Discovery

The specimen (ML1188, ML – Museu da Lourinhã, Portugal) is composed of a large number (> 500) of clustered eggshell fragments forming an assemblage 65 cm in diameter, containing embryonic bones and teeth (Fig. 1; Supplementary Note 1). No other vertebrate remains or eggshell fragments were found in the immediate area. The assemblage was discovered on a grey mudstone layer (39° 13′ N; 9° 20′ W; exact coordinates available upon request) in August 2005 by A. Walen at Porto das Barcas in the Sobral Member of the Lourinhã Formation. Excavation was performed in September 2005 and May 2006. A single block containing the entire assemblage was jacketed in the field using a specific technique previously described16. The specimen was prepared at the Museu da Lourinhã in 2009. Another clutch (ML565) previously described5, referred to as the Paimogo nest, was found in penecontemporaneous deposits of the Lourinhã Formation in 1993 by I. Mateus, less than 10 km away from the area where ML1188 was collected. The Paimogo nest was recovered from an iron-rich mudstone layer. Despite the dubious phylogenetic position of the avetheropod Lourinhanosaurus antunesi6,17, the Paimogo nest was tentatively ascribed to this taxon.

Figure 1 (a) Right maxilla, dentary and jugal of Torvosaurus sp. (ML 1188) in medial view. (b & c) Anterior part of right maxilla of Torvosaurus sp. (ML 1188) in medial view. (d) Right dentary of Torvosaurus sp. (ML 1188) in medial view. (e) Anteriormost maxillary teeth in medial view. (d) Anteriormost dentary teeth in medial view. Abbreviations: amp, anteromedial process; anr, anterior ramus; aof, antorbital fenestra; cx, cervix dentis; d, dentary; d1, isolated first dentary tooth; d2, second dentary tooth; d3, third dentary tooth; d5, fifth dentary tooth; dt7, seventh dentary tooth; dca, distal carina; idp, interdental plate; j, jugal; jur, jugal ramus; lac, lacrimal contact of the maxilla; m, maxilla; mca, mesial carina; mf, Meckelian fossa; mfo, Meckelian foramina; mg, Meckelian groove; ms, mandibular symphysis; mx1,first maxillary tooth; mx2, second maxillary tooth; mx5, fifth maxillary tooth; nug, nutrient groove; pdg, paradental groove; ro, root. Scale: 10 mm for (a), 5 mm for (b), (c) & (d); 2 mm for (e) & (f). Full size image

Clutch

In spite of post-burial compression18, the observation of continuous patches of eggshells allows discerning eggshell orientations, which indicate the presence of different eggs. In Supplementary Note 1a the assemblage can be divided into three different mounds: one at the top, one at the middle and one at the bottom. The top mound has eggshell patches with N-S fracture orientation, whereas the middle and lower mounds E-W fracture patterns. All the eggs are heavily crushed and individual boundaries between the eggs are difficult to distinguish (Supplementary Note 1). Nevertheless, this assemblage forms a clutch (ML1188) because it consists of several eggs (> 3) mingled with embryonic bones. There are generally one or two eggshells vertically stacked, but rarely three or four eggshells are found piled up (Supplementary Note 1). Embryonic material in the clutch consists of five dispersed and isolated teeth, a maxilla preserving four teeth, a dentary with four teeth in situ, an isolated dentary tooth, a series of three articulated centra and other unidentifiable bones (Fig. 1; Supplementary Note 1, 2, 4 and 6). ML1188 was found as an isolated and concentrated clutch without any surrounding dispersed eggshells.

