CidarisRevista Ilicitana de Paleontología y Mineralogía

G R U P O C U L T U R A L P A L E O N T O L Ó G I C O D E E L C H E

Núm. 302010

VIII Encuentro de Jóvenes Investigadores en PaleontologíaVOLUMEN DE ACTAS

Cidaris (2010).30 - VIII EJIP, pág. 299-304 299

DINOSAUR NEURAL SPINE ELONGATION AND DRY PERIODS, NEW EVIDENCES FROM THE ARCILLAS DE MORELLA FORMATION

ALARGAMIENTO DE LA ESPINA NEURAL EN DINOSAURIOS Y PERIODOS SECOS, NUEVAS EVIDENCIAS EN LA FORMACIÓN

ARCILLAS DE MORELLA

Andrés Santos-Cubedo1, Carlos de Santisteban2, Maite Suñer2 and Begoña Poza3

1Grup Guix, c/ Santa Lucia 75, E-12540 Vila-real, Castellón (ES), santos.cubedo@gmail.com2Departament de Geologia, Universitat de València, Av. Dr. Moliner s/n, E-46100

Burjassot, València (ES), carlos.santisteban@uv.es, maite.sunyer@uv.es3Consorci Ruta Minera, Carretera de Ribes, 20, E-08698 Cercs, Barcelona (ES), bego.poza@gmail.com

ABSTRACT

It is studied a caudal vertebra found at ANA site (Cinctorres, Castellón, Spain) with an elongated neural spine and its pos-sible relationship with some dry periods, or periods of marked aridity. These dry periods are documented in Arcillas de Morella Formation where the fossil was recovered, for example for the presence of calcareous paleosoils of the caliche type. In the case of the specimen 2ANA61, the presence of an elongated neural spine supposed that exist a hump. This hump has been related to an adaptation to locomotion (as counterbalance and attachment site for muscles) and/or to be energy storage depots (for sustenance during dry seasons or periods of migration, and for production of eggs in the case of females). This adaptation could help to survive this great ornithopod in poorly shaded paleoenvironments in which moisture and productivity were seasonally limited, as was the case of semideciduous arboreal biomes with prominent xeromorphic elements.

Keywords: Dinosaurs, neural spines, dry periods, ANA, Lower Cretaceous.

RESUMEN

Se estudia una vértebra caudal recuperada en el yacimiento ANA (Cinctorres, Castellón), con una espina neural alargada y su posible relación con periodos secos, o de marcada aridez. Estos periodos secos se encuentran documentados en la Formación Arcillas de Morella, donde se recuperó el fósil, por la presencia de paleosuelos calcáreos del tipo caliche. En el caso de 2ANA61, la presencia de una espina neural alargada supone la existencia de una corcova en el animal. Ésta se ha relacionado con una adaptación a la locomoción (como contrapeso y lugar de inserción de músculos) y/o también con depósitos de almacenamiento de energía (como reservas en épocas de sequía o períodos de migración, y para la produc-ción de huevos en el caso de las hembras). Esta adaptación permitiría sobrevivir a este gran ornitópodo en paleoambientes calurosos en el que la humedad y la productividad estacional estaría limitada, como es el caso de los biomas arbóreos semideciduos con elementos dominantes xeromorfos.

Palabras clave: Dinosaurios, espinas neurales, periodos secos, ANA, Cretácico Inferior.

1. INTRODUCTION

The presence of elongated neural spines in dinosaurs (Fig. 1) has been documented in several studies (Bailey, 1997; Rauhut et al., 2005). This elongation occurs not only in dorsal vertebrae and also it is found in cervical, sacral and caudal vertebrae. In a classic study on the pos-sible origin and function of this elongation and its com-parison with reptiles and mammals, Bailey (1997) bid by the presence of a hump in dinosaurs, similar to that of the

bisons, rather than sailbacks similar to that of the pelyco-saurs. Despite this work, it still makes some dinosaurs with elongated neural spines with dorsal, sacral, or caudal sailbacks, as in Jurassic Park 3. Bailey (op. cit.) believes that these humps can be adapted to: 1) energy storage, maintenance of gigantothermy, and heat-shielding in un-shaded habitats; 2) long-distance migration from feeding to nesting grounds across terrains of variable productivity; and 3) lipid conservation for production of large clutches of eggs at the nesting site.

