Spinophorosaurus

Spinophorosaurus Temporal range: Middle Jurassic?,
Reconstructed skeleton in the Naturkundemuseum Braunschweig; the fossils on the floor are authentic
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	†Sauropodomorpha
Clade:	†Sauropoda
Clade:	†Gravisauria
Genus:	†Spinophorosaurus Remes et al., 2009
Species:	†S. nigerensis
Binomial name
†Spinophorosaurus nigerensis Remes et al., 2009

Spinophorosaurus (meaning "spine-bearing lizard") is a genus of early sauropod dinosaur that lived in what is now Niger. Two specimens were discovered in the 2000s by German and Spanish teams under harsh conditions. The naming of the sauropod is due to its distinctive dual tail spikes, an uncommon feature among sauropods. The holotype specimen was around 13 metres (43 ft) in length when measured along the vertebral column. Spinophorosaurus was one of the most basal sauropods, along with for example Vulcanodon and Tazoudasaurus. Estimates are that the herbivore existed during the Middle Jurassic period.

History of discovery

The PALDES team in Niger, 2007

The rich dinosaur fauna of Niger was brought to scientific attention through French and Italian excavations in the 60s and 70s, leading to the description of new genera such as Ouranosaurus from Lower Cretaceous rocks. An older succession of rocks, the Tiourarén Formation, was explored by American Paleontologist Paul Sereno, who conducted a large-scale excavation campaign in Niger between 1999 and 2003. Although initially thought to fall within the Lower Cretaceous, the formation is now thought to be of a much older Middle Jurassic age. Sereno named new dinosaurs such as the sauropod Jobaria and the theropod Afrovenator from the Tiourarén; most finds were discovered along a cliff known as the Falaise de Tiguidit in the southern Agadez Region. In Marendet, Sereno left partial Jobaria skeletons in the field as a tourist attraction. Starting in 2003, the PALDES project (Paleontología y Desarrollo – "Palaeontology and development") was conducting excavations in the southern Agadez Region. A coorperation between a number of Spanish science and humanitary institutions, PALDES aimed to combine paleontological research with a developmental program for the region, such as the improvement of infrastructure, education structures, and the promotion of tourism, including the planned construction of a new palaeontological museum in Tadibene. Spinophorosaurus stems from a new locality about thirty kilometres north of the Falaise de Tiguidit and near the town of Aderbissinat in the Agadez Region. This locality is part of the Argiles de l'Irhazer, a geological formation below (and thus slightly older than) the Tiourarén.

Early in 2005, the German explorers Ulrich Joger and Edgar Sommer explored the semi-deserts south of Agadez after Tuareg had informed Sommer about the occurrence of large bones in the region. Sommer is the founder of CARGO, a relief organisation specialised in improving the local education system for the Tuareg people, while Joger is biologist and the director of the State Natural History Museum, Braunschweig, Germany. On their return route, they chatted with a group of Tuareg who then directed them to a hilly area nearby that was littered with small bone fragments. After a one-hour search, Joger discovered a rounded bone tip sticking out of the surface, which after further excavation turned out to be a complete femur (upper thigh bone) of what would later become the holotype of Spinophorosaurus. An associated scapula (shoulder blade) and a vertebra were discovered soon after. The sediment, a hard but brittle siltstone, could be removed from the bones using light hammer blows. Joger and Sommer then hired local Tuareg for support and, after two days, had uncovered most of the specimen, which included a virtually complete, articulated vertebral column and several limb and pelvic bones. The vertebral column formed an almost complete circle, with the tip of the tail located where the skull would have been expected, which was not found. Lacking equipment and excavation permit, Joger and Sommer covered the specimen with debris for protection and returned to Germany, now planning a full-scale scientific excavation to be carried out by the Braunschweig museum. An official excavation permit was promised to the museum in 2006; in return, the museum was to built and equip a new school for local Tuareg children in the settlement of Injitane. In autumn 2006, Sommer and Joger, together with other associates of the Braunschweig museum, revisited the site in preparation for the excavation, putting one of the pelvic bones in plaster to test equipment and methodology. Sponsors for financing both the school and excavation were found early in 2007, and the official campaign started on March 1, 2007, when two trucks with equipment left Braunschweig for Niger, taking a route via Spain, Morocco, Mauritania, and Mali. The German team included ten permanent members.

