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Ameghiniana

versión impresa ISSN 0002-7014

Ameghiniana vol.47 no.1 Buenos Aires ene./mar. 2010

 

ARTÍCULOS ORIGINALES

First data on dinosaur eggs and clutches from Pinyes locality (Upper Cretaceous, Southern Pyrenees)

Bernat Vila1, Frankie Jackson2 and Àngel Galobart1

1Institut Català de Paleontologia, Edifici ICP, Campus de la Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Barcelona. bernat.vila@icp.cat, angel.galobart@icp.cat
2Dept of Earth Sciences, 211 Traphagen Hall, Montana State University, Bozeman, MT 59717. frankiej@montana.edu

 


Abstract. We describe a new megaloolithid locality discovered near Coll de Nargó (Catalonia, northeastern Iberian Peninsula). Eggs at the Pinyes site occur within extensively developed, vertically-stacked paleosols of an alluvial system. Eight clutches were excavated and mapped and biological and taphonomic aspects recorded. Three-dimensional maps were constructed using a Trimble total station, supplementing traditional planview mapping to allow more accurate interpretation of egg and clutch distribution. The Megaloolithus siruguei eggs are compressed, presenting an ellipsoidal shape. The three-dimensional geometry of two large clutches suggests that the eggs were laid in shallow, bowl-shaped depressions. Tangential thin sections of eggshells also reveal abundant pores, consistent with incubation in a high humidity/ low oxygen environment. These data support previous inferences that M. siruguei eggs were buried in shallow pits.

Resumen. Primeros datos sobre huevos y puestas de dinosaurios de la localidad de Pinyes (Cretácico Superior, sur de los Pirineos). Se describe una nueva localidad con huevos del tipo megaloolithido cerca de Coll de Nargó (Cataluña, noreste de la Península Ibérica). Los huevos en el yacimiento de Pinyes se hallan dentro de paleosuelos extensa y verticalmente desarrollados en un sistema aluvial. Se excavaron y cartografiaron ocho puestas y se registraron aspectos biológicos y tafonómicos. Se construyeron cartografías tridimensionales con una estación total Trimble, complementando así las cartografías en planta tradicionales para permitir una interpretación más adecuada de la distribución de huevos y puestas. Los huevos Megaloolithus siruguei están comprimidos, presentando una forma elipsoidal. La geometría tridimensional de dos grandes puestas sugiere que los huevos fueron depositados en depresiones de poca profundidad en forma de bol. Las secciones delgadas tangenciales de las cáscaras también revelan abundantes poros, consistentes con una incubación en un ambiente de alta humedad/baja concentración de oxígeno. Estos datos confirman las inferencias que afirmaban que los huevos M. siruguei eran enterrados en hoyos de poca profundidad.

Key words. Megaloolithidae; Eggs; Clutches; 3D reconstruction; Pyrenees; Excavated pits.

Palabras clave. Megaloolithidae; Huevos; Puestas; Reconstrucción en 3D; Pirineos; Hoyos excavados.


 

Introduction

The south Pyrenean mountain range of the north-eastern Iberian Peninsula contains excellent outcrop exposures of Upper Cretaceous terrestrial sediments and a rich fossil record of dinosaur faunas (Casanovas et al., 1987; López-Martínez et al., 2001, Vila et al. 2006a, among others). From the mid 1950s, discoveries in the Tremp basin in Catalonia included dinosaur bones, track horizons, and fossil eggs (Lapparent, 1958). Beginning in the late 1980s, the importance of the south-Pyrenean basins for studies of the Cretaceous extinction event brought renewed interest in the fossil egg localities. The Late Cretaceous Tremp and Arén Sandstone formations comprise more than 40 localities that yield important information on egg microstructure, paleobiology, and biochronology (Moratalla, 1993; López-Martínez, 2003; Bravo et al, 2005; Sander et al., 2008). In situ eggs and clutches were mapped from at least six localities: Basturs (Sanz et al., 1995; Sander et al., 1998); Coll de Nargó (Sander et al., 2008); Suterranya (López-Martínez, 1999); Biscarri (López-Martínez et al., 2000); Faidella (Bravo et al., 2000); and Font del Bullidor (Vila et al., 2009b). However, traditional two-dimensional maps that illustrate these sites provide only partial information on the three-dimensional architecture of the clutches. In 2003, the Pinyes megaloolithid nesting locality was discovered in out-crops approximately 20 km west of the village of Coll de Nargó, Lleida Provence, Spain (Escuer et al., 2003). Here, we present preliminary data on our study of the Pinyes locality. This work was originally presented at the Third International Symposium on Dinosaur Eggs, Babies, and Developmental Biology, Plaza Huincul, Neuquén, Patagonia, Argentina, Abril 1-15, 2006.

