Eolambia

Eolambia (meaning 'dawn lambeosaurine') is a genus of herbivorous hadrosauroid dinosaur from the early Late Cretaceous of the USA. It contains a single species, E. caroljonesa, named by paleontologist James Kirkland in 1998. The type specimen of Eolambia was discovered by Carole and Ramal Jones in 1993; the species name honors Carole. Since then, hundreds of bones have been discovered from both adults and juveniles, representing nearly every element of the skeleton. All of the specimens have thus far been found in Emery County, Utah, in a layer of rock known as the Mussentuchit Member of the Cedar Mountain Formation. Measuring up to 6 metres (20 ft) long, Eolambia is a large member of its group. While it closely approaches the Asian hadrosauroids Equijubus, Probactrosaurus, and Choyrodon, in traits of the skull, vertebrae, and limbs, it may actually be more closely related to the North American Protohadros. This grouping, based on the straightness of the quadrate bone and scapula, would represent an isolated, endemic radiation of hadrosauroids. Despite resembling hadrosaurids – lambeosaurine hadrosaurids in particular – in several features, leading to its initial identification as one of them, these similarities have been rejected as either entirely convergent or misinterpreted. Eolambia would have lived in a forested environment at the edge of lakes in a humid floodplain environment, feeding on gymnosperms, ferns, and flowering plants. The water levels in the lakes changed over time with cyclical wet and dry spells caused by the precession of the Earth, reflected by alternating bands in the sediments of the Mussentuchit Member. As a juvenile, Eolambia would have been preyed upon by large crocodylomorphs residing in the lake waters. With increasing age, however, they became impervious to the crocodylomorphs, and mature individuals (at least eight to nine years in age) were preyed on by large theropods such as the neovenatorid Siats. Eolambia is a large hadrosauroid. Initial estimates placed the length of its skull at 1 metre (3 ft 3 in) in length, although this was due to a disproportionately long snout that was later corrected by the discovery of additional material. In 2016, Gregory S. Paul estimated a body length of 6 metres (20 ft) and a weight of 1 tonne (1.1 tons) for Eolambia, which agrees with a prior body length estimate of 6.1 metres (20 ft) by Thomas R. Holtz Jr. in 2012. Earlier, in 2008, an adult specimen was estimated as having a length of 5.2 metres (17 ft) and a height at the hip of 2 metres (6 ft 7 in). The crestless skull of Eolambia has a similar overall shape to those of Equijubus and Probactrosaurus. The front of the snout is highly roughened, being punctuated by many foramina. At the tip of each premaxilla, there are two tooth-like structures known as denticles, which is also seen in its closest relative Protohadros. Further back, the rear portion of the lower branch of the premaxilla abruptly projects upwards, closing off the nostril at the rear as in Probactrosaurus, Protohadros, and other hadrosauroids. This part joins with the two finger-like processes of the maxilla, which is similar to Protohadros. The body of the maxilla itself does not bear a recess or any indication of an antorbital fenestra, like Equijubus, Protohadros, and other hadrosauroids. One of the characteristics used to distinguish Eolambia is the concave profile of the tooth row of the maxilla when viewed from the side, which is like Equijubus, Probactrosaurus, and several other hadrosauriforms but unlike Protohadros. Like Probactrosaurus and other hadrosauroids, the back of the maxilla connects to the jugal – which borders the bottom of the eye socket and infratemporal fenestra – through a finger-like projection that fits into a recess. The bottom margin of the jugal bears a strong flange beneath the level of the infratemporal fenestra; this is also seen in Equijubus, Probactrosaurus, Protohadros, and several other hadrosauroids. Connecting to the jugal from above is the postorbital, which has a roughened surface where it borders the eye sockets (like Protohadros), but the side of the bone is otherwise smooth. At the back of the skull, the quadrate articulates with the squamosal with a joint that is D-shaped when viewed from the top. The left and right squamosals would have contacted each other extensively, being only separated at the back by a small process of the parietal. The supraoccipital bone, which forms the top portion of the back of the skull, is flat and nearly vertical, as is the case in Probactrosaurus and other hadrosauroids. As with the premaxilla, the predentary of Eolambia bore denticles. There is a prominent dorsomedial process, a tab-like structure also seen in Probactrosaurus and other hadrosauriforms. Several additional tab-like denticles were present on either side of the dorsomedial process, which are likewise present in Probactrosaurus. The predentary is joined at the back by the dentary, which constitutes most of the lower jaw. There is a short recess, or diastema, between the articulation of the predentary with the dentary and the first tooth position on the dentary, which is observed in Equijubus, Probactrosaurus, and other hadrosauroids. The front of the dentary characteristically deepens, as in Protohadros, Ouranosaurus, and Bactrosaurus. Two bulges are present on the outer surface of the dentary, one of them representing the coronoid process as in Probactrosaurus and other hadrosauroids. Like Protohadros and several other hadrosauriforms but unlike Probactrosaurus, only the front end of this process is thickened. As in Equijubus, Probactrosaurus, and various other iguanodonts, a small foramen (or opening) is present on the side of the surangular, which is located behind the dentary. Similar to other iguanodonts, the teeth of Eolambia are arranged in tightly-spaced and interlocking rows. At any given time, each of the 32 maxillary tooth sockets holds three teeth, while each of the 30 dentary tooth sockets holds four teeth. Out of these, two of the teeth in each socket are replacement teeth like those of Probactrosaurus; the others are active teeth. Every active tooth has one wear facet. Like Probactrosaurus, Gongpoquansaurus, Protohadros, and other hadrosauroids, each maxillary tooth crown has only one ridge, which is slightly offset towards the midline of the mouth. Meanwhile, each dentary tooth crown characteristically bears a primary ridge, and an accessory ridge closer to the midline of the mouth, a condition which is also present in Protohadros and other hadrosauroids. All of the crowns also bear small, denticle-like serrations on the front and rear edges, which is also seen in Probactrosaurus.