Embryonic remains

The medial side of the incomplete right maxilla is on the surface of the clutch (Fig. 1, Supplementary Note 2). The partial maxilla comprises the anterior ramus, an incomplete ascending process and the jugal ramus whose the posterior part is separated from the rest of the maxillary bone. The main body has a Subtriangular anterior ramus projecting moderately anteriorly from the antorbital fenestra (Fig. 1). The anterior margin of the anterior ramus is convex and forms a right angle with the ventral margin of the maxilla. The anterodorsal margin of the anterior ramus is slightly concave and confluent with the ascending process, thus, there is no step delimiting the two structures. The jugal ramus is broken in two pieces, the posterior part lying one centimeter below the maxilla. Once digitally restored (Supplementary Note 3), the jugal ramus is elongated with some parts in the middle of the ramus missing. The dorsal margin of the horizontal ramus, corresponding to the ventral rim of the antorbital fenestra, is straight and subparallel to the tooth row in its anterior part whereas the posterior part of the ramus has a sigmoid outline and slopes ventroposteriorly towards the jugal contact of the ramus. The posteriormost part of the jugal ramus corresponds to a tongue-like process delimited by a small concavity on its dorsal margin and the main axis passing through this posterior process is oriented ventroposteriorly. The ventral margin of the jugal ramus is straight and parallel to the anteroposterior axis of the jugal process. Little information can be extracted from the jugal but the contact between the jugal and maxilla extends one third on the lateral side of the jugal ramus. The articular surface for the lacrimal is visible on the anteromedial part of the jugal, at the level of the posterior process of the jugal ramus. The lacrimal was also contacting the maxilla along one third of the jugal ramus. The ascending process is thick at its base and tapers dorsally. The anteroventral margin of the ascending process is convex, almost forming an obtuse angle and the main axis passing through this process angles 28° with the ventral margin of the maxilla. Two elongated and subparallel ridges are present at the dorsal tip of the ascending process and delimit the lacrimal contact of the maxilla. The antorbital fenestra is parabolic in outline. There is no medial antorbital fossa. The medial surface of the maxilla bears parallel stripes corresponding to vascular canals. On the main body of the maxilla, these canals are anteroposterioly aligned but, on the ascending process they are parallel to its main axis. However, the canals are more irregularly placed on the anterior part of the ascending ramus. Although the anteromedial surface of the maxilla is damaged, the surface is composed of solid bone, thus no maxillary and promaxillary fenestrae, maxillary antrum and pneumatic excavations are present. At the level of the teeth roots, only one isolated interdental plate has been preserved in between the second and fourth maxillary teeth. This suggests that the interdental plates of the maxilla were unfused. The plate is subrectangular and its surface is punctuated. The anteromedial process has been crushed at the level of the anterior margin of the maxilla. The anteromedial process is located on the anterodorsal border of the anterior ramus, being a long finger-like projection parallel to the anterodorsal rim of the anterior ramus. Four maxillary teeth are preserved on the anterior part of the maxillary body and there is an isolated tooth near the maxilla and two more loose teeth located some distance away from this bone. The maxillary tooth count is difficult to estimate because the posterior part of the main body bears no teeth. However, on the anterior ramus of the maxilla there were probably five teeth. All teeth are strongly elongated (Crown Height Ratio > 2.5), their apices are sharply acute and both mesial and distal carinae lack serrations. The mesial and distal profiles are recurved distally and the crowns are subconical. Nevertheless, the lingual surface of the crowns is mostly flattened except the basal surface which is concave. This concave area on the medial surface of the crown received the erupting tooth of the maxilla. The enamel surface of the teeth is smooth and does not bear transversal undulations, enamel wrinkling, longitudinal grooves, or wear facets.

A right dentary is lying on its lateral side just below the anterior part of the maxilla (Fig. 1, Supplementary Note 4). The bone is almost complete and only part of the bone posterior to the opening of the Meckelian fossa is missing. It is fractured in the middle by a transversal fissure. The dentary is massive, with a ventrodorsally large medial wall remaining the same width along the bone. Four fully erupted teeth are present on the dentary but only two of them, the third and seventh dentary teeth, are complete. A fifth isolated tooth, most likely the first dentary tooth, is lying beside the anteroventral corner of the dentary. Based on the positions of the remaining teeth, we estimate a total of eight alveoli on this portion of the dentary. The interdental plates are unfused. Three interdental plates are preserved in between the first and second dentary teeth, the second and third and the fifth and seventh teeth. Variation occurs in the shape of the interdental plates along the tooth row as the two anteriormost plates form a vertical rectangle whereas the third one, more posteriorly located, has a horizontal rectangular outline. The mandibular symphysis is short, smooth and forms an elongated triangle on the anteriormost part of the dentary. The paradental groove separating the medial wall of the dentary with the interdental plates is large, gently concave and seems to have been open in between those two structures. The Meckelian groove is well visible on the dentary, running along the bone just above the dorsal margin of the dentary. This longitudinal groove is filled with sediment and seems to extend anteriorly until the third dentary teeth. The step between the Meckelian groove and Meckelian fossa occurs at the level of the seventh dentary tooth. Only the anterior part of the Meckelian fossa is preserved. A large concavity at the level of the third dentary tooth is present just above the ventral margin of the dentary and includes a short anteroposterioly oriented ridge. Two small grooves located in between this concavity and the mandibular symphysis are interpreted to be Meckelian foramina (Fig. 1). The anteroventral margin of the dentary is rounded and almost subrectangular, with the inflexion point close to the level of the anteriormost point of the dentary. The anteroventral margin is unexpanded with no articular brace forming a chin and the ventral margin appears to have been fairly straight. The teeth of the dentary are unserrated and the crowns are pointed, strongly basoapically elongated and recurved posteriorly and devoid of enamel structure.

