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Ameghiniana

versión impresa ISSN 0002-7014

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

 

ARTÍCULOS ORIGINALES

The "Tustea Puzzle": hadrosaurid (Dinosauria, Ornithopoda) hatchlings associated with Megaloolithidae eggs in the Maastrichtian of the Hateg Basin (Romania)

Dan Grigorescu1

1University of Bucharest, Faculty of Geology and Geophysics, Laboratory of Paleontology, 1 Blvd. Nicolae Balcescu, 010041 Bucharest, Romania, dangrig@geo.edu.ro

 


Abstract. Since 1991 when the coinciding occurrence of megaloolithid eggs and hatchlings of the hadrosaurid Telmatosaurus transsylvanicus was suggested by few limb bone remains found close to the first discovered eggs in the Hateg Basin, tens of eggs grouped in clutches together with neonate bones were unearthed within the place that become "Tustea nesting site". In the meantime the original vertical escarpment near the village of Tustea was leveled and converted into a horizontal platform. By their external characters and eggshell microstructure all the eggs belong to the Megaloolithus oogenus, closer to M. siruguei; similarly, all the baby remains whose state of preservation allows their taxonomic assignment belong to Telmatosaurus transsylvanicus, one of the common dinosaur species from the rich Maastrichtian faunal assemblage of the Hateg Basin (Romania), and the basalmost hadrosaurid. Most of the baby remains consist of isolated bones and teeth, but partial skeletons with articulated elements were also found. The preliminary ontogenetic study reveals a rapid growth of the hatchlings, a fact also supported by the discovery of most of the skeletal remains outside the egg clutches, but in their close vicinity. In few cases bones were found within the egg clutches and even inside the egg. It is worth to mention that no sauropod remains were found within the nesting horizon from Tustea.

Resumen. El "Enigma de Tustea": pichones de hadrosáuridos (Dinosauria, Ornithopoda) asociados con huevos Megaloolithidae en el Maastrichtiano de la Cuenca Hateg (Rumania). Desde 1991, cuando la coincidencia de la aparición de huevos megaloolítidos y pichones del hadrosáurido Telmatosaurus transsylvanicus fue sugerida sobre la base de unos pocos huesos apendiculares encontrados en proximidad a los primeros huevos descubiertos en la Cuenca Hateg, decenas de agrupaciones de huevos junto con huesos de neonatos fueron desenterrados del sitio que pasó a llamarse "sitio de nidificación Tustea". Mientras tanto, el escarpado vertical original próximo a la Villa de Tustea fue nivelado y convertido en una plataforma horizontal. Por sus caracteres externos y microestructura de cáscara todos los huevos pertenecen al género Megaloolithus, cercano a M. siruguei; del mismo modo, todos los restos de pichones cuyo estado de preservación permite su asignación taxonómica pertenecen a Telmatosaurus transsylvanicus, una de las especies comunes de dinosaurios de la rica asociación faunística del Maastrichtiano de la Cuenca Hateg (Rumania), y el hadrosáurido más basal. La mayoría de los restos de pichones consiste en huesos y dientes aislados, aunque esqueletos parciales con elementos articulados fueron también encontrados. El estudio ontogenético preliminar revela un rápido crecimiento de los pichones, un hecho también soportado por el descubrimiento de una mayoría de restos esqueletales fuera de las agrupaciones de huevos, pero en su proximidad. En unos pocos casos fueron encontrados dentro de las agrupaciones de huevos, y aún en el interior de los huevos. Es importante destacar que no se han hallado restos de saurópodos dentro del horizonte de nidificación de Tustea.

Key words. Dinosaur eggs; Megaloolithidae; Hatchlings; Hadrosaurids; Telmatosaurus; Maastrichtian; Hateg Basin; Romania.

Palabras clave. Huevos de dinosaurios; Megaloolithidae; Pichones; Hadrosáuridos; Telmatosaurus; Maastrichtiano; Cuenca Hateg; Rumania.


 

Introduction

The first dinosaur eggs and egg remains from Romania were found in 1988, ninety years after the first dinosaur bones were recorded in the Uppermost Cretaceous continental deposits of the Hateg Basin by Franz Nopcsa. The eggs disposed in clutches, were found in a vertical escarpment, near the village of Tustea, in the north-western part of the basin (figure 1.2). The egg and eggshell characters correspond to the Megaloolithidae, the most widespread oofamily during the Late Cretaceous (mostly in the Campanian and Maastrichtian), with a large number of oospecies found in South America, Europe and Asia.

