Eolambia

Initial discoveries and description In 1979 Peter Galton and James A. Jensen described a fragmentary right femur, BYU 2000, belonging to a hadrosaurian dinosaur discovered in sediments belonging to the Cedar Mountain Formation in Arches National Park, Utah. Though poor material, it was important for it (alongside a second North American femur described in the paper) was the first hadrosaur specimen from the Lower Cretaceous anywhere in the world. Galton and Jensen hypothesized more complete remains of a hadrosaur may be found from the formation in the future. Various hadrosauroid teeth had also been found in quarries of small vertebrates in the western region of the San Rafael Swell, near Castle Dale in Emery County, Utah; they were described in 1991 by J. Michael Parrish. Subsequently, in 1993, Carole Jones and her husband Ramal Jones discovered fragmentary bones in a fossil site located in the northwestern region of the Swell. They brought the site to the attention of Donald Burge, director of the institution then called the College of Eastern Utah Prehistoric Museum (CEUM). The site, which is formally known as CEUM Locality 42em366v, would subsequently be named Carol's Site (sic) in her honour. The fossils, stored under the specimen number CEUM 9758, represent the partial remains of an adult hadrosauroid, including parts of the skull, vertebrae, ischium, and leg. CEUM 5212, a partial skull and forelimb from an adult, was found nearby in CEUM Locality 42em369v. In a 1998 paper published in the bulletin of the New Mexico Museum of Natural History and Science, Kirkland formally named the specimens as a new genus, Eolambia, with the type and only species being Eolambia caroljonesa. The generic name combines the Greek prefix eos/ἠώς ("dawn", "morning", implying "early") with the suffix lambia (derived from Lambeosaurus, which is in turn named after Canadian paleontologist Lawrence Lambe). In all, the genus name means "dawn (or early) lambeosaurine", in reference to its supposed position as the most basal lambeosaurine. Meanwhile, the specific name honors Carole Jones. The name Eolambia was suggested by paleoartist Michael Skrepnick; it replaced the informal name "Eohadrosaurus caroljonesi", which was used by Kirkland before the 1998 paper. However, the epithet caroljonesa, following the ICZN, is incorrect and in this case the correct spelling would be caroljonesae. Subsequent discoveries Additional excavations since 1998 have revealed additional skeletal elements belonging to Eolambia. It is now the most completely-known iguanodont from between the Cenomanian and Santonian stages. Nearly every single skeletal element belonging to the taxon is represented by multiple fossils across existing collections. At least twelve individuals, including at least eight juveniles, Bentonite is also present within the sandstone itself throughout the Mussentuchit. as well as studies of fossil megaflora and invertebrates, had initially suggested that the uppermost Cedar Mountain Formation – including the Mussentuchit – dated to the Albian stage. Subsequently, in 2007, James Garrison and colleagues dated the volcanic ash layer passing through the Cifelli #2 Quarry – which is likely the same layer dated previously by Cifelli and colleagues – to 96.7 ± 0.5 million years ago. At an approximate level, the Cifelli and Garrison estimates agree with each other, thus indicating a Cenomanian age for Eolambia. Garrison and colleagues also re-analyzed the pollen assemblage from localities throughout the Mussentuchit, concluding that the assemblage spans the Albian-Cenomanian boundary. ==Description==

