Subtiliolithus sabathi

Varricchio, David J. & Barta, Daniel E., 2015, Revisiting Sabath’s “ Larger Avian Eggs ” from the Gobi Cretaceous, Acta Palaeontologica Polonica 60 (1), pp. 11-25 : 13-20

Styloolithus sabathi nov.

Figs. 1A , 3 , 4A .

Etymology: After Karol Sabath (1963–2007) in recognition of his initial description of these specimens and his stimulating contributions to the study of fossil eggs.

Holotype: ZPAL MgOv-II/7a–e, a clutch of at least four eggs associated with adult remains.

Type locality: Volcano locality, Bayn Dzak, South Gobi Aimak, Mongolia.

Type horizon: Djadokhta Formation, Upper Cretaceous.

Material. —ZPAL MgOv-I/19, MgOv-I/21a–c, and MgOv-

30 40 50 60 70 80 30 Length

40 50 60 70 80 Length

/25c–d, from Khulsan locality, South Gobi Aimak, Mongolia, Barun Goyot Formation, Upper Cretaceous; MgOvII/6a–g and MgOv-II/25 from Bayn Dzak, South Gobi Aimak, Mongolia, Djadokhta Formation, Upper Cretaceous.

Diagnosis.—Differs from Gobioolithus minor and G. major by large size (70 × 32 mm), higher elongation index

2.0–2.3), and clutches consisting of likely more than four eggs, sub-vertically oriented and tightly spaced ( Figs. 1–3 ). Potentially differs from G. minor , G. major , and all non-avi-

an theropod eggs by the presence of a third layer (possible external zone) thicker than its mammillary layer, and nearly as thick as its second layer (possible squamatic zone). Continuous layer (= second and third layers) to mammillary layer ratio is 3.1:1 rather than 2:1 ( Fig. 4 ).

Description.—The largest intact piece for ZPAL MgOv-II/7 includes four partial eggs standing with their long axes nearly parallel in well-sorted fine sandstone and adjacent to one another. Immediately above them is an articulated hind limb with distal femur, and proximal tibia and fibula. Assuming the bones were flat lying puts all four eggs steeply inclined with plunges of 45–70°. All trend in nearly the same direction. The eggs are elongate and smooth. Three well-preserved eggs retain complete eggshell circumferences over their lowermost 60–70%. Weathering has reduced the fourth egg to one partial side. All four eggs appear to have broken upper ends. One egg has a larger than 1.5 cm piece of displaced eggshell lying across its broken upper end. A second retains a portion of eggshell wrapping over to form the blunt end but matrix overlies other portions of the cross-section. Matrix covers the broken upper ends of the other eggs as well.

Two eggs still have their bottoms intact, but egg dimensions are still difficult to assess on these matrix bound eggs, and better proportions come from other specimens. Preserved lengths are 55 and 59 mm. Estimated original length was likely around 70 mm. One egg provides a maximum available diameter of 29 mm, but is clearly partially distorted. Normal maximum diameter likely exceeded 30 mm. Associated with the matrix-bound eggs are four additional egg bottoms. The uniform and asymmetric orientation of the matrix-bound eggs suggests that if the eggs within the clutch were arranged symmetrically, then at least four eggs would be needed to complete the clutch. Hence, these isolated eggs likely belong to the clutch and the minimum number of eggs was eight. These pieces demonstrate that the eggs should have a circular cross-section, but that this is often distorted due to lithostatic compaction. One egg has matrix covering the upper broken end. This distorted specimen has a length of 52 mm to the break, and a maximum diameter of 35 mm with a 30 mm perpendicular diameter. Consequently, the original estimate of 70 by 32 mm is likely accurate ( Sabath 1991).