Holotype skeleton during excavation by the PALDES team in 2007

In the meantime, a Spanish team of the project PALDES, led by the Paleontological Museum of Elche, was working in the region. Early in 2007, Mohamed Echika, mayor of Aderbissinat, allowed the PALDES team to excavate the skeleton previously discovered by the Germans; the skeleton was subsequently shipped to Spain. Unaware of these activities, the vanguard of the German team found an empty dig site upon their arrival on March 16; the trucks arrived on March 20. Although disappointed, the German team discovered a second specimen, the future paratype, 15 metres apart from the first, on March 17. An exploratory trench within an area littered with small bone fragments soon revealed jaw and tooth fragments; on the next day, ribs, vertebrae, a humerus (upper arm bone) and a scapula (shoulder blade) could be identified. Eight African excavation helpers joined the group on March 19. On March 20, before the arrival of the trucks, the freshwater reserve of initially 200 litres was depleted as the African helpers had used it for washing the night before, causing the members of the team to faint. Excavation was usually interrupted between 12 AM and 15 PM, when temperatures reached 43–45 °C. On March 25, all but two of the German team members were ill, suffering diarrhoea and circulation problems. Throughout the excavation, progress was documented with photographs and field notes.

Paratype skeleton during excavation by the German team in 2007

By March 27, the humerus, scapula, and most ribs had already been wrapped in protective plaster and extracted. Although no further bones were apparent beneath the skeleton, the team removed an additional 60 to 80 cm of sediment to make sure that all fossils had been collected. Excavation was completed on April 2, and the fossils were packed for transport to the port of Cotonou on April 3. On the same day, Echika revealed to the team that the first skeleton had been excavated by a Spanish group with his permission, and promised to lead the team to another fossil site as compensation, located around 80 km south of Agadez at the cliff of Tiguidit. There, the team opened camp on 5. April after the discovery of sauropod bones, which would later be confirmed to represent the rear part of a skeleton of Jobaria. Excavation ended on 8. April, when the team was forced to leave the largest block, a pelvis of more than half a tonne, in the field until the next season. In an attempt to discourage others from collecting the fossil, an explosive dummy was fabricated and attached to the fossil, labeled with a warning in Spanish. The German team retrieved the block the next season in 2008; the PALDES team had canceled their excavation plans upon outbreak of the Tuareg rebellion (2007–2009).

The two specimens were provisionally housed in the Spanish and German museums, but were to be managed by the Musée National d'Histoire Naturelle, in Niamey, Niger, in the future. The paratype specimen arrived in Germany on 18. March 2007; for its preparation, which took two and a half years, the museum rented a separate factory building. In parallel, a joint paper was prepared by the now cooporating German and Spanish teams. The German team digitized prepared bones and fragments of both specimens in 3D using laser scanning. These models could be digitally repaired and undeformed and printed in 3D, to be assembled to a mounted cast skeleton for the museum's exhibition. Additionally, a life-sized model of a living Spinophorosaurus, nicknamed "Namu" (after the museum's name), was put up in front of the main entrance of the museum. The Spanish team produced separate 3D models of the holotype using photogrammetry for study; a caudal vertebra was put on display at the Elche museum in 2018.

Spinophorosaurus model outside the Braunschweig Museum

A skeleton (divided between the Spanish museum, where it was catalogued as GCP-CV-4229, and the German museum, catalogued as NMB-1699-R) was made the holotype of the new genus and species Spinophorosaurus nigerensis by Kristian Remes and colleagues in 2009. The generic name is composed of spina, Latin for "spike", phoro, Greek for "to bear", and sauros, which means lizard; the name refers to the presence of osteoderms bearing spikes. The specific name refers to Niger, where the taxon was discovered.

The holotype specimen (GCP-CV-4229 and NMB-1699-R) consists of a braincase, a postorbital bone, a squamosal, a quadrate, a pterygoid, a surangular, and a nearly complete postcranial askeleton which lacks the sternum, antebrachium, manus, and pes. The second, paratype specimen (NMB-1698-R), consists of a partial skull and incomplete postcranial skeleton. Elements not preserved in this specimen but not the holotype include a premaxilla, maxilla, lacrimal, dentary, angular, the dorsal ribs of the right side, the humerus, and a pedal phalanx. The two specimens were considered to belong to the same taxon since the skeletal elements that overlapped between them were identical, and due to the proximity of the skeletons in the same stratigraphic layer. At the time it was described, Spinophorosaurus was the most completely known Middle Jurassic sauropod from North Africa.