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In this paper we study a caudal vertebra found at ANA site (Cinctorres, Castellón, Spain) with an elongated neu-ral spine and its relationship with some dry periods. The vertebra was classifi ed as belonging to a large ornithopod not determined.

2. METHODOLOGYFollowing the methodology from Bailey (1997), he-

rein we use the ratio of neural spine length (maximum elongation of the neural spine axis) to centrum height (height of the anterior articular surface) of the caudal vertebrae. Bailey (op. cit.) did not use the caudal ver-tebrae so we have calculated the range for the caudal vertebrae for several ornithopods. Results are shown in Fig 2.

3. MATERIALS2ANA61 is a caudal vertebra (Fig. 3), stored at Co-

lección Museográfi ca de Cinctorres (Castellón, Spain) and recovered in the second fi eld campaign at ANA site (Aptian, Lower Cretaceous). The centrum is platycoelous. The height of the anterior articular surface of the centrum is 133 mm. The sides of the centrum are concave antero-posteriorly, fl at dorsoventrally, slightly higher than long. The dorsal margin of the centrum has a strong transverse process, outward, horizontal and slightly inclined bac-kwards. The neural arch prezygapophyses have a wing shape. They are advanced beyond the level of the ante-rior surface of the centrum. The postzygapophyses are less developed and they slightly overhang the posterior surface of the centrum. In the ventral part of the centrum

there are two articular faces for the chevrons. The neural spine is long, backward inclined, transversely fl attened, with anterior and posterior edge thin. In cross section it is oval-shaped. The neural spine widens slightly toward its distal end. This end has a thickened edge. It is important to remark that this vertebra has a long neural spine (390 mm from the centrum to the top). The characteristics of this vertebra seem to indicate that this is one of the last anterior caudal vertebrae.

The morphology of caudal vertebrae of many ornitho-pods is similar, which does not permit to differentiate spe-cies only with a caudal vertebra (Taquet, 1976; Norman, 1980; Norman, 1986; Weishampel and Bjork, 1989). In lack of another material, the characteristics present in this fossil allow to determine it as Iguanodontia indet.

At the same formation in which 2ANA61 has been re-covered, other iguanodontoids have been found (Royo y Gómez, 1920; Suñer et al., 2008). Caudal vertebrae of two of these species have been compared with 2ANA61 (Fig. 2). 2ANA61 falls within the range given in Fig. 2 for Mantellisaurus. However, this species corresponds to a medium-sized iguanodontoid, but 2ANA61 belongs to a large ornithopod. For example, the 4th caudal verte-bra of Mantellisaurus specimen IRSNB 1551 (Norman, 1986) has a length of 79 mm, width of 79 mm and height of 84 mm, and the 12th has a length of 72 mm, width of 62 mm and height of 68 mm. 2ANA61 has a length of 124 mm, width of 158 mm and height of 133 mm. In the absence of more diagnostic characters, the size difference is not a character that drives the allocation of the species 2ANA61 to Mantellisaurus atherfi eldensis, or to another iguanodontoid.

4. GEOLOGY AND CLIMATOLOGYSediments from the Arcillas de Morella Formation

were deposited on the northern margin of the Maestrat Basin. This basin was one of several sedimentary basins developed in the Iberian. Through, in the eastern part of the Iberian plate, during the Mesozoic (Caja et al., 2005).

Figure 1. Some long-spined dinosaurs. Modifi ed from Bailey (1997).

Figure 2. Ratio of neural spine length to centrum height in caudal anterior vertebrae of some ornithopods. The range of variation is calculated from the fi rst fi fteen caudal vertebrae. From the data published in: Parks, 1922; Ostrom, 1970; Taquet, 1976; Maryanska and Osmólska, 1981 and Norman, 1986.