Figure 1. 1, Location of the Hateg Basin in Romania; 2, Geological map of the western part of the Hateg Basin (F, the Tustea nesting site); 3, Lithostratigraphic column (in meters) from Tustea and the facies interpretation / 1, Ubicación de la Cuenca Hateg en Rumania; 2, Mapa geológico de la parte oeste de la Cuenca Hateg (F, el sitio de nidificación Tustea); 3, Columna litoestratigráfica (en metros) de Tustea y la interpretación de las facies. Ch, channel deposits /depósitos de canal; Cs, crevasse deposits / lóbulos de desbordamiento; Fr, reddish fines / fangolitas rojizas(paleosoil with calcrete concretions / paleosuelo con concreciones carbonáticas).

In southern France, where no less than eight megaloolithid oospecies were described (Garcia and Vianey-Liaud, 2001) this type of egg was tentatively assigned to the titanosaurid sauropod Hypselosaurus Matheron, 1869 based on some occasional co-occurrences of the eggs with adult bone remains. This referral not consistently proved since nowhere in France or elsewhere in Europe embryo or hatchling remains were found closely linked with megaloolithid eggs. The assignment remained questionable, until 1998, when the discovery from Auca Mahuevo in Argentina of numerous megaloolithid eggs containing titanosaurid sauropod embryonic remains was announced (Chiappe et al., 1998).
The close similarity of the eggs from Tustea with the French ones, in both macroscopic and microscopic aspects, made us to presume at the beginning that the Romanian eggs were laid by Magyarosaurus Huene, 1932 the only sauropod genus known at that time in the dinosaur assemblage of the Hateg Basin, without excluding the possibility that other dinosaurs, including the hadrosaur Telmatosaurus Nopcsa, 1900 might have laid the eggs (Grigorescu et al., 1990). Soon after that a few hatchling bones, undoubtedly belonging to the hadrosaur Telmatosaurus transsylvanicus Nopcsa, 1900 were found closely associated to the first discovered egg clutches in Tustea.
The first announcement on the connection of megaloolithid eggs and hadrosaurid embryonic and hatchling skeletal remains from Tustea was made in few successive articles (Weishampel et al., 1991; Grigorescu, 1993; Grigorescu et al., 1994). The fact was (and still is) regarded with natural suspicion. This is well expressed in the Summary of the "Dinosaur eggs and babies" book, edited by Carpenter, Hirsch and Horner in 1994, that includes one of the above-quoted articles: "This conflict (suggested by the Tustea case) between identity of the embryonic remains and the shell microstructure could be a simple misidentification of the eggs as hadrosaur, or it could reveal that the diversity of eggshell type in the Hadrosauria is much more diverse than hitherto realized" (Carpenter et al., 1994, pp. 367, 369).
The number of neonatal and embryo remains increased after the original vertical outcrop from Tustea was converted into a horizontal platform by the help of a bulldozer. The searches made on this platform revealed more than a dozen of new egg clutches close to which and in few cases inside whom hatchling bones were found. All the bones whose stage of preservation allow the taxonomic assignment belong to Telmatosaurus, while no sauropods, babies or adults, were found in the nesting site as well as in the entire geological section from Tustea.
The case of Tustea highlights once again the problem of identity of the egg-laying dinosaur taxa based on egg morphology and eggshell microstructure.
The purpose of this paper is to review the evidences of this intriguing situation that can be named "the Tustea puzzle".