Three bones lie in articulation across the upper portions of the four eggs. In their current state of preservation and preparation, the femur and fibula provide only few details. The poorly preserved distal femur consists of a shaft fragment eroded to expose the large medullary cavity that expands slightly distally for the development of the condyles. Maximum diameter is 8.4 mm. The preserved portion of the tibia measures over 40 mm, and with the impression of its diaphysis, 63 mm. However, these are likely far short of the actual length of the original element. The mediolateral width at the proximal end is 9.7 mm, and at the level of the crista fibularis, 8.1 mm. The mediolateral and anteroposterior diameters of the distal most shaft are 5.7 mm and 3.5 mm. Proximally, the cnemial crest appears robust despite portions of its anteriormost projection being broken off. The cnemial crest angles anterolaterally. Distally the crest thins rapidly and disappears into the shaft by about 29 mm down the shaft. The crista fibularis is thin anteroposteriorly but projects as far as the lateral margin of the proximal end. The medial margin of the tibia is slightly concave until a point equivalent to the end of the cnemial crest. The shaft is anteroposteriorly compressed with a just slightly convex anterior aspect, and more convex posterior one. Portions of the anterior aspect of the fibula are damaged, however some morphology remains visible. Proximally, the fibula has a rounded, mediolaterally compressed articulation. The shaft arcs medially and expands only to taper rapidly and end at the level of the proximal crista fibularis.

These elements provide a few features likely relevant to the taxonomic identification: the size and breadth of the crista fibularis, the arc of the proximal tibia, and the relatively short and unfinished condition of the fibula. Nanantius valifanovi Kurochkin 1996 : fig. 10), now likely Gobipteryx minuta Chiappe et al. 2001 ), exhibits all of these features. However these elements in ZPAL MgOv-II/7 are nearly twice the linear dimensions, have a more prominent cnemial crest, and appear to lack the nutrient foramen on the Nanantius tibia.

Three other bone fragments were collected with this specimen, but their relationship to the eggs is unclear. These include a shaft cross-section (2.7 by 4.2 mm), the end of a compressed element, and a relatively large (18 by 23 mm) flattish fragment, which seems too robust to correspond with the leg elements.

ZPAL MgOV-II/25 represents a second Bayn Dzak specimen that preserves bone in association with eggs. The specimen consists of a sandstone block with one near complete egg and a small impression of a second. Above these, as in ZPAL MgOV-II/7, lie one or perhaps two skeletal elements. However, these elements have sustained substantial recent erosion. The good egg has a similar shape and surface texture to those of the type specimen, being elongate and smooth. The incomplete egg is 57 mm from its intact narrow pole to the upper broken edge, marked by a broken and displaced fragment. The diameter is 28.6 mm. Adjacent to this egg is small patch where eggshell once sat. It is unclear if this represents a remnant of an entire egg or simply a displaced fragment. Sabath (1991) considered there to be a third possible egg, but this was not apparent in the current condition of the specimen.Assuming the limb bone was horizontal, the intact egg would have a plunge of 45°.

The bone sits 15 mm above the egg and consists merely of a shaft fragment and an impression of bone separated by a gap of 11 mm. The trend is consistent with the two parts representing a single element as indicated by Sabath (1991: pl. 16: 2). The represented length of the element would be 52 mm, likely something far short of the element’s true length. Because the cross-section changes from more triangular to transversely expanded, the bone likely represents a tibia. Shaft dimensions are 3.6 by 5.0 mm and more distally 2.3 by 5.1 mm.

Another egg, curated under this same specimen number, ZPAL MgOv-II/25, is particularly complete, providing a good view of the egg size and shape. The egg is clearly asymmetric with both narrow and blunt ends and with the maximum diameter closer to the blunt end.This egg measures 69.1 by 30.3 mm. With the previous eggs, this provides an estimate of the elongation index for S. sabathi of on average 2.2.

ZPAL MgOv-II/6 consists of a collection of eggs from the Ruins at Bayn Dzak. Likely only some of these eggs belong to S. sabathi . Among the ZPAL MgOv-II/ 6 eggs is a complete but distorted egg with a length of 71.1 mm and an average diameter of 25.5 mm. However, externally this and the other eggs have irregular boundaries showing crushing, lateral shifting and bulging related to telescoped portions of the eggs. Broken portions reveal that internally these eggs are largely filled with crystals and open spaces in contrast to the earlier described specimens.

