Trigonoolithus amoae

Moreno-Azanza, Miguel, Canudo, José I. & Gasca, José M., 2014, Unusual theropod eggshells from the Early Cretaceous Blesa Formation of the Iberian Range, Spain, Acta Palaeontologica Polonica 59 (4), pp. 843-854 : 845-848

Trigonoolithus amoae oosp. nov.

Figs. 2–4.

2009 Prismatoolithidae indet.; Moreno-Azanza et al. 2009a: 151 A.

2010 Prismatoolithidae indet.; Canudo et al. 2010: 218–219, fig. 6.4.

2013 Prismatoolithidae indet.; Moreno-Azanza et al. 2013: 22, figs. 1c, d, 2d, 3c, d.

Etymology: After the late Olga Amo-Sanjuan, pioneer in the study of the Lower Cretaceous eggshell in the Iberian Range and sadly deceased.

Holotype: MPZ 2012 /737, an eggshell fragment.

Type locality: La Cantalera 1 site, near the village of Josa, Teruel Province, Spain.

Type horizon: Lower Cretaceous (early Barremian), Blesa Formation.

Material.— 25 eggshell fragments gold-coated and mount- ed for SEM, numbered MPZ 2012 / 728 to MPZ 2012 / 736, MPZ 2012 / 738 to MPZ 2012 / 745, MPZ 2012 / 765 to MPZ 2012 / 767, and MPZ 2012 / 843 to MPZ 2012 / 846; 22 eggshell fragments prepared in radial thin sections, numbered MPZ 2012 / 847 to MPZ 2012 / 868; 300 additional eggshell fragments numbered MPZ 2012 / 428 to MPZ 2012 /727. All from type locality.

Diagnosis.— Prismatoolithidae eggshell with three different layers, with gradual transition between mammillary and prismatic layer and between prismatic and external layer, and angusticanaliculate pore system. Outer surface strongly sculptured with ornamentation made up of well-formed triangle-shaped and/or rounded unconnected protuberances. Eggshell thickness 330–1040 μm (mean 672 μm, standard deviation 106.46 μm).

Description.— Eggshell thickness ranges between 330–1040 μm including ornamentation (n = 300). The eggshell exhibits a strongly sculptured outer surface, with numerous independent protuberances with sections that vary from round to isosceles triangles (Figs. 2A, 4A, D). A few eggshells present

Fig. 2. Prismatoolithid eggshell Trigonoolithus amoae oogen. et oosp. nov. from La Cantalera 1 site, Early Barremian. A. MPZ 2012/740, external surface with ornamentation consisting of tightly packed sub-triangular protuberances. Arrow points to sub-circular pore opening. B. MPZ 2012/846, outer shell surface is at the top of the image. Radial section shows a straight pore (arrow) characteristic of angusticanaliculate pore system. C. MPZ 2012/737, holotype, radial section showing mammillary layer (ML), prismatic layer (PL) and external layer (EL) (C 1). White lines show approximate position of gradual transition between layers. Arrows point to vesicles that are present in the mammillary and prismatic layer, but absent in the external layer. Detail →

both morphologies in the same fragment. Protuberances range from 80–300 μm, with a density of 30 protuberances per mm 2.

The straight pore canals are sparse and occur around the ornamentation; pore openings are circular, with an average diameter of 10 μm. The pore system, therefore, is angusticanaliculate (Fig. 2B).

The columns are narrow and their boundaries faint, with a height to width ratio of 4:1. The eggshells have three distinct structural layers, with a gradual transition between mammillary and prismatic layers and between prismatic and external layers (Fig. 2C). The prismatic layer to mammillary layer thickness ratio ranges from 3:1 to 4:1, and the prismatic layer to external layer ratio varies from 2:1 to 3:1. Mammillary, prismatic and external layers exhibit tabular ultrastructure (Fig. 2D). In the mammillary layer, crystals radiate outward from the organic cores (Fig. 2D 2) until they reach their neighbours from adjacent mammillae (see Moreno-Azanza et al. 2013, for a full description of this process). Crystals sub-parallel to the direction of eggshell growth survive to form the prismatic and external layers. Crystals in the prismatic layer show a poorly developed squamatic structure, with few vesicles (Fig. 2C). Prisms in the third structural layer are wider than those of the prismatic layer,

of the triangular ornamentation (C 2), where compact calcite structure can be seen. Vesicles are absent. Arrow points to tabular structure. Detail of the mammillary layer (C 3), showing tabular crystals and gradual transition between mammillary layer and prismatic layer. Scale bars: A, 1000 μm; B, 200 μm; C

1

, 100 μm; C

2

, C

3

, 50 μm.

with more defined crystals and identifiable, calcite crystal faces and boundaries (Fig. 2D).