Description

Skeletal reconstruction (size based on the holotype) with known elements in white and a human for scale

The holotype specimen was around 13 metres (43 ft) in length when measured along the vertebral column, while the paratype was about 13 percent larger, measuring around 14 metres (46 ft). A 3D photogrammetry model of the holotype skeleton measures 11.7 metres from head to tail, with proportions differing from estimates based on 2D skeletal reconstructions. The shoulder height reached by these individuals was estimated at around 4 metres (13 ft); the weight at about 7 metric tons (7.7 short tons). The endocranial and neurocentral sutures (in the skull and vertebrae, respectively) of the holotype specimen are unfused, which indicates it was a subadult, whereas the second specimen has fully fused neurocentral sutures.

Skull

Skull elements of the holotype: braincase (A–C), right quadrate and pterygoid bones (D, E), and upper end of the right quadrate (F, G)

The frontal bones of the skull roof were fused in the midline, unlike the other sutures of the skull, and bore a small pineal foramen at the middle top, about 10 mm in front of the suture between the frontal and the parietal bones, features which characterised the skull. It had an open postparietal notch, a feature otherwise only known in dicraeosaurids and Abrosaurus. A notch following the midline at the front of the conjoined frontal bones indicates there might have been a prong extending from each nasal bone to between the frontals, as may have been the case for Nigersaurus, but unusual among sauropods in general. The occipital condyle at the back of the skull was concave on the sides, like in Shunosaurus. The basal tubera were enlarged and were directed to the sides, unlike what is known in other sauropods. The quadrate did not have a concavity on the hind-side, an ancestral condition otherwise only known in Tazoudasaurus among sauropods. The spatulate teeth were unique in possessing enlarged, spaced denticles around the top of the crown, with a higher count of denticles on the mesially facing carinae.

The braincase of Spinophorosaurus was short from front to back and moderately deep, broad, and of relatively large size overall. It differed significantly form those of other Jurassic sauropods, possibly except Atlasaurus. They shared a basipterygoid process on their basisphenoids that were similar in being directed strongly backwards. A CT endocast of the brain cavity showed that it was similar to most sauropods in having pontine and cerebral flexures that were well-marked, a pituitary fossa that was large and oblong, and in that the structure of the brain was obnscured by meninges that were relatively thick and dural venous sinuses. The labyrinth of the inner ear was characteristic in having long and slender semicircular canals, similar to those of Massospondylus and Giraffatitan. Its neuroanatomy was in some ways intermediate between that of basal sauropodomorphs and that of neosauropods.

Vertebrae and ribs

Vertebrae and clavicle (D)

The vertebral column is almost completely known, and Spinophorosaurus is one of the few sauropods preserving a complete neck. The neck was composed of 13 cervical vertebrae, and the trunk of 12 dorsal and 4 sacral vertebrae. The tail comprised more than 37 caudal vertebrae. Complex elements, individual vertebrae are composed of a lower part, the centrum, and an upper part, the neural arch. Important landmarks of the neural arch include the upwards projecting neural spine (spinous process) and the sidewards projecting diapophyses, which together give the vertebra a t-shape in front and rear views. Pairs of articular processes connecting with neighboring vertebrae are protruding from the front (prezygapophyses) and rear (postzygapophyses).

The cervical vertebrae were similar to those of Jobaria and Cetiosaurus. They were moderately elongated compared to sauropods in general, but generally longer than in other basal forms. Their elongation index, the length-to-width ratio of the centrum, was approximately 3.1. The cervical centrum had large excavations on their sides that deepened towards the front; such pleurocoels were also developed in Jobaria and Patagosaurus. Unlike in Jobaria, the pleurocoels were not subdivided by a oblique bony ridge. A midline keel was present on the underside of the front end of the centrum, which is absent in Cetiosaurus. The tips of the prezygapophyses had a triangular extension which is also seen in Jobaria, although it is deeper in that genus. Above the postzygapophyses were comparatively large epipophyses, bony projections for muscle attachment. The cervical vertebrae were more different from those of basal sauropods from South America and India. The diapophyses (sidewards facing processes of the neural arch) were inclined to face slightly downwards and had triangular flanges on their rear margins—features unseen in those southern forms. Furthermore, the neural spines were rugose on their rear and front surfaces and, close to the base of the neck, broader in side view and less high. In side view, a U-shaped depression is present between the centrum and the neural arch, which is an autapomorphy of Spinophorosaurus.