Eolambia is a large hadrosauroid. Initial estimates placed the length of its skull at in length, which agrees with a prior body length estimate of by Thomas R. Holtz Jr. in 2012. Earlier, in 2008, an adult specimen was estimated as having a length of and a height at the hip of . The first few dorsal vertebrae are similar to the cervical vertebrae, but have taller and more prong-like neural spines. The rest are amphiplatyan, meaning that they are flat at both ends. They also have postzygapophyses which are less elongate and more pedestal-like, in addition to taller and more rectangular neural spines. The parapophyses, which are depressions instead of projections, have moved off from the transverse processes to the neural arch, between the transverse processes and the articular processes known as the prezygapophyses at the front of the vertebrae. However, they move back onto the base of the transverse processes in the last few dorsals. In the third or fourth dorsal, the parapophysis is located very close to the suture between the neural spine and the centrum, which is unlike the other dorsals but similar to the first few dorsals in Edmontosaurus. Also like Edmontosaurus, the median ridge separating the prezygapophyses become more pronounced in the rear dorsals. Among the Eolambia specimens found to date, the best-preserved sacrum includes seven vertebrae. Given that this individual is immature, and hadrosaurs increase their sacral vertebra count with age, adults may have had more sacrals. The caudal (tail) vertebrae are amphicoelous, having both the front and back ends being concave. They have pedestal-like prezygapophyses and tab-like postzygapophyses, the latter of which are separated by a depression. The prezygapophyses lengthen to become stalks and the postzygapophyses shrink in the middle and rear caudals. Furthermore, the neural spines transition from rectangular to strongly curved (concave in front and convex behind), the centra become more elongated, and the transverse processes disappear as well. Limbs and limb girdles Like Probactrosaurus, the scapular blade of Eolambia is nearly rectangular, with the sides of the blade being straight and meeting the end of the blade at an almost-right angle. The humerus is bowed towards the midline, and the deltopectoral crest on the humerus is prominently thickened. A knob is present on the outer surface of the joint with the radius on the humerus. Both the ulna and the radius are relatively straight bones. The top of the ulna bears three "prongs", namely the olecranon process and two additional triangular flanges. Meanwhile, the radius bears a flattened inner surface at the top end, and a distinct platform on the inner surface at the bottom end, both for articulating with the ulna. The hand has five digits. Among the metacarpal bones of the hand, the second is curved, the third is straight, and the fourth is strongly curved. Like Iguanodon, Probactrosaurus, and other basal iguanodonts, the claw on the first digit is distinctively conical. The second and third claws are hoof-like, with the third claw bearing roughened flanges on its sides – a trait also shared with other iguanodonts. The preacetabular (i.e. located in front of the hip socket) process of the ilium bears a horizontal "boot" which forms an obtuse angle with the bottom of the process. Additionally, the top edge of the bone projects outwards to form a rim located above the ilium-ischium articulation on the bottom edge of the bone. These traits are also seen in Probactrosaurus and other iguanodonts. The hip socket itself is quite large relative to the ilium for an iguanodont. A distinctive flange is present along the top surface of the ilium, a trait which is shared with an unnamed hadrosauroid from the Woodbine Formation. The postacetabular (i.e. located behind the hip socket) process of the ilium tapers at its rear with no break in its top margin. On the pubis, the forward-projecting process expands towards the front, like Probactrosaurus, Gongpoquansaurus, and other iguanodonts. The shaft of the ischium is characteristically straight in adults (juveniles have a curved shaft, which is more common among iguanodonts), and the bottom of the shaft bears a compressed "boot" that is expanded forwards. Like the humerus, the femur is bowed, but outwards. However, characteristically, the bottom half of the femur is straight. The fourth trochanter, located halfway down the femur, is vertically tall and triangular, similar to Probactrosaurus, Gongpoquansaurus, and other iguanodonts but dissimilar to more basal ornithopods. Like the ulna and radius, the tibia and fibula have straight shafts, and they articulate with each other; a roughened projection on the bottom of the fibula fits between two bulb-like condyles located just behind the cnemial crest on the bottom end of the tibia. The top of the tibia is rough, indicating the presence of a cap of cartilage between the femur and tibia. The foot has four digits. Like the metacarpals, the second metatarsal is curved, the third is straight, and the fourth is strongly curved. The phalanges of the foot are very stout, with the terminal phalanges near the end of each digit being particularly stout. As with the second and third claws on the hand, the second, third, and fourth claws of the foot are uniformly broad, flat, and hoof-like. ==Classification==