Stratigraphical setting

The sequence that include the Tustea nesting level belongs to the middle member of the Densus-Ciula Formation (D-C F), that outcrops in the north-western part of the Hateg Basin, a subsiding intermountain basin, formed in connection with the main thrusting phase of the Laramian tectogeny within the Southern Carpathians (figure 1.2). The D-C F represents continental molasse deposits and includes a wide range of siliciclastic rocks, from conglomerates to mudstones. The deposits were accumulated in a braided-river system, concomitantly to volcanic eruptions, whose fine and coarser products are intimately associated with the terrigenous elements. The regional stratigraphic context indicate a Maastrichtian age for the lower and middle members of the D-C F (Grigorescu and Melinte, 2002) that roughly agrees with the conclusion based on the palynologic content of the deposits (Antonescu et al., 1983).
The egg clutches are located in a red, mica rich mudstone, 4 m. thick that is divided by a thin layer of greenish grey sand (figure 1.3). The sedimentologic environment of the mudstone is a well drained flood-plain, subjected to pedogenetic processes (Grigorescu and Csiki, 2002). All the clutches correspond to a single incubation horizon, located at less than a meter below the uneven base of a thick grey-greenish, matrix-supported conglomerate bed.
The overall paleontological content of the red mudstones includes invertebrates, mostly fresh-water and terrestrial gastropods and more abundant and diverse vertebrates. The vertebrates are found as disarticulated bones or as small fragments in micropaleontologic samples, attributed to discoglossid frogs, albanerpetontids, lizards, crocodilians, turtles, pterosaurs, theropod and ornithopod dinosaurs and multituberculate mammals. It is worth to mention that sauropods, either adult or young are absent from the entire section at Tustea; the only dinosaur from which neonatal remains were recorded being the hadrosaur Telmatosaurus.
In regards of the eggshells, except the megaloolithid type, characteristic of the egg-laying horizon, only rare and small fragments of geckoid and possible bird eggshells were found at different levels of the mudstone bed.
The taphonomic features of the elements indicate the mixture within the fossil assemblage of autochthonous (floodplain dwelling taxa) with allochthonous ones, whose remains, more fragmented, were brought into the floodplain episodes together with the sediments during flood events.

Eggs and nests

The first collection of eggs made in 1988 in the vertical escarpment from Tustea includes 14 egg remains, most of them preserving only parts of the lower halves, more or less fragmented and crushed due to the burial. All, except one egg that is almost complete with the upper part preserved, seem to have been hatched. The collection increased consistently after the area was leveled and a horizontal platform was created allowing systematic excavation. On a rather small surface of only 180 m2, 18 egg clutches including 2 to 10 eggs more or less fragmented, were excavated up to date; the total number of eggs is around 90 (figure 2).


Figure 2. Distribution of egg clutches in Tustea nesting horizon. The baby remains are indicated by stars. (The numbers indicate the chronology of discoveries.) / distribución de las agrupaciones de huevos en el horizonte de nidificación de Tustea. Los restos de pichones son indicados por estrellas. (Los números indican la cronología de los descubrimientos.)

The individual eggs external characters and internal microstructure of the eggshell indicate they belong to the Megaloolithidae oofamily. The eggs are subspherical, the larger diameter ranging between 14 and 16 cm (figure 3.1). The calculated eggs volume varies between 1000 and 1500 ml. The eggshell thickness ranges from 2.1 to 2.6 mm, although it typically has 2.3-2.4 mm thickness. The outer surface of the shell is covered by closely packed, rounded tubercles, variable in size, from 0.4 to 1.1 mm with a mean diameter, calculated on eggshell areas of different eggs, of 0.78 mm. The tubercles may coalesce in small chains, but usually they are separated by narrow spaces, some of these occupied by the pore openings. The internal surface of the eggshell displays a hieroglyph pattern given by the coalesced bases of the growth units, encrusted by secondary calcite. Some eggshells show internal cratering of the mammillary knobs, in some other cases the cratering is missing.