Geological setting

The Pyrenean mountain belt formed as a result of Late Cretaceous to late Oligocene crustal contraction, due to the collision of the European and Iberian plates (Vergés et al., 2002). After the initial stages of collision, two major thrust systems and foreland basins (northern Aquitanian and southern Ebro basins) developed, one on either side of a central double-wedge of basement rocks. The southern thrust systems produced various structural features (e.g., Tremp, Àger-Fontllonga, Coll de Nargó and Vallcebre synclines; figure 1.1) where dinosaur fossil bearing formations are now exposed. The Arén Sandstone is transitionally overlain by the Tremp Formation; together, these units deposited during late Campanian and Maastrichtian time record the progressive regression from marine to lagoonal settings, and finally to entirely continental environments.

Figure 1. 1, Regional geologic map of Pyrenees showing Upper Cretaceous-Paleocene outcrops; 2, stratigraphic section at Pinyes locality, highlighting four egg beds. / 1, Mapa geológico regional de los Pirineos mostrando los afloramientos del Cretácico Superior; 2, sección estratigráfica en la localidad de Pinyes, destacando las cuatro capas con huevos.

The Tremp Formation in the Coll de Nargó area

Continental Upper Cretaceous-lower Paleocene sediments comprise the Tremp Formation (Mey et al., 1968), a formation historically referred to as the "Garumnian" facies of the southern Pyrenees (see review in Rosell et al., 2001). The Tremp Formation can be subdivided into two major units. The first unit attains a maximum thickness of 500 meters. The lower portion consists of thin coals, calcareous mudstones, and limestones. The colour of these facies changes from grey to red up-section. The mudstones intercalate with small to medium-sized sandstone bodies (< 4 m thick) and include mottling, caliche nodules, and evidence of extensive bioturbation. These deposits are interpreted as sediment that accumulated within lagoon, marsh, and fluvial environments (Díaz Molina, 1987). In the Coll de Nargó area the lower unit of the Tremp Formation yields abundant eggs and clutches (Sander et al., 1998, 2008; Peitz, 2000; Escuer et al., 2003; Panadés i Blas, 2005). The Pinyes locality falls within the lower middle portion of this unit.
The second and upper unit of the Tremp Formation includes the lacustrine Vallcebre Limestone, mudstones, sandstones, conglomerates and other limestones deposited in various continental environments. These strata also exhibit mottling, bioturbation, and caliche nodules. Dates for the lower Tremp Formation vary from late Campanian to entirely Maastrichtian, although most workers consider the upper portion of the formation as early Paleocene
in age. Below, we report preliminary sedimentologic and taphonomic data from the 2005 excavation at the Pinyes locality.

Methods and materials

Field data acquisition

Eight egg sites containing one or more clusters were documented and/or collected. Excavation consisted of egg exposure and identification of discrete egg clusters. Schematic maps were produced by traditional mapping methods, using metric grid and graph paper. In addition, five of these sites were mapped with a high-accuracy Trimble total station (table 1). Taphonomic data for each egg cluster was recorded in plan and cross sectional views (e.g., contact between eggs, eggshell orientation, egg arrangement). Eggshells were removed from an egg at site 18E02 and half of each freshly broken specimen was coated with 10 nm of gold and examined by scanning electron microscope (SEM). The remaining half of the eggshell was prepared as a radial thin section (30 µm thick) and studied by light microscopy.

Table 1. List of documented and interpreted clutches at Pinyes locality. / lista de las puestas documentadas e interpretadas en la localidad de Pinyes.

Processing data

The dataset generated by the Trimble total station consists of more than 800 points (x, y, z coordinates) from five egg clusters. Using Rhinoceros® software, we designed a 3-D model of egg distribution, augmented by field observations of egg size and shape. Eggs were modelled as ellipsoids and placed in the corresponding outline sections recorded by the total station (see detailed information in Fortuny et al., 2007). A surface for the bedding plane was created by joining the topographic points, thereby illustrating the true dip of the layer. This 3-D model allows visualization of the position of each egg from any perspective.

Results

Sedimentology and facies analysis

Three lithofacies are present in the 18 m-thick measured stratigraphic section (figure 1.2): (A) laterally extensive, massive calcareous silty mudstones, (B) massive, very fine- to fine-grained sand bodies, and (C) a medium to coarse-grained, parallel stratified sandstone. Repeated intervals of facies A and B occur within the lower 10.5 m of the section, with the thickness of the sandstone units generally decreasing in an upward direction. Extensive bioturbation, macroscopic fine root traces, sparse organic material, and abundant overlapping, branching and un-branched pedotubules are common in Facies A and B (figures 3.1 to 3.5). An erosional contact separates the uppermost 6.5 m-thick mudstone of facies A from overlying medium- to coarse-grained sandstone (facies C) that preserves primary sedimentary structures.