Three articulated amphiplatyan vertebrae were fully prepared (Supplementary Note 6). The ventral part of the centrum bears two paired pits identified as neurovascular foramina. The anterior and posterior faces of the vertebrae show evenly distributed small pits, consistent with an early developmental stage8. The centrum faces are expanded (~ 38% relative to the mid-centrum) and bear confluent striations, whereas in the median part of the centrum, the striations are parallel. Although other fragmentary bones are present and scattered within the clutch none are identifiable, thus, no further information could be obtained.

Eggshell characterization

The total eggshell thickness of the ML1188 eggshells is approximately 1.2 mm. They bear (i) anastomizing ornamentation resembling to some degree the patterns of linearituberculate1 ornamentation, consisting of sub-circular to sub-elliptical grooves separated by interconnected sharp ridges (Fig. 2c, 3a), (ii) acicular, elongated blade-shaped, calcite crystals radiating from the base of the mammillae to the outermost part of the eggshell (Fig. 2b) and (iii) only one primary layer (Figs. 2a,b, 3a,b). Growth lines are visible across the entire thickness of the eggshells, but tend to be concentrated at the base and develop perpendicularly to the acicular calcite crystals of the wedges (Fig. 2b). The mammillae height, which is difficult to measure for this eggshell morphotype, averages 166 μm, corresponding to about 17% of the total thickness of the eggshell (Figs. 2b, 3b).

Figure 2 ML1188 eggshell (a) SEM micrograph of the eggshell radial section (b) optical micrograph eggshell radial section showing acicular crystals and a single layer; (c) SEM micrograph of the external surface of the eggshell and (d) SEM micrograph of the internal surface of the eggshell. Full size image

Figure 3 SR-μCT image of the eggshell in (a) external, (b) internal and (c) transverse view. Full size image

The results presented in Fig. 3 show that SR-μCT is a useful technique for non-destructive imaging of the eggshell morphology. The morphology of the pores is provided by high-quality data that is difficult to visualize by SEM or thin-sections. The pores are irregular canals that ramify and vary in width along their length (i.e., resembling prolatocanaliculate1). Their diameter is highly variable because the contour is irregular, ranging between ~ 100 μm and 500 μm. Pores anastomose with adjacent pores close to the outer surface (Fig. 2, 3). In fact, the pores seem to form an interconnected system (Fig. 3c and Supplementary videos). All eggshells have equivalent pore density, which indicates that all the eggs were buried under a homogenous incubating medium19. Eggshell ornamentation consists of well-defined ridges and islets with prominent and sub-vertical walls, which show no flattening or smoothing of their sharp edges (Fig. 3a). An external zone (Supplementary Note 1) resembling an additional layer may be present in the eggshells. However, the SR-XRD analysis of the external and internal surface zones of the eggshell fragments shows the presence of quartz and phyllosilicates (Fig. 4). Furthermore, SR-XRD (Fig. 4) and micro-PIXE (Supplementary Note 8) data indicate that pore-filling sediment is composed mainly of phyllosilicates. SR-XRD shows that the major compound of the eggshell is calcite (Fig. 4). The inner surface shows some degree of flattening and in some points partial fusion of the mammillae (see upper right corner of Fig. 2d). The eggshells do not luminesce under the impact of accelerated electrons by the CL equipment (Fig. 5). Luminescence is only observed on the pores. The micro-PIXE analysis of the eggshell (excluding pores) revealed the presence of Mg, Fe, Mn, Si (0.33%, 0.27%, 0.18% and 0.10%, respectively) and several trace elements (Na, Al, S, K and Cl), with a corresponding loss of Ca (Table 1, Supplementary Note 8).

Table 1 Compositional results for the eggshell and pore inclusions as determined by micro-PIXE technique. The remaining percentage includes oxygen and carbon Full size table

Figure 4 SR-XRD patterns obtained for an eggshell from ML1188 at the topmost zone (external surface zone), at the middle and at the bottommost zone (internal surface zone) of the eggshell. The data were acquired in transmission mode (beam closely parallel to the outer/inner surfaces of the eggshell). The identification of the compounds present in each zone was performed by comparing the measured SR-XRD patterns with data sets from the PDF-2 database. Full size image