Figure 3. Megaloolithid eggs and Telmatosaurus transsylvanicus babies from Tustea. 1, Individual egg (larger diameter 16 cm); 2, Radial thin section of eggshell in polarized light showing fan-shaped units and weathered mammillary zone; 3, Left femora of a hatchling- (left) (FGGUB R 1850) and of an adult (right) (BMNH R 4914) (scale bar=2 cm); 4, Right distal femur of a hatchling (FGGUB 248) in lateral, cranial and ventral views (from left to right) (scale bar=1 cm); 5, Right dentary tooth of a hatchling found isolated (FGGUB R 2089) (scale bar=1 mm); 6, Right dentary tooth found within an egg matrix together with incompletely ossified embryonic skeletal remains of a nearterm embryo (FGGUB R 2091) (scale bar=0.5 mm); 7, Fragment of a left dentary (FGGUB R 1850) (scale bar=1 mm) / huevos megaloolítidos y pichones de Telmatosaurus transsylvanicus provenientes de Tustea. 1, Huevo individual (diámetro mayor 16 cm); 2, Sección delgada radial de cáscara de huevo vista con luz polarizada mostrando las unidades en abanico y la zona mamilar erosionada; 3, Fémures derechos de un pichón (izquierda) (FGGUB R 1850) y de un adulto (derecha) (BMNH R 4914) (Barra de escala=2 cm); 4, Porción distal de fémur derecho of un pichón (FGGUB 248) en vistas lateral, craneal y ventral (de izquierda a derecha) (Barra de escala=1 cm); 5, Diente dentario derecho aislado de un pichón (FGGUB R 2089) (Barra de escala=1 mm); 6, Diente dentario derecho hallado dentro de la matriz de un huevo junto con restos esqueletales incompletamente osificados de un embrión ya maduro (FGGUB R 2091) (Barra de escala=0.5 mm); 7, Fragmento de un dentario izquierdo (FGGUB R 1850) (Barra de escala=1 mm) .

The eggshell microstructure is discretispherulitic; in radial section the eggshell is composed of fan-shaped units with variable widths, often individualized and delimited by well-defined margins (figure 3.2). The fine growth lines, arched upwardly cross continuously among adjacent bundles. The herring-bone pattern is sometimes present. The analysis in cathodoluminescence reveals that the eggshell microstructure is not deteriorated by diagenetic changes.
The pore pattern corresponds to the tubocanaliculate system, consisting of a complicated network of sinuous tubes, with branches and anastomosis among them. The diameter along the pore length varies between 0.13 and 0.38 mm. The pore geometry and distribution was studied through seriated thin
sections across the entire eggshell thickness, the successive sections being spaced at approximately 0.2 mm interval. The study revealed a high pore density, expressed by ca.100 pores/cm² at the surface; the total area occupied by pores representing 3.7 % of the total eggs surface. From all the known Megaloolithidae species the eggs from Tustea seems closer to Megaloolithus siruguei Vianey-Liaud, 1994 from Southern France (in Vianey-Liaud et al., 1994).
More recently, 11 clutches with approximately 40 eggs, in all regards very similar with the Tustea ones and closely resembling Megaloolithus siruguei were discovered in superposed levels of grayish and reddish calcretic paleosoils on the Raul Mare valley, near the Nalat Vad village, southwards from Tustea (Codrea et al., 2002).

Nesting pattern and incubation environment

In most of the cases the number of eggs in a clutch varies from 3 to 7, rarely clutches with more or less eggs were unearthed; only one egg was found isolated. The largest clutches include respectively 10 and 13 eggs (nrs. 12 and 20 in figure 2), disposed in two parallel rows (clutch nr. 12), or randomly (clutch nr 20). Most of the eggs in the two clutches were obviously hatched, preserving only their bottom halves, or only large eggshell fragments. Tree eggs in the clutch nr.12 are almost completely preserved, with slightly deteriorated upper caps, however the CT scans did not indicate embryonic material inside. The eggs are filled with red mudstone material, identical with that of the encasing rock.
The eggs within the clutches are more randomly than regularly disposed. Sometimes the eggs are linearly aligned, in straight or slightly curved rows. More frequently they are grouped in clusters with a random disposal of the eggs. As Cousin has mentioned in France, the random position of eggs in clutches might represent a regroupement during or after the hatching of the eggs that originally were disposed in rows (Cousin, 2002). The eggs in the clutches are closely packed; sometime the neighbor eggs are tangent or overlap by their margin. The slightly uneven position of the eggs in the clutches suggests that they were laid either in the existing holes on the ground or the mothers dug such superficial holes for laying the eggs. The distance among the neighbor clusters varies from 0.5 to 3 m.
The clutches seem to represent original nests not much affected after hatching, except the crushing and flattening produced by the pressure and compaction of the surrounding sediments.
Based on sedimentologic, clay mineralogy and stable isotope analysis (Bojar et al., 2005) the environment of incubation was a humid one that confirms the conclusion drawn a decade ago based on the high pore density and pore geometry of the eggs (Grigorescu et al., 1994). The eggs were laid within a fine-grained substrate of paleosol, formed in the distal part of a well-drained floodplain, well vegetated by small swampy plants and bioturbated. The incubation took place concomitantly to the evolution of pedogenetic processes on the ground, which explains the location of the nesting level in a calcrete rich paleosol. The calcrete nodules are frequently found associated with the bottom portions of the eggs.
The rarity of completely crushed eggs within the clutches and generally, the small quantity of isolated eggshell fragments in the nest indicate that the babies did not remain for a long time in the nest, a fact that is sustained by the position of the hatchling remains in the nesting horizon (see below).