Restoration

Although the front dorsal vertebrae showed deep pleurocoels in their centra, these openings became much shallower towards the rear of the trunk. The rearmost dorsal vertebrae were also proportionally short. In Amygdalodon and Patagosaurus, in contrast, the rearwards dorsals were more elongate and had pronounced pleurocoels. The neural canal of the dorsal vertebrae was very narrow but high. Hyposphene-hypantrum articulations (accessory articular processes) were present in all dorsals. The neural spines had marked rugosities on their front and back sides, as in other basal sauropods. The frontmost caudal vertebrae had the same rugosities on the neural spines that are seen in the dorsals, a feature otherwise only seen in Omeisaurus. In the hind part of the tail, neural spines are strongly inclined backwards and extend over the front part of the succeeding vertebra, similar to some East Asian sauropods, Barapasaurus, and Jobaria.

The ribs of the second to fifth dorsal vertebrae were flattened and backwards directed, while those of the sixth to eleventh dorsal were more circular in cross-section and more vertically oriented. The ribcage can therefore be clearly subdivided into a pectoral and a lumbar section; such a differentiation has only been described in a single other sauropod, the dicraeosaurid Brachytrachelopan. The ends of the pectoral ribs furthermore show attachment sites for the sternal ribs, which connect to the sternum. In the front part of the tail, the chevrons (paired bones below the vertebral centra) were blade-like, which is the primitive condition. In the rear part of the tail, however, the chevrons were rod-like, with the left and right counterparts separated from each other. These rod-like chevrons would have been closely attached to the bottom edges of the centra. They articulated with the preceding and succeeding chevrons at the midlength of the vertebral centra, thus bracing the vertebral joint, restricting bending of the tail.

Girdles and limbs

Spike-like osteoderm (A–C), girdle, and limb elements

The scapula (shoulder blade) was characterised by an broadened upper end and a bony flange extending from its rear margin. In these respects it was similar to mamenchisaurids from Asia, but different from the Gondwanan forms Vulcanodon, Barapasaurus, and Patagosaurus, in which the upper end was only weakly broadened, and the rear flange lacking. The coracoid, which articulated to the lower end of the scapula, showed a distinct kidney-shape, which is considered an autapomorphy. It had a large biceps tubercle to which the biceps brachii muscle attached. The clavicle was robust, although slenderer than in Jobaria. Of the forelimb only the humerus (upper arm bone) is preserved. The lower end of the humerus was asymmetrical and had enlarged enlarged accessory condyles (forward directed projections on the lower front margin of the bone)—features otherwise only seen in mamenchisaurids. The pubis and ischium of the pelvis were robust, with the latter being broadened at its end. The upper femur (thigh bone) was characterised by the presence of a lesser trochanter on its upper end—a bony projection serving as attachment site for muscles that draw the hind leg forwards and inwards. The fourth trochanter, which projected from the back surface and anchored muscles that draw the hind leg backwards, was especially large in Spinophorosaurus. Close to the fourth trochanter was a large opening that is absent in other sauropods, and thus an autapomorphy. The tibia (shinbone) was similar to other basal sauropods, and the fibula (calf bone) was robust. Of the ankle, the upper side of the astragalus hat facets for articulation with the tibia and fibula that were not separated by a bony wall, and as much as eight nutrient foramina (openings that allow blood vessels to enter the bone).