Other specimens consist of only partial eggs. For example, ZPAL MgOv-I/19 consists of a box of 26 egg halves or smaller parts collected in 1970 from Khulsan. Thirteen of these consist of the narrow pole and varying amounts of the egg. A few represent cross-sections, and others simply assorted egg fragments. One ornamented piece clearly does not belong. Three egg fragments contain abundant small shell fragments nested within each other in the bottom of the egg, in a rosette-like pattern. A fourth egg bottom has a nearly intact bottom, but its upper portion consists of small highly fractured shell. The accumulation of shells within the egg bottoms is consistent with an upright egg orientation. Potentially, as eggs were exposed subaerially in the Cretaceous, erosion and breakage of the eggshell could lead to the build up of fragments in the egg bottom.

As noted by Sabath (1991), the eggshell is poorly preserved. Typically, eggshell catalogued under ZPAL Mg0v- II/6 measures 0.25 mm thick and displays three possible structural layers. We were unable to examine the ultrastructure of these layers with scanning electron microscopy (SEM); thus, interpretations of Styloolithus sabathi eggshell must remain tentative at this time. Because of this uncertainty, our use of the terms “second layer” and “third layer” in the following eggshell description should not be treated as necessarily equivalent to the avian and non-avian theropod eggshell terms “squamatic zone” and “external zone”, respectively. However, we provide comparisons of these internal divisions within S. sabathi eggshell to the squamatic zone and external zone of better-known eggshells and suggest their possible homology. Mikhailov (1997a) defines the continuous layer of avian and non-avian theropod eggshell to consist of both the squamatic zone and external zone. Our use of “continuous layer” to refer to Styloolithus eggshell in the Eggshell comparisons section below is for ease of comparison with previous measurements of this layer only, and refers to the combined thickness of the second and third layers, our uncertainty about the ultrastructure of which is described above and in the following paragraphs. We interpret a fourth layer as diagenetic overgrowth ( Fig. 4A ).

The innermost layer is rather thin, averaging 0.06 mm. Though nucleation sites are not clearly preserved, the few remaining altered mammillary cones are spaced, on average, 0.08 mm apart, as measured center-to-center from radial thin sections. Faint radiating crystal structures arising from the knob-shaped mammillae may be present as in Gobioolithus minor ( Mikhailov 1991: pl. 35: 2a, b, 3), though the diagenetic alteration and lack of SEM images for these specimens do not allow for more definite confirmation of this observation. Future examination of the eggshell with cathodoluminescence and SEM could clarify the extent of alteration and other features of S. sabathi eggshell described below.

The transition between the first and second layers is fairly gradual and is marked by the first appearance of dark-colored horizontal banding that increases in density throughout the 0.10 mm thick second layer. The prismatic shell units are clearly visible in the second layer under polarized light, but largely hidden in plain light where the strong horizontal banding predominates. The second layer exhibits dark-colored, dense horizontal lines in thin section that may represent the lamellar arrangement ( Dennis et al. 1996) of the organic matrix of squamatic ultrastructure. The second layer also appears similar to that of a Cretaceous avian egg from the Neuquén locality of Argentina ( Fig. 4C, D ) where SEM examination confirms the presence of the squamatic ultrastructure in this layer ( Schweitzer et al. 2002: fig. 1). Squamatic ultrastructure is commonly found to varying degrees of development in the squamatic zone of the continuous layer of derived maniraptoran and avian eggshells ( Zelenitsky et al. 2002). Despite the above evidence suggesting the presence of squamatic ultrastructure in the second layer of S. sabathi eggshell, we note that SEM is necessary to confirm the presence of the individual scale-shaped grains of calcite that characterize squamatic ultrastructure ( Mikhailov 1991). As the term “squamatic zone” properly refers to the second layer of eggshell for which squamatic ultrastructure has been observed ( Mikhailov 1991; 1997a), we use the term “second layer” in the absence of SEM observation of squamatic ultrastructure in Styloolithus sabathi eggshell.