Hatchlings and embryos

Two distal parts of femora, two proximal and one distal tibiae, the distal half of a fibula were the only neonate remains found in the original, vertical escarpment from Tustea within the level which provided the first egg clutches, at about half meter laterally from one of these clutches.
Tens of new neonatal and possible embryonic bones were found within the Tustea egg horizon after the vertical section was converted into a horizontal platform. The distribution of the baby remains within the incubation horizon is shown by small stars in figure 2. Most of the remains were found, as in the case of the first discovery, outside the egg clutches, but very close to them, usually within one meter of the eggs; in a single case the bones were found at more than three meters from the nearest clutch. Rarely baby bones were found within egg clutches and in a single case a dentary tooth was recovered from within the matrix of a broken egg, together with very small, porous, apparently representing the stage of replacement of the embryonic cartilage by bone tissue. Most of the remains consist in isolated bones and teeth, more or less completely preserved. They include elements from all skeletal regions: skull, vertebrae, ribs, girdles and limbs.
Beside the numerous isolated elements, three partial skeletons of grown hatchlings, with closely associated bones from the same skeleton region (figure 4). The diagnostic characters shown by all the elements are definitely not of a sauropod but of the hadrosaurid Telmatosaurus transsylvanicus (Weishampel et al., 1991; Grigorescu et al., 1994), all the skeletal remains, including dentition and individual teeth supporting this assignment.


Figure 4. Skeletal remains of a hatchling with bones in almost articulated position (FGGUB R 2088) / restos esqueletales de pichones con huesos en posición casi articulada. pg, pelvic girdle / cintura pélvica; fe, femur / fémur; ti, tibia; ve, vertebra / vértebra.

Telmatosaurus was the first dinosaur from the Hateg assemblage described by Nopcsa (1900) under the name of Limnosaurus. Telmatosaurus was reviewed by Weishampel et al. (1993) and interpreted as the basalmost hadrosaur, which lacked several apomorphic features of the other, more derived members of Hadrosauridae. The study includes references to the first hatchling remains, femora and tibiae, from Tustea that were compared with adult bones from different collections. As in adults of Telmatosaurus, the femur (figure 3.4) has well formed distal condyles that start to contact each other cranially, enclosing the tunnel-like intercondylar groove; also, as in adults, the caudal intercondylar groove is very deep and includes a small condylid on the lateral condyle. Similar to the adult condition, the fourth trochanter is positioned towards the middle of the shaft, it is blade shaped, prominent, well-extended from the shaft and slightly pendent at its tip.
The size and stage of preservation differ slightly among the skeletal remains, even in the case of elements found associated in the same place, indicating minor differences in age of the hatchlings or nearterm embryos. For example, in the case of the first discovered remains, which include two distal femora, two proximal and one distal tibiae, the surface texture, the preservation of articular surfaces and the size of the bone fragments indicate that they come from distinct individuals, with slightly different stages of ontogenetic development. The best preserved femur (Faculty of Geology and Geophysics of the University of Bucharest -FGGUB R.248) (figure 3.4), a proximal (FGGUB R.250) and the distal tibia (FGGUB R.246), both, apparently from the same individual represent hatchling elements, while the other femur (FGGUB R.249) and tibia (FGGUB R.245) seem to come from nearterm embryos or early hatchlings.
Generally, the bone remains which display a very porous surface texture and, in the case of the appendicular elements, lack ossified articular ends are interpreted as nearterm embryos or early hatchlings, while the limb bones of older hatchlings are indicated by well developed articular morphology, as well as by the presence of an external cortical layer that gives a shiny appearance to the bone surface.
The hatchling limb bones that are more numerous have allowed a preliminary ontogenetic study based on comparative analysis of hatchling limb bones and adult bones (Grigorescu and Csiki, 2006). Besides this close similarity in shape and proportions between hatchlings, adults and sub-adults of Telmatosaurus transsylvanicus (figure 3.3) the study highlighted some allometric changes that occurred during growth in the appendicular skeleton of Tematosaurus. This study also suggested a high growth rate during early post-natal development. The measurements of limb bones coming from different hatchling or possible newborn individuals are shown in table 1.

