KROKOLITHIDAE Kohring and Hirsch, 1996 [24]

Suchoolithus portucalensis oogen. et oosp. nov.

ZooBank Life Science Identifier (LSID) for the oogenus: urn:lsid:zoobank.org:act:C5B99229-C8C5-4322-B7B7-9E827DBF8F3B

As combined characters: ellipsoid eggs, size approximately 42 x 26 mm; ornamented outer surface with very small bumps; average shell thickness of 160 μm; trapezoidal shell units tightly packed together and wider than taller, with almost no interstices at the bases of the shell units.

The shell thickness is 160 μm (n = 80, sd = 17 μm). Microscopically, in radial section, the wedges of the shell units are clearly visible, interlocked and closely packed together with little space between them, with a wider top and gradually narrowing until the darker basal knobs ( Fig 5C ). A very thin, discontinuous diagenetic layer covers the outer surface. No growth lines are visible. The basal plate groups are present ( Fig 5C ), although not observable through the whole radial section. There is a very thin, darker line at two thirds of the eggshell thickness. The shell units are sometimes domed in the upper part, resulting in the bumpy outer surface ornamentation, and a blocky extinction pattern is present when observed under cross-polarized light ( Fig 5C ). In Fig 5D , with transmitted light, on a tangential section, darker areas, corresponding to the basal plate groups, are clearly visible and show a distribution identical to what is observed in extant crocodyloid eggs [ 9 ].

The eggs are ellipsoid in shape, with blunt ends, measuring approximately 42 mm long and 26 mm wide, with an elongation index (EI) of 1.62. The external surface is lightly sculptured by slightly uneven, tiny bumps ( Fig 5A ). No evidence of extrinsic degradation can be appreciated. The internal surface shows a tight packing of the basal knobs ( Fig 5B ). No pore openings have been observed.

Macroscopically, the holotype FCT-UNL706 is a clutch with 13 eggs ( Fig 4 ), seven of which are well preserved and mostly intact. The remaining are incomplete, composed of aggregates of eggshell fragments in situ. Some of the eggs are truncated, either by erosion or excavation of the clutch or by hatching, although the undisturbed aspect of the clutch allows us to consider the first scenario as the most likely. It was found in a fallen block of fine sandstone, and no sedimentological features of the block allow polarity orientation, other than the truncation of eggs, where the truncation of upper halves of the egg is more probable. Considering this orientation, most of the eggs are shown in the bottom part of the specimen, with three of the eggs only visible on the upper of the clutch. Except for two eggs, which are oriented vertically, all the others are oriented horizontally. The eggs are dark brown, standing out from the very fine, light gray sandstone matrix, and show a fractured and cracked surface. Nonetheless, the clutch is well preserved and, even though there is truncation in some of the eggs which is a form of fossil diagenetic alteration, the eggs do not show any signs of any other severe post-burial disarrangement. Furthermore, there is no evidence of recrystallization or replacement of the original composition of the eggs.

Diagnosis sensu [ 46 ] and amended to include the thinner eggshells described in this study (as combined characters): eggshell with outer surface smooth to undulating; straight pore canals ending between shell units in deep interstices; ellipsoidal eggs with two blunt ends; egg size 68–50 mm and 44–30 mm, shell thickness 170–760 μm.

Description.

ML760. Macroscopically, ML760 is a crushed egg (Fig 6) found in association with a theropod nest [58]. The egg is encased in a small block of reddish mudstone with some caliche nodules. Even though crushed, the egg retains a characteristic ellipsoid shape with blunt ends. It measures 70 mm in length and 40 mm in width, with an EI of 1.75. The external surface is smooth and the dark gray shell is very fractured. No pores were identified in a macroscopic observation.

Eggshell thickness is 248 μm (n = 80, sd = 14 μm). Microscopically, in radial section, (Fig 7B), the basal knobs and nucleation centers are evident, but the trapezoidal shell units are faint and in most cases, hard to define due to a strong sub-horizontal fracturing that prevents a clear observation of the tabular growth structure which is barely visible (Fig 7B). The basal plate groups make up approximately 20% of the eggshell thickness. A diagenetic layer, with a thickness of 71 μm, of diagenetic secondary deposits of calcite and recrystallization, covers the external surface. The eggshell has an extremely low porosity (less than one pore per cm2). In cross-polarized light (Fig 7B2), the irregular triangular extinction pattern is clearly visible.

