, Saurolophinae and Lambeosaurinae. Prieto-M arquez et al. find evidence in support of nasal arches as the ancestral crest morphology, from which other derived types of supracranial ornaments evolved within Saurolophid Hadrosaurids.

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