Figure 3. Pedogenic features. 1, Bifurcating tubules with grey mottling in red mudstone. Note egg in lower right corner of photo; 2, corkscrew-shaped rhizolith; 3, complex intersecting pedotubules; black arrow indicates cross-cutting relationship of structures, and white arrow shows meniscate burrow, scale bar equals 1 cm; 4, meniscate burrow exhibiting a pelleted appearance; 5, cross-cutting burrow with arcuate structure; 6, black arrows indicate burrow that follows egg conture, note similar mottling inside egg; 7, egg removed from a clutch during excavation, note lower surface, relative to bedding plane, exhibits tubular structure / caracteres pedogénicos. 1, Túbulos bifurcados con moteado gris en arcilla roja, obsérvese el huevo en el extremo inferior derecho de la fotografía; 2, rizolito en forma de sacacorchos, 3, pedotubulos complejos e intersectados, la flecha negra indica el cruce de las estructuras y la flecha blanca muestra una huella de excavación meniscada; barra de escala: 1 cm; 4, huella de excavación meniscada exhibiendo una apariencia de pellet, 5, huella de excavación cruzada con estructura curvada; 6, las flechas negras indican la huella de excavación que sigue el contorno del huevo, nótese el moteado similar dentro del huevo; 7, huevo extraído de una puesta durante la excavación, nótese la superficie inferior, en relación al plano de capa, exhibiendo estructura tubular.

Facies A consists of brownish grey to chocolate-brown, silty mudstone; the weathered exterior appears dark red in colour. These massive mudstone units throughout the section retain no evidence of sedimentary structures and are characterized by ubiquitous bioturbation. Extensive mottling varies in colour from blue-grey or grey to yellow-brown. Small (~ 1.0 cm), dispersed calcareous nodules occur within most mudstone facies. In contrast, a marked absence of calcareous nodules distinguishes the mudstone unit that occurs from 10.6 to 14.8 m within the section (figure 1.2). Stratigraphically upwards from 14.8 m to the erosional contact with the overlying sandstone, the strata are characterized by increasingly abundant and larger caliche nodules (~2.0 mm).
Facies B consists of massive, highly bioturbated, very fine to fine-grained red sandstone; the thickness and continuity varies laterally, with units thinning in an upward direction within the section. Approximately 6.5 and 8.5 m above the base of the section, these sandstones contain calcium carbonate
nodules (~1 cm) that are similar in size to those present in most mudstone facies at this locality. Contacts between facies A and B are generally gradational, and these units display similar colour, texture, and extensive bioturbation. The burrows occur both within the sediment-filled fossil eggs and the surrounding mudstone. Egg clutches are present within or on the upper surfaces of the facies B units.
Facies C is a fining-upward, medium to coarse-grained, poorly-sorted, dark brown sandstone. This sandstone is separated from the final mudstone in the sequence by an erosional contact, approximately 17 m above the base of the section. The base of the sand-stone includes mud rip-up clasts, small carbonate nodules, and abraded eggshell fragments. The sand-stone exhibits parallel stratification and rare, faint ripple cross-laminations. Primary sedimentary structures are occasionally disrupted by minor bioturbation. The well-cemented calcareous sandstone contains moderately abundant scattered fossil eggshell fragments, minute macerated plant remains, and vertical to sub-vertical, bifurcating, three-dimensional pedotubules.

Egg horizons

Rocks at the Pinyes locality exhibit a penetrative foliation in a NE-SW direction and dip steeply to the north at 30 degrees. The steep dip of the beds contributes to rapid erosion of the fine-grained deposits. Although truncated by erosion, the egg horizons thin to the east but are laterally traceable for approximately 1 km, until covered by vegetation. At least four egg horizons occur within the measured section, at approximately 1.5, 5.0, 6.2 and 7.5 meters above the base of the outcrop (figure 1.2, table 1).

Eggs

The Pinyes eggs are referable to oospecies Megaloolithus siruguei Vianey-Liaud et al., 1994 based on the eggshell microstructure (figure 2.1). Abundant pores characterize the eggshells (see Jackson et al., 2008 for further details). The long axes of the eggs typically vary between 16 to 24 cm in length whereas the short axes range from 8 to 20 in length (figures 2.2, 2.3). As a consequence, most specimens exhibit an elliptical to subcircular outline in plan view (figure 4.3). The mean long axis direction of 41 eggs is N42º and therefore consistent with tectonic stress orientation in the region.