ML195. ML195 are small, dark gray, eggshell fragments, less than 25 mm2 each, also found in association with a theropod nest. The outer and inner surfaces are smooth, with no discernible internal bumps of the basal knobs or pore openings on direct observation.

The eggshell thickness is 250 μm (n = 80, sd = 8 μm), with a 14 μm diagenetic layer overlaying the outer surface. Under the microscope, the radial section (Fig 7A) shows the darker basal knobs align along the inner surface of the eggshell, about 30 μm thick (approximately 14% of the total shell thickness). The trapezoidal shell units are very faint and barely distinguishable, but still present. The horizontal lamination or tabular structure is unevenly distributed, with a lighter colored portion and fainter growth lines just above the basal knobs (Fig 7A) making up about 160 μm of the total shell thickness (approximately 62%). Just above it, there is a thin darker band of more compacted horizontal growth lines (Fig 7A), approximately 60 μm thick (about 24% of the shell thickness). In cross-polarized light, the blocky extinction is present (Fig 7A2), although less conspicuous than in ML760, ML1795 and ML1194.

ML1194. Macroscopically, ML1194 are small fragments, also found in association with theropod eggs [64], very similar to ML195, both in dimensions and morphology. The inner and outer surfaces are smooth. In macroscopic observation, pores were not observed.

Eggshell thickness is 220 μm (n = 80, sd = 7 μm). Microscopically, in radial section, the shell units (Fig 7D) are very faint and hard to distinguish. The basal knobs and nucleation centers make up approximately 16% of total shell thickness, with 35 μm, and are characterized by a darker coloration (Fig 7D). About 140 μm thick (approximately 63% of shell thickness), there is a portion of the eggshell characterized by a horizontal tabular lamination that shows an increase inits density from the bottom to the top (Fig 7D). A darker zone is visible (Fig 7D) just above the previous, with the tabular growth more evident, more tightly packed together, about 50 mm thick (approximately 21% of total shell thickness). The diagenetic layer is very thin and sparse, not observable throughout the whole section, and at most 20 μm thick. With cross-polarized light (Fig 7D2), the extinction triangles are visible. Pores are very few (less than a pore per cm2), and have a subcircular opening, with a diameter of 110 μm, and straight long canals (Fig 9).

ML1795. ML 1795 includes four crushed, very fragmented eggs (Fig 8A), and eggshell fragments, found slightly south and above of ML760, and associated with a theropod nest [58]. Because the specimen is so fragile, it is still partially encased in its plaster jacket. Some of the dark brown mudstone matrix is present (Fig 8A). As with ML760, ML195, and ML1194, pores are undistinguishable macroscopically.

Eggshell thickness is 250 μm (n = 80, sd = 15 μm). In microscopic observation, the shell unit wedges are clearly observable, with basal knobs and nucleation centers visible (Fig 7C), and measuring about 60 μm (approximately 25% of shell thickness). The horizontal tabular lamination is present in the middle portion of the eggshell (Fig 7C), about 100 μm thick (approximately 40% of shell thickness). A darker area, just above the latter layer, showing a more compact lamination (Fig 7C), can be differentiated (approximately 90 μm thick, about 35% of shell thickness). A diagenetic layer (Fig 7C), about 140 μm, covers the external surface of the eggshell. Pores are very scarce (less than one per mm2) and hard to observe. Still, in Fig 8C, an obstructed pore opening can be seen. The pore diameter is 42 μm. The internal openings are not visible in the samples. With cross-polarized light, the irregular triangular extinction is observable (Fig 7C2). Under the stereomicroscope and using transmitted light, the darker areas corresponding to the basal knobs and tips of the shell units are evident in the tangential section (Fig 8B). Under the SEM, the nucleation centers are noticeable (Fig 8D).