The third structural layer has an average thickness of 0.09 mm and is separated from the underlying layers by an abrupt transition, visible primarily as a color difference between the darker second layer and the more translucent third layer. The position of this transition varies slightly among adjacent shell units. The margins of a potential pore are visible through the entire thickness of layers one through three ( Fig. 4A ). We interpret the third layer as an external zone, sensu Mikhailov (1991), despite its great relative thickness, based on its similar appearance to the external zone of some avian eggshells as viewed in thin sections (Jackson et al. 2010: fig. 3A, B; 2013: fig. 3D, E). The eggshells figured in Jackson et al. (2010, 2013) share a darker continuous layer that abruptly transitions to a lighter-colored overlying external zone. Mikhailov (1991) notes that such translucence and a lesser amount of organic content compared to the continuous layer is characteristic of the external zone. The observed lack of horizontal banding and continuation of the columnar extinction pattern through the third layer of Styloolithus sabathi eggshell in thin section concurs with descriptions of extant avian external zones ( Dennis et al. 1996; Fraser et al. 1999), thus demonstrating their probable homology, though SEM is needed to test this. Previous authors ( Zelenitsky et al. 2002; Varricchio and Jackson 2004a; Jackson et al. 2010) often refer to the external zone in theropod and avian eggshell as an “external layer”, but Mikhailov (1997b) suggests that the external zone can be treated as a separate structural layer only when better demarcated from the squamatic zone, as in paleognaths and members of the Galloanserae. We note that all of these authors, regardless of terminology, refer to the same general structure, one that is likely homologous among derived maniraptorans and birds, but absent in oviraptorosaurs ( Elongatoolithidae ), for example ( Zelenitsky et al. 2002; Varricchio and Jackson 2004a).

The absence of a third layer in some thin sections catalogued under ZPAL Mg-Ov-II/6 is likely a taphonomic phenomenon ( Fig. 4A ). In these specimens the thickness of the remaining eggshell corresponds to the combined thickness of the mammillary and second layers in complete specimens. Further, they exhibit an outer surface with an irregular texture and some loose calcite grains that appear to have separat- ed from the outer surface of the exposed second layer. Given that the external zone of fossil eggshells is often found partially separated from the underlying squamatic zone, it has been inferred that a plane of weakness likely exists between the squamatic and external zones in theropod and avian eggshell ( Jackson and Varricchio 2010; Jackson et al. 2013). We have observed a similar separation in modern goose eggshell (uncatalogued MSU ES specimen). These examples highlight the potential for errors in interpreting an external zone as absent in taphonomically altered fossil eggshell ( Zelenitsky et al. 2002; Jackson and Varricchio 2010).

An outermost fourth layer is separated by a clearly demarcated line from the remainder of the eggshell and displays a continuation of the columnar extinction pattern of the shell units throughout its thickness. Many of the crystals are deflected at a slight angle at the boundary between the third and fourth layers, and some display a blockier morphology with irregular boundaries.

Evidence in favor of a diagenetic origin for the fourth layer includes the abrupt transition with the third layer at its base, its inconsistent thickness, its relative lack of dark-colored, presumably organic matter compared to all of the underlying layers, and the occasionally irregular shape of some of its crystals compared to the underlying columns ( Fig. 4A ). Though the fourth layer exhibits crystallographic continuity with the third layer, diagenetic calcite can grow in structural continuity with biogenic calcite ( Grellet-Tinner et al. 2010).

Eggshell comparisons.—Microstructure, egg size, and elongation index distinguish Styloolithus sabathi from all avian and non-avian theropod eggs from the Cretaceous (Table). Styloolithus sabathi differs from the Mongolian elongatoolithid oogenera Macroolithus and Elongatoolithus by its smaller egg size, lack of surface ornamentation, possible presence of an external zone, and clutches with subvertically oriented eggs. Though they share a similar elongation index and asymmetrical shape, S. sabathi differs from all members of the Mongolian prismatoolithid oogenus Protoceratopsidovum Mikhailov, 1994 , by its smaller egg size, thinner eggshell, and possible presence of an external zone. It further differs from Protoceratopsidovum fluxuosum by its lack of surface ornamentation. S. sabathi has a greater continuous layer (CL): mammillary layer (ML) thickness ratio than Protoceratopsidovum sincerum (PIN 614-58/1, PIN 3143/121; Fig. 4B ), and Protoceratopsidovum minimum (PIN uncatalogued).

Styloolithus sabathi differs from eggs referred to the Mongolian troodontid Byronosaurus jaffei , and the Mongolian oospecies Oblongoolithus glaber , Laevisoolithus sochