Europasaurus

''Europasaurus'' is a basal macronarian sauropod, a form of quadrupedal herbivorous dinosaur. It lived during the Late Jurassic (middle Kimmeridgian, about 154 million years ago) of northern Germany, and has been identified as an example of insular dwarfism resulting from the isolation of a sauropod population on an island within the Lower Saxony basin.

Discovery and naming

In 1998, a single sauropod tooth was discovered by private fossil collector Holger Lüdtke in an active quarry at Langenberg Mountain, between the communities of Oker, Harlingerode and Göttingerode in Germany. The Langenberg chalk quarry had been active for more than a century; rocks are quarried using blasting and are mostly processed into fertilisers. The quarry exposes a nearly continuous, thick succession of carbonate rocks belonging to the Süntel Formation, that ranges in age from the early Oxfordian to late Kimmeridgian stages and have been deposited in a shallow sea with a water depth of less than . The layers exposed in the quarry are oriented nearly vertically and slightly overturned, which is a result of the ascent of the adjacent Harz mountains during the Lower Cretaceous. Widely known as a classical exposure among geologists, the quarry had been extensively studied, and visited by students of geology for decades. Although rich in fossils of marine invertebrates, fossils of land-living animals had been rare. The sauropod tooth was the first specimen of a sauropod dinosaur from the Jurassic of northern Germany.

After more fossil material was found, including bones, excavation of the bone-bearing layer commenced in April 1999, conducted by a local association of private fossil collectors. Although the quarry operator was cooperative, excavation was complicated by the near-vertical orientation of the layers that limited access, as well as by the ongoing quarrying. The sauropod material could not be excavated directly from the layer but had to be collected from lose blocks that resulting from blasting. The exact origin of the bone material was therefore unclear, but could later be traced to a single bed (bed 83). An excavation conducted between July 20–28 of 2000 rescued ca. of bone-bearing blocks containing vertebrate remains. Fossils were prepared and stored in the Dinosaur Park Münchehagen (DFMMh), a private dinosaur open-air museum located close to Hanover. Due to the very good preservation of the bones, consolidating agents had to be applied only occasionally, and preparation could be conducted comparatively quickly as bone would separate easily from the surrounding rock. Bones of simple shape could sometimes be prepared in less than an hour, while the preparation of a sacrum required a workload of three weeks. By January 2001, 200 single vertebrate bones had already been prepared. At this point, the highest bone density was found in a block measuring 70 x 70 cm, which contained ca. 20 bones. By January 2002, preparation of an even larger block had revealed a partial sauropod skull – the first to be discovered in Europe. Before complete removal of the bones from the block, a silicon cast was made of the block to document the precise three-dimensional position of the individual bones.

Part of the ''Europasaurus'' fossil material got damaged or destroyed by arson fire in the night from the 4th to the 5th of October, 2003. The fire destroyed the laboratory and exhibition hall of the Dinosaur Park Münchehagen, resulting in the loss of 106 bones, which account for 15% of the bones prepared at the time. Furthermore, the fire affected most of the still unprepared blocks, with firefighting water hitting the hot stone causing additional crumbling. Destroyed specimens include DFMMh/FV 100, which included the best preserved vertebral series and the only complete pelvis.

In 2006, the new sauropod taxon was formally described as ''Europasaurus holgeri''. The given etymology for the genus name is "reptile from Europe", and the specific name honours Holger Lüdtke, the discoverer of the first fossils. Given the comparatively small size of the bones, it was initially assumed that they stem from juvenile individuals. The 2006 publication, however, established that the majority of specimens were adult, and that ''Europasaurus'' was an island dwarf. The number of individual sauropod bones had increased to 650 and include variously articulated individuals; the material was found within an area of squared. From these specimens, the holotype was selected, a disarticulated but associated individual (DFMMh/FV 291). The holotype includes multiple cranial bones (premaxilla, maxilla and quadratojugal), a partial braincase, multiple mandible bones (dentary, surangular and angular), large amounts of teeth, cervical vertebrae, sacral vertebrae and ribs from the neck and torso. At least 10 other individuals were referred to the same taxon based on overlap in material.