Figure 2. Eggshells, eggs and clutches at Pinyes. 1, Radial section of Megaloolithus siruguei eggshell (scale bar: 100 μm); 2, ellipsoidal egg from Pinyes (scale: 10 cm); 3, Clutch 2 17E04 cluster (scale: 10 cm grid). / cáscaras, huevos y puestas en Pinyes. 1, Sección radial de una cáscara de Megaloolithus siruguei (barra de escala: 100 μm); 2, huevo elipsoidal de Pinyes (escala: 10 cm.); 3, puesta del punto 17E04 (escala: cuadrícula de 10 cm.).


Figure 4. Eggs and clutch distribution. 1-3, Clusters and clutches; 4, three-dimensional reconstruction of 17E04 site; 5, lateral view of 17E04 egg distribution with interpreted bowl-shaped geometry. Arrows indicate in situ egg clutches. Scale bar: 1 m. / distribución de huevos y puestas. 1-3, Acumulaciones y puestas; 4, reconstrucción tridimensional del punto 17E04; 5, vista lateral de la distribución de huevos en el punto 17E04 con la geometría interpretada en forma de bol. Las flechas indican puestas de huevos in situ. Barra de escala: 1 m.

Egg clusters

Egg clusters mapped in situ preserved between 2 and 28 eggs (see table 1), with most containing between 8 and 11 eggs; however, two clusters (17E02; 17E06) contained 5 or fewer eggs. Laboratory preparation and new fieldwork also revealed that some clusters (e.g., 17E06, 18E01, 18E02) contained additional eggs not previously identified in field. These clutches require further preparation in order to accurately assess geometry and clutch size. In plan view, the eggs in most of clusters appear to be grouped in small accumulations (see arrows in clusters 17E04 and 17E05; figure 4), although some eggs show a more random distribution. Interestingly, some large accumulations (e.g., 17E04 in figure 4.3) display an elongated or linear arrangement, a pattern also observed in cluster 18E04 (figure 4.1). In most clusters the eggs occur in close contact with one another, and at least four clusters (18E01, 18E02, 17E04, 17E06) show superimposed specimens. None of the eight clusters preserve fossil plant material. Finally, clusters at five sites (18E01, 18E02, and 18E04; 17E04 and 17E05) contain from one to possibly four intact eggs.

Discussion

Depositional setting of the nesting sites

The Pinyes section records deposits of a fluvial environment, located some distance from an active stream channel. The facies consist of highly bioturbated, calcareous mudstone (facies A), incised by laterally discontinuous, bioturbated, small- to medium-sized (< 5 m thick) channel fill or crevasse splay deposits (facies B). The thick, red, clayey profiles of facies A and B preserve bifurcating rootlets, corkscrew-shaped rhizoliths (figure 3.2), and drab-coloured, tapering tubular structures that occasionally contain plant remains. These overbank deposits are characterized by abundant Spirographites ellipticus Astre, 1937 traces, interpreted as sediment reworking by arthropods (Mayoral and Calzada, 1998). Deposits of the uppermost three meters of the section accompanied increasingly drier conditions that produced more abundant and larger caliche nodules within the soil profile. This may be linked to the absence of eggs. Finally, the fining upward, medium- to coarse-grained, parallel-stratified sandstone (facies C) that overlies the sequence is interpreted as a channel sand deposit. The basal lag layer of mud chips, caliche nodules, and dispersed eggshell fragments indicate reworking of floodplain sediments.