Table 1. Measurements of selected Telmatosaurus limb bones. * estimated measurements; ** dimensions as preserved / medidas de huesos apendiculares selectos de Telmatosaurus. * medidas estimadas; ** dimensiones de la parte preservada. L, length; PW, DW, MW, proximal, distal, medial width. 1, additional measurements: deltopectoral crest length, in humerus; distance of 4th trochanter from proximal end, in femur.

The dentition of hatchlings and embryos also revealed close similarity with adult Telmatosaurus transsylvanicus. Few individual dentary teeth were found isolated (figure 3.5), one of these within an egg matrix (figure 3.6). The apex of the teeth is pointed as in the adults, but strait or very slightly recurved distally; the margin of the teeth are denticulate, the denticles are irregular in size. The enameled lingual face of each tooth display a strong median carina, some teeth presents also a lateral, slightly bent ridge. In all the cases, teeth do not show signs of wear.
A dentary fragment (14.4 mm long) was found together with forelimb remains, vertebrae and ribs, apparently of the same individual early hatchling (figure 3.7). Like in all hadrosaurids, the dentition is organized into dental batteries; the fragment preserves seven erupted teeth and the apex of an unerupted tooth; the teeth are disposed in three rows. The teeth are leaf-shaped (6.5 mm high and 3.0 mm width), covered by a thick layer of enamel and crossed medially by a crest, less prominent or even absent in the lowermost part of the crown. The two sides around the medial crest are either flat or banded towards the crest. Except the lower part of the crown, the teeth margins are denticulate with 8 to 10 slightly rounded denticles not supported by marginal ridges, as is the case in adults.

Conclusions

The taxonomic identification of the hatchlings associated with megaloolithid eggs within the incubation horizon from Tustea as belonging to the hadrosaurid Telmatosaurus transsylvanicus has been confirmed by D. Weishampel (Weishampel et al., 1991, 1993), J. Horner and Ph. Currie (personal communication, 1991). All those colleagues who could not visit the site have questioned the association of eggs and babies in the same level, suggesting that, possibly, the egg clutches and baby bones represent different stratigraphic levels. To answer to this question we mention that the "marker bed" we used as a stratigraphic control in the Tustea outcrop is a paleosol level, with a constant position towards the base of the thick conglomerate bed that dominates the Tustea geological section (see figure 1.3). This "marker bed" includes the egg clutches and baby remains and preserves its identity throughout the entire outcrop. Besides this stratigraphic aspect, the discoveries of the last years have increased the number of hatchling remains found inside the egg clutches, while teeth and skeletal fragments were also identified within the egg matrix. All these facts support the idea that the megaloolithid eggs from Tustea were laid by the hadrosaurid Telmatosaurus transsylvanicus. We are not prepared to give a consistent explanation to this convergence in egg morphology and eggshell microstructure between sauropods and this "most basal hadrosaurid dinosaur" (Weishampel et al., 1993) that is Telmatosaurus transsylvanicus.

Acknowledgements

I thank T. Niculescu for preparing the egg in which embryonic remains, including the tooth were found. I also thank R. Dimitrescu and M. Dumbrava for the drawings of figs. 1 and 3 (E-G). I am grateful to Z. Csiki for discussions and assistance in organizing the text and illustrations of the present paper. Finally, I am indebted to R Coria and L.Chiappe for the opportunity to visit the famous Auca Mahuevo nesting site and for their special hospitality. I am grateful to L. Salgado for the Spanish translation of the Abstract (Resumen) and the help in organizing the final version of the paper. This contribution is part of the Proceedings of the Third International Symposium on Dinosaur Eggs, Babies, and Developmental  Biology, held in Plaza Huincul, Neuquén, Patagonia, Argentina, April 13-15, 2006.

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Recibido: 19 de noviembre de 2008.
Aceptado: 15 de noviembre de 2009.