Radial thin sections greater than 30 μm in thickness show faint growth lines in the lower half of the eggshell, namely in the mammillary and lower part of the prismatic layers ( Fig. 3A ). Column boundaries are visible in the mammillary layer, but indistinguishable in the prismatic layer under a light microscope ( Fig. 3B 1 ). The inner shell surface exhibits tightly packed mammillae, with more than 40 mammillae per mm 2. The mammillae display radiating tabular crystals, and are strongly cratered in most specimens and completely eroded in others (Figs. 2E, 3B 1). Only a few eggshell fragments exhibit an eisospherite, and in these the central core appears eroded.

Remarks.—The presence of the prismatic structure, with a gradual transition between the mammillary and prismatic layers, represents a synapomorphy of the Prismatoolithidae and a character shared with most modern and fossil neognath bird eggs ( López-Martinez and Vicens 2012). In addition, Trigonoolithus amoae has a similar eggshell thickness to other Prismatoolithidae eggs ( Table 1).

Prismatoolithidae includes five oogenera: Preprismatoolithus Hirsch, 1994 ; Prismatoolithus Zhao and Li, 1993 ; Protoceratopsidovum Mikhailov, 1994 ; Spheruprismatoolithus Bray, 1999 ; and Sankofa López-Martínez and Vicens, 2012 .

Trigonoolithus shares its angustiprismatic morphotype and angusticanaliculate pore system with Sankofa , Prismatoolithus , and Protoceratopsidovum , unlike Preprismatoolithus , which has oblique pore canals that cross columns. Protoceratopsidovum eggs have a smooth surface or fine linearituberculate ornamentation, whereas Prismatoolithus oospecies have a smooth to finely sculptured surface (Zelenitsky and Hills 1996). Only a single oospecies, Prismatoolithus caboti Garcia, Feist, Cabot, Valentin, and Vianey-Liaud, 2000 from the Upper Cretaceous of France, shares with Trigonoolithus a well-developed ornamentation. However, the P. caboti shell is thinner and presents dispersituberculate ornamentation with spherical nodes up to four times larger than those present in Trigonoolithus .

Bray (1999) described the oogenus Spheruprismatoolithus as a Prismatoolithidae eggshell with outer surface ornamentation.The ornamentation of this ootaxon is described as sculptured, with nodes that coalesce to form ridges on the outer shell surface, resulting in a dominant ramotuberculate to dispersituberculate ornamentation ( Bray 1999: fig. 3), as opposed to the individualized protuberances that are present in Trigonoolithus . Also, Spheruprismatoolithus exhibits a lower prismatic layer to mammillary layer ratio (1:1) and a combination of angusticanaliculate and rimocanaliculate pores that are not present in Trigonoolithus . Spheruprismatoolithus presents widely spaced nucleation sites, and shell units that flare towards the outer surface of the eggshell and have a sweeping extinction pattern. Indeed, the inclusion of Spheruprismatoolithus in Prismatoolithidae has been questioned by Jackson and Varricchio (2010), who point out that the described features of the structure of this ootaxon are not compatible with prismatic theropod eggs.

The oogenus Sankofa displays a smooth surface and a lower eggshell thickness. In addition, Trigonoolithus does not show the characteristic jagged central structure in the palisade layer (sensu López-Martínez and Vicens 2012).

Triprismatoolithus stephensi eggs from the Upper Cretaceous Two Medicine Formation of Montana display a prismatic structure, but were not assigned to the oofamily Prismatoolithidae by Jackson and Varricchio (2010) due to the presence of well-developed ornamentation and the abrupt contact between the second and third structural layers. Recently, Agnolin et al. (2012) defined a new oofamily, Arriagadoolithidae , and Arriagadoolithus , a similar three-layered eggshell found in the Upper Cretaceous of Patagonia. Trigonoolithus differs from Arriagadoolithidae in displaying a gradual transition between the second and third structural layers.

Geographic and stratigraphic range. —Early Barremian of the Iberian Range.Blesa (Oliete subbasin), El Castellar (Galve subbasin) and Mirambel Formations (Morella subbasin).

Figures

Fig. 3. Thin-sections of prismatoolithid therapod eggshell Trigonoolithus amoae oogen. et oosp. nov. from La Cantalera 1 site, Early Barremian. A. MPZ 2012/856, light microphotograph. B. MPZ 2012/862, light microphotograph (B 1), microphotograph showing eggshell under cross-polars (B 2). C. MPZ2012/850, cathodoluminescence image showing dark blue luminescence except for some orange tones in the mammillary layer, probably due to the high amount of organic matter in this area. Scale bars 500 μm.

Fig. 1. Geographical and geological location of the site of La Cantalera 1 (early Barremian, Teruel, Spain), modified from Canudo et al. (2010). A. Simplified geological map of the Iberian Peninsula. B. Palaeogeographic subbasins (Ol, Oliete; Pa, Las Parras; Ga, Galve; Mo, Morella; Pe, Perelló; Sa, Salzedella; Pg, Peñagolosa) within the Maestrazgo Basin and active faults during Early Cretaceous sedimentation, modified from Salas et al. (2001). C. Detailed location of the La Cantalera site near the village of Josa, Teruel. D. Geological map of the La Cantalera area with the local geological units.