Tail spikes

A left and a right osteoderm (bone formed in the skin) was found with the holotype skeleton. These bones had a roundish base from which a spike-like projection protruded; the inner surfaces were rugose and concave. Although found within the pelvic region, they are thought to have been situated on the tip of the tail in the living animal, a distinguishing feature of the genus. Evidence for this position can be derived from the fact that the left and right elements were found closely together, suggesting that they came from near the midline of the body. Furthermore, the stiffening of the hind part of the tail by elongated chevrons is also observed in various other dinosaurs showing tail clubs or spikes. Similar spines were part of a tail club in the related sauropod Shunosaurus; such a tail club was, however, likely not present in Spinophorosaurus, as the hindmost caudal vertebrae became too small. The right osteoderm was somewhat larger than the left and slightly different in shape. This indicates that they did not form a pair, in which case they would probably be simply the mirror-inverted counterpart of one another. Rather, these differences indicate that two pairs of spines were present originally. In a 2015 conference abstract, Vidal and colleagues presented preliminary results of an ongoing restudy of the spines, suggesting that they were not osteoderms and might not have formed a thagomizer after all.

Classification

Dentary tooth of the paratype; the enlarged, spaced denticles near the top of the crown are a unique feature of this genus

The initial phylogenetic analysis presented by Remes and colleagues suggested Spinophorosaurus to fall among the most basal sauropods known, being only slightly more derived than Vulcanodon, Cetiosaurus, and Tazoudasaurus. In this position, it would form the sister taxon of Eusauropoda, a clade comprising all more derived sauropods. The authors noted, however, that support for this very basal position was weak, and discussed several alternative placements within eusauropods that would explain anatomical similarities with other sauropods from northern Africa and Laurasia. A similarly basal position outside of Eusauropoda was suggested by several subsequent studies, which recovered Spinophorosaurus as the sister taxon of Tazoudasaurus or Volkheimeria. In a 2013 conference abstract, however, palaeontologist Pedro Mocho and colleagues re-evaluated the phylogenetic relationships of the genus by incorporating additional information from newly prepared bones, arguing that Spinophorosaurus was nested within eusauropods. According to this analysis, the genus was more derived than Shunosaurus and Barapasaurus and close to Patagosaurus and mamenchisaurids. A much more derived systematic position within Eusauropoda was also proposed by a 2015 study, which found Spinophorosaurus to be the sister taxon of Nebulasaurus.

Below is a cladogram found by Nair and Salisbury in 2012 showing the position of Spinophorosaurus within Gravisauria:

Gravisauria	Vulcanodon Tazoudasaurus Spinophorosaurus Barapasaurus Rhoetosaurus Eusauropoda Shunosaurus Patagosaurus Omeisaurus Mamenchisaurus Turiasauria Losillasaurus Neosauropoda

Palaeobiology

CT scans of the holotype braincase, with endocast of the brain and inner ear below

Since Spinophorosaurus is one of the most completely known primitive sauropods, it is a good model for biomechanical studies. Manipulation of a 3D model of the holotype skeleton showed that the frontmost 20 tail vertebrae could deflect up to 20º individually before stopping, and 8-10º in sideways flexion, which indicates their range of movement were limited rather by soft tissue. Movement was limited by the large chrevrons, and in the hindmost part of the tail by the chevrons overlapping (as in dromaeosaurids or ankylosaurids). The function of these tail-vertebra features is unknown. The 3D model of the holotype skeleton shows a body body plan with inclination towards vertical lengthening, which is consistent with high browsing, but more testing of for example the animal's range of motion has to be done to support this idea.

Spinophorosaurus and some other sauropodomorphs did not have reduced vestibular apparatuses of the inner ear, tough this might have been expected in a lineage that lead to heavy, plant-eating quadrupeds. It is unknown why Spinophorosaurus retained this feature, but the size and morphology of sauropodomorph labyrinths may be related to for example neck length and mobility. It is possible that expansion of the vestibular apparatus is an indicator of the importance of vision and coordinated eye, head, and neck movements, though interpretation of sauropod vestibular features are still uncertain.

Palaeoenvironment

Congruence between Middle Jurassic sauropod distribution and paleoclimatic zones; ★ (middle) is the Spinophorosaurus type locality

It was recovered in a rock formation belonging to the lower part of the Argiles de l'Irhazer, the age of which is unknown but may be Middle Jurassic or older. Dromaeosaur tracks were found close to the Spinophorosaurus site.

See also

• Dinosaurs portal

External links

• The Dino Project – Google Arts & Culture photo series about the excavations in Niger