A large-scale excavation campaign commenced in the summer of 2012, with the goal to excavate ''Europasaurus'' bones not only from lose blocks but directly from the rock layer. Access to the bone-bearing layer required the removal of some 600 tons of rock using excavators and wheel loaders, and the constant pumping out of water from the base of the quarry. Excavations continued in spring and summer 2013. The campaign resulted in the discovery of new fish, turtle, and crocodile remains, as well as valuable information of the bone-bearing layer; additional ''Europasaurus'' bones, however, could not be located. By 2014, around 1300 vertebrate bones had been prepared from bed 83, the majority of which stemming from ''Europasaurus''; an estimated 3000 additional bones await preparation. A minimum number of 20 individuals was identified based on jaw bones.

Description

''Europasaurus'' is a very small sauropod, measuring only as an adult. This length was estimated based on a partial femur, scaled to the size of a nearly complete ''Camarasaurus'' specimen. Younger individuals are known, from sizes of to the youngest juvenile at . Gregory S. Paul estimated that a long individual would have weighed .

Distinguishing characteristics

Aside from being a very small neosauropod, ''Europasaurus'' was thought to have multiple unique morphological features to distinguish it from close relatives by its original describers, Sander ''et al.'' (2006). The nasal process of the premaxilla was thought to curve anteriorly while projecting upwards (now known to be preservational), there is a notch on the upper surface of the centra of cervical vertebrae, the scapula has a prominent process on the posterior surface of its body, and the astragalus (an ankle bone) is twice as wide as tall.

When compared to ''Camarasaurus'', ''Europasaurus'' has a different morphology of the postorbital where the posterior flange is not as short, a short contact between the nasal and frontal bones of the skull, the shape of its parietal (rectangular in ''Europasaurus''), and the neural spines of its vertebrae in front of the pelvis are unsplit. Comparisons with ''Brachiosaurus'' (now named ''Giraffatitan'') were also mentioned, and it was identified that ''Europasaurus'' has a shorter snout, a contact between the quadratojugal and squamosal, and a humerus (upper forelimb bone) that has flattened and aligned proximal and distal surfaces. There were finally quick comparisons to the potential brachiosaurid ''Lusotitan'', which has a different ilium and astragalus shape, and ''Cetiosaurus humerocristatus'' (named ''Duriatitan''), which has a deltopectoral crest that is less prominent and extends across less of the humerus.

Skull

Nearly all external skull bones have been preserved among ''Europasaurus'' specimens, except the prefrontals. Some additional bones are only represented by very fragmented and uninformative fossils, such as the lacrimals. Eight premaxillae are known, with a generally rectangular snout shape as found in ''Camarasaurus''. The anterior projection of the premaxilla identified in Sander ''et al.'' (2006) was re-identified as a preservational artifact in Marpmann ''et al.'' (2014), similar to the anatomy found in ''Camarasaurus'' and '' Euhelopus'' to a lesser degree. The dorsal projection of the premaxilla, the one which contacts the nasal bone, begins as a postero-dorsal projection, before becoming straight vertical at the point of the subnarial foramen, until it reaches the nasal. This weak "step" is seen in ''Camarasaurus'' and ''Euhelopus'', and is present more strongly in ''Abydosaurus'', ''Giraffatitan'' and a possible skull of ''Brachiosaurus''. These latter taxa also have a longer snout, with more distance from the first tooth until the nasal process of the premaxilla. As well, ''Europasaurus'' shares with the basal camarasauromorphs (brachiosaurids, ''Camarasaurus'', ''Euhelopus'' and ''Malawisaurus'') a similarly sized orbit and nasal fenestra, whereas the nasal opening is significantly reduced in derived titanosaurs ('' Rapetosaurus'', ''Tapuiasaurus'' and ''Nemegtosaurus'').