SystematicsTheropoda Acrocanthosaurus atokensis Spinosaurus aegyptiacus Suchomimus tenerensis Baryonyx walkeri Metriacanthosaurus parkeri Yangchuanosaurus shangyouensis Yangchuanosaurus magnus Becklespinax altispinax

Sauropoda Dicraeosaurus hansemanni Dicraeosaurus sattleri Brachytrachelopan mesai Amargasaurus cazaui Rebbachisaurus garasbae Rebbachisaurus tamesnensis Apatosaurus louisae

Ornithopoda Mantellisaurus atherfi eldensis Ouranosaurus nigeriensisOrnithopoda (Continuación) Gryposaurus incurvimanus Saurolophus angustirostris Bactrosaurus johnsoni Hypacrosaurus altispinus Hypacrosaurus stebingeri Barsboldia sicinskii

Lambeosaurus lambei Parasaurolophus walkeri Parasaurolophus cyrtocristatus Corythosaurus casuarius Tenontosaurus tilletti Tsintaosaurus spinorhinus

Ceratopsia Protceratops andrewsi Montanoceratops cerorhynchus

Stegosauria Stegosaurus armatus Stegosaurus stenops Wuerhosaurus homheni

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Others paleohabitats where Ouranosaurus, Spinosaurus, Hypacrosaurus or Tenontosaurus, among others, were found, had seasonal climates, with a dry summer season (Bailey, 1997). For example, Wealden deposits of Surrey (UK) had vegetation adapted to a seasonal climate of al-ternating wet and dry periods, including periods of mar-ked aridity (Sladen and Batten, 1984; Allen, 1998; Nye et al., 2008). In fact, Wright et al. (2000) noted that the abundance of fossil charcoal and the absence of in situ peat was the evidence of seasonal aridity and wildfi res.

In these areas, southern of the nemoral zone, as in the Early Cretaceous of Els Ports, there were semideciduous arboreal biomes with a prominent xeromorphic element. It should be noted that many ornithopods (“hypsilophon-dontids”, dryosaurids, camptosaurs and iguanodontoids) had in the central part of the mouth a battery of teeth with the ability to grind plant fi bers (Norman and Weishampel, 1985; Weishampel and Norman, 1989). They also had a special mechanism known as Pleurokinetic hinge sys-tem, that allows crushing xeromorphic plants. In fact, the xeromorphic cycadophyte–brachyphyllous shrublands expansion over the Mesozoic drylands coincided chro-nologically with the appearance of the giant dinosaurian herbivores and was probably enhanced by overbrowsing and deforestation (Nye et al., 2008).

In the case of Tereschenko (2008) he wrote that some Protoceratopoids might have some adaptations for swi-mming. This idea was proposed in a paper by Brown and Schlaikjer (1940). Tereschenko (2008) proposed a

Red clays, grey marls and mudstones and sandstones mainly compose the deposits of this unit. According with Santisteban and Santos-Cubedo (2008 and references the-rein) they were formed in a wave dominated delta. This delta had a similar extent, as the modern Ebro delta (eas-tern Spain), which present extensive beaches and spits along the coast. The sedimentary record of the Arcillas de Morella Formation is composed of four to six sequences with a transgressive - regressive character. Transgressive deposits contain marine marls and tropical organic car-bonates. Regressive deposits are formed mainly by con-tinental red-beds containing calcareous paleosoils of the caliche type. Trangressive deposits are placed inside of incised valleys formed during regressions (Santisteban and Santos-Cubedo, 2008; Santisteban and Santos-Cube-do, 2009). These sequences are due to relative sea-level changes on the order of few meters to thirty meters. The changes in the depositional environments, from marine tropical to continental semi-arid (Mediterranean type), also refl ect climate changes. There is no evidence that existed during the Mesozoic permanent ice at the poles, and although temperatures were higher than at present (at least in the oceans). There was a zoned climate by latitude with seasonal variations. In fact Tethys seas would have sustained a low latitude anticyclone that made the Me-so-Mediterranean zone drier than its modern equivalent (Krassilov, 2003). This is demostrated by the fossil fl ora record. South of the nemoral zone (Fig. 4), there is a semi-deciduous arboreal biomes with a prominent xeromorphic element. The climatic conditions were warm-temperate to subtropical, of the Mediterranean type, with a dry summer season (Krassilov, op. cit.).

5. DISCUSSIONBailey (1997) described some functional and biome-

chanics features for the humps in dinosaurs. He bid by the presence of a hump in dinosaurs, similar to that of the bisons, rather than sailbacks similar to that of the pelycosaurs. Tereschenko (2008) refutes these ideas and suggests that the increased height of the neural spines of protoceratopoids was connected with the expansion of the area for the attachment of epaxial muscles of the tail, and this is an indicator of adaptation of protoceratopoids for swimming.

We will analyze both cases and compared with 2ANA61.

In the case of Bailey (op. cit.) more interesting data in his work are the relationships between paleohabitats and humped dinosaurs. For this author several of the hum-ped taxa (Fig. 1) show a suggestive association with po-orly shaded tropical to subtropical paleoenvironments in which moisture and productivity were seasonally limited. We found in Arcillas de Morella Formation evidences of long periods of semi-arid climate, as the presence of cal-careous paleosoils of the caliche type. The presence in this formation of dry periods and humped dinosaurs suggested the correlation of these structures with unshaded habitats.

Figure 3. 2ANA61, Iguanodontia indet., caudal vertebra with elongated neural spine in left lateral view. Scale bar = 5 cm.

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mostly aquatic mode of life for Bagaceratops, a semia-quatic for Protoceratops, facultatively aquatic for Uda-noceratops and predominantly terrestrial for Leptoce-ratops. He analyzed some morphological characteristics, the mobility of the vertebral column and the probable adaptive signifi cance of these features. This idea is dis-puted by Bailey (1997) and presents some problems, like the incipient fl exibility of the tail, the absence of deve-loped transverse processes for the attachment of caudo-femoral muscles or that these dinosaurs have been found in a semi-arid basin. However, isotope studies could be applied to these Protoceratopoids, to see if they had aquatic habits. This technique has been applied recently by Amiot et al. (2010) in which they proposed amphi-bious habits for spinosaurid theropods. They proposed that their apparent lack of anatomical adaptation to aqua-tic habits is the result of a thermoregulatory strategy. Although Amiot et al. (2010) proposed for some African spinosaurs to have a more opportunistic habitat use by alternating aquatic and terrestrial life. Under this con-text that these animals spent long periods in water, and being homeotherms (mammals and theropod dinosaurs are both considered as homeotherms; Luck and Wright, 1959; Fricke and Rogers, 2000; Amiot et al., 2006), it is better sailbacks or dorsal humps?. Immersed in water, fat humps help to maintain body heat (heat shields), while sailbacks increasing relative body surface to the volume, functioning as a bad thermoregulator.

6. CONCLUSIONS

Sailbacks or dorsal humps in dinosaurs?. Whether for the attachment of epaxial muscles or for adipose tissues, it is more likely that most of the dinosaurs with elongated neural spines of Fig. 1 had humps.

In the case of the specimen 2ANA61, the possible functions of an enlarged tail neural spine would be: 1) lo-comotion: counterbalance and attachment site for the cau-dofemoralis longus and the extensor caudae lateralis; 2) energy storage depots for sustenance during dry seasons or periods of migration; 3) lipid conservation for produc-tion of large clutches.

In fact, 2ANA61 was a large ornithopod with a feeding mechanism capable of crushing the xeromorphic vege-tation. Probably part of this energy processed would be stored as fat in the tail, forming a tail hump. Thus, to pos-sess reserves of fat in the tail, allows this large ornithopod from ANA site to survive dry periods and even migration processes in search of food or breeding and nesting sites. These dry periods are recorded in Arcillas de Morella For-mation as calcareous paleosoils of the caliche type.

However, we do not know if this elongation of the cau-dal neural spines in ANA specimen were present in the sacral and dorsal vertebrae, because these elements have not been hitherto recovered from the site.

7. ACKNOWLEDGMENTSF. Ortega (UNED, Madrid) and J.I. Ruiz-Omeñaca

(Museo del Jurásico de Asturias, Colunga) kindly pro-vided helpful comments on the manuscript. This work has been carried out in the framework of the projects: CGL2008-06533-C03/BTE and 2009-SGR-59.

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