Initial hadrosaurid identification In Kirkland's initial description of Eolambia, he considered it to be a member of the Hadrosauridae, as defined by David B. Weishampel, David B. Norman, and Dan Grigorescu in 1993. Weishampel and colleagues used seven unifying characteristics to define the Hadrosauridae: the upward expansion of the ascending process of the maxilla; the absence of the paraquadrate foramen, which separates the quadrate and quadratojugal; the location of the angular on the inner surface of the lower jaw; the absence of the surangular foramen on the surangular; the narrow teeth of the maxilla; the presence of three or more teeth in each dentary tooth position; and the reduction of the top margin of the scapular blade. The first, fifth, sixth, and seventh of these traits were recognized in Eolambia, with the rest being unknown due to missing material. Kirkland further assigned Eolambia to the Euhadrosauria, defined by Weishampel and colleagues to include the common ancestor of Hadrosaurinae (now the Saurolophinae) and Lambeosaurinae – the two primary branches of hadrosaurids – and all of its descendants. the absence of a foramen on the premaxilla; the (at least partial) enclosure of the nostril by the premaxilla; the development of a shelf on the maxilla; the very tall neural spines of the caudal vertebrae; the robustness of the humerus; and the large "boot" of the ischium in adults. Kirkland found that two additional traits separated the crestless Eolambia from other, crested lambeosaurines, which were considered as being related to the development of the crest: the elevation of the nasal cavity above the eye socket, and the shortening of the parietal. If Eolambia were to be recognized as a lambeosaurine, then the Lambeosaurinae would have to be redefined to exclude those two traits. Alternatively, if Eolambia was instead the sister group of the Lambeosaurinae, then it would represent a morphology in hadrosaurids that is close to the divergence between hadrosaurines and lambeosaurines. This possibility is supported by the presence of a groove on the bottom of the sacrum of Eolambia, which was recognized by Kirkland as a defining trait of hadrosaurines He thus remarked that adult Eolambia specimens would resolve this issue. ==Paleobiology==

Eggshells possibly belonging to Eolambia have been discovered at various localities alongside isolated teeth. They have reticular (net-like) surface patterns, and have a thickness of . Overall, the growth rate of Eolambia appears to have been comparable to that of Hypacrosaurus and Maiasaura, which reached maturity after seven or eight years. ==Paleoecology==

Environment and taphonomy Sedimentological data from the Mussentuchit Member indicates that Eolambia lived on a poorly-drained floodplain, around a system of large, perennial lakes. The lakes fluctuated between high and low water levels in Milankovitch cycles of roughly 21,000 years, caused by the Earth's axial precession. These cycles is reflected by the alternating layers of muddy sandstone and mudstone in present-day rock deposits. During dry periods, the exposed lakebed formed a broad beach devoid of plants. The cracked mud of the lakebed produced mud clasts, while organic debris and scavenged carcasses were scattered around the water's edge as the lake progressively retreated. Meanwhile, during periods in which the sea level rose, small rivers invaded the floodplain, which reworked and displaced the debris that had been deposited during the dry season. With a paleolatitude of 46° N, the Mussentuchit would have had a warm mean surface temperature of , about warmer than the present day, according to general circulation models. Ecosystem Crocodylomorphs are abundant in the Mussentuchit deposits, being represented largely by teeth. The largest known teeth have been referred to the Pholidosauridae; they are broad with rounded bases, and some of them bear longitudinal ridges. However, given the uniform small size of these teeth, and the co-occurrence of these morphotypes in some crocodilians, which was discovered by expeditions from the FMNH. Teeth belonging to more basal ornithopods have also been discovered, with similar teeth having been referred to the Pachycephalosauria or to toothed birds by Cifelli in 1999. A giant oviraptorosaur has also been discovered, alongside teeth from ceratopsians and sauropods. These plants would have provided an abundant source of food for Eolambia. This alternative interpretation suggests that the common ancestor of Eolambia, Protohadros, and more derived hadrosauroids was not exclusively Asian, but instead distributed across Asia and North America. McDonald and colleagues further suggested that Eolambia and Protohadros represent a localized group of hadrosauroids endemic to North America, much like other groups of styracosternans throughout the Cretaceous. Such contemporary groups included a central Asian group of Shuangmiaosaurus, Zhanghenglong, and Plesiohadros. ==Notes==