A single maxilla is present in the well-preserved material of ''Europasaurus'', DFMMh/FV 291.17. This maxilla has a long body, with two elongate processes, a nasal and a posterior process. There is only a weak lacrimal process, like in most sauropods except ''Rapetosaurus''. The nasal process is elongate and covers the anterior and ventral rim of the antorbital fenestra. This process extends about 120º from the horizontal tooth row. The base of the nasal process also forms the body of the lacrimal process, and at their divergence is the antorbital fenestra, similar in shape to those of ''Camarasaurus'', ''Euhelopus'', ''Abydosaurus'' and ''Giraffatitan'', but about 1/2 taller proportionally. The pre-antorbital fenestra, a small opening in front of or beneath the antorbital opening, is well developed in taxa like ''Diplodocus'' and ''Tapuiasaurus'', is nearly absent, like in ''Camarasaurus'' and ''Euhelopus''. There were about 12–13 total teeth in the maxilla of ''Europasaurus'', fewer than in more basal taxa (16 teeth in ''Jobaria'' and 14–25 in '' Atlasaurus''), but falling within the range of variation in Brachiosauridae (15 in ''Brachiosaurus'' to 10 in ''Abydosaurus''). All of the unworn teeth preserved display up to four small denticles on their mesial edges. A small amount of the posterior tooth crowns are slightly twisted (~15º), but much less than in brachiosaurids (30–45º).

Among the nasal bones of ''Europasaurus'', several are known, but few are complete or undistorted. The nasals are overlapped posteriorly by the frontal bones, and towards the side, they articulate bluntly with the prefrontals. Unlike the nasals of ''Giraffatitan'', those in ''Europasaurus'' project horizontally forwards, forming a small portion of the skull roof over the antorbital fenestrae. Four frontals are known from ''Europasaurus'', three being from the left and one being from the right. Because of their disarticulation, it is likely that the frontals never fused during growth, unlike in ''Camarasaurus''. The frontals form a portion of the skull roof, articulating with other bones such as the nasals, parietals, prefrontals and postorbitals, and they are longer antero-posteriorly than they are wide, a unique character among a eusauropodan. Like in diplodocoids ('' Amargasaurus'', '' Dicraeosaurus'' and ''Diplodocus''), as well as ''Camarasaurus'', the frontals are excluded from the frontoparietal fenestra (or parietal fenestra when frontals are excluded). The frontals are also excluded from the supratemporal fenestra margin (a widespread character in sauropods more derived than '' Shunosaurus''), and they only have a small, unornamented participation in the orbit. Several parietal bones are known in ''Europasaurus'', which show a rectangular shape much wider than long. the parietals are also wide when viewed from the back of the skull, being slightly taller than the foramen magnum (spinal cord opening). The parietals contribute to about half the post-temporal fenestra (opening above the very back of the skull) border, with the other region enclosed by the squamosal bones and some braincase bones. Parietals also form part of the edge of the supratemporal fenestra, which is wider than long in ''Europasaurus'', like in ''Giraffatitan'', ''Camarasaurus'' and '' Spinophorosaurus''. Besides the before mentioned fenestra, the parietals also have a "postparietal fenestra", something rarely seen outside of Dicraeosauridae. A triradiate postorbital bone is present in ''Europasaurus'', which evolved as the fusion of the postfrontal and postosbital bone of more basal taxa. Between the anterior and ventrally projecting processes the postorbital forms the margin of the orbit, and between the posterior and ventral processes it borders the infratemporal fenestra.

Multiple jugals are known from ''Europasaurus'', which are more similar in morphology to basal sauropodomorphs than other macronarians. It forms part of the border of the orbit, infratemporal fenestra and the bottom edge of the skull, but does not reach the antorbital fenestra. The posterior process of the jugal are very fragile and narrow, showing a bone scar from the articulation with the quadratojugal. There are two prominences projecting from the back of the jugal body, which diverge at 75º and form the bottom and front edges of the infratemporal fenestra. Like in ''Riojasaurus'' and '' Massospondylus'', two non-sauropod sauropodomorphs, the jugal forms a large part of the orbit edge, from the back to the front bottom corner. This feature has been seen in embryo