Eggs and clutches

Six of the eight documented clusters are distinct and clearly separated from others (17E02, 17E03, 17E06, 18E01, 18E02, 18E04 and figure 4.1). Because of the close contact of the eggs within the cluster, their arrangement, and the depositional setting, we interpret these groups as in situ egg clutches, representing oviposition by a single individual. In contrast, some clusters (17E04 and 17E05) include larger egg accumulations, comprised of possible subgroups of 8 to 20 eggs (see figures 4.2, 4.3, 4.4). The large sub-group in 17E04 (and possibly other sites) has similar geometry as 18E04; therefore, we interpret 17E04 and 18E04 as complete clutches.
Due to the absence of lithologic evidence of a nesting trace such as truncation of primary sedimentary structures, a raised rim, or compositional differences in sediment surrounding the eggs compared to the nest structure (Varricchio et al., 1999; Chiappe et al., 2004), most megaloolithid localities provide little evidence of nest architecture. Typically, a nest interpretation has been based on plan view and cross sectional maps (Kérourio, 1981; Sanz et al., 1995; Sander et al., 1998, 2008; Bravo et al., 2000; Mohabey, 2005, among others). With the exception of Cousin and Breton (2000) and Vila et al. (2009b), very few studies of European megaloolithid eggs include complete excavation and detailed mapping that is necessary to demonstrate clutch and nest structure. In contrast, high-resolution 3-D documentation of the location of individual eggs within the clutch can better define clutch architecture and allow inference of nest geometry from the partially exposed specimens. Data points recorded around each exposed egg with the Trimble total station at site 17E04 allows correction for dip and identification of multiple, superimposed egg horizons (figure 4.5). Furthermore, the superimposed eggs that comprise at least two layers in 17E04 form a concave-up outline in cross section, indicating that the eggs were laid in a shallow, elongated bowl-shaped depression. These superimposed eggs and the clutch geometry are far less apparent in traditional, two-dimensional maps. Because of the steep dip of the beds, typical of mountainous regions, the excavation often intersects the bedding plane, there-by obscuring the true clutch geometry.

Egg incubation mode

At the Pinyes site two lines of evidence strongly suggest that clutches occur within shallow, excavated pits and egg incubation took place underground. First, the above-mentioned geometry suggests a pit-like structure excavated in the substrate and second, the high water vapour conductance rate calculated for a Pinyes egg (Jackson et al., 2008). The later is consistent with a high-humidity/ low oxygen incubation environment, thus supporting previous inferences of underground incubation for European megaloolithid eggs (Deeming, 2006 and references therein).

Spatial clutch distribution

Eroded gullies at the Pinyes locality truncate the laterally continuous egg horizons, thus providing little opportunity to measure the distance between clutches. In addition, the precise stratigraphic position of the clutches is difficult to determine because of the homogeneous nature of the enclosing sediment. Further, whether clutches occur on the same or slightly different paleosurfaces within the egg-bearing horizon is also difficult to assess with two dimensional maps. However, data from the Trimble total station allows greater accuracy in correlating between adjacent clutches, and five clutches (17E02, 17E03, 17E04, 17E05, 17E06) most likely occur at the same stratigraphic level. Where measurable, distances between these clutches vary from 3 to 6 m. Clear, unambiguous identification of nesting surfaces is critical for interpretation of reproductive behaviour. For example, small differences in vertical stratigraphic position may represent hundreds or perhaps thousands of years that separated the nesting events. Therefore, interpretation of gregarious nesting of animals at a site is not possible from two-dimensional maps. However, restoring the steeply dipping strata to horizontal orientation and locating clutches accordingly can differentiate paleosurfaces that may represent nesting events over a more restricted time interval.

Conclusions

Preliminary conclusions regarding the sedimento-logic and taphonomic characteristics documented at the Pinyes locality near Coll de Nargó are as follows:

1) Eggs occur within extensively developed, vertically-stacked paleosols in a fluvial setting on the floodplain, at some distance from an active stream channel. At least four egg-bearing horizons occur within the lower eight meters of the study area, and the eggs are referable to oospecies Megaloolithus siruguei.
2) Eight egg sites containing one or more clutches were mapped using both traditional mapping and high-accuracy documentation provided by a Trimble total station. Construction of a 3-D model allows visualization of egg and clutch position, relative to the bedding plane, providing greater accuracy for interpretations of reproductive biology, compared to traditional 2-D mapping.
3) Most clutches contain 8-11 eggs, and some large clutches may include up to 20 eggs. The superimposed eggs and clutch geometry indicate that the eggs were laid in shallow excavated pits. Tangential thin sections also reveal abundant pores, consistent with incubation in a high humidity/ low oxygen environment (Jackson et al., 2008).
4) Egg shape was significantly affected by the tectonic setting, resulting in their ellipsoidal shape.

Acknowledgements

Our special thanks go to J. Fortuny and the 2005 field crew, especially J. Peralba, R. Jackson, R. Gaete, A. M. Bravo and members of the ADAU team. Thanks also to L. Salgado and C.M. Magalhaes Ribeiro for their reviews. Funding is provided by CGL2005-07878-C02-01, 02 (Ministerio de Educación y Ciencia) and CGL2008-06533-C03-01/BTE (Ministerio de Ciencia e Innovación), the grants from the Servei d'Arqueologia i Paleontologia (Departament de Cultura i Mitjans de Comunicació de la Generalitat de Catalunya), projects and grants to FJ from the Jurassic Foundation and the College of Letters and Sciences, Montana State University-Bozeman.

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Recibido: 19 de noviembre de 2008.
Aceptado: 5 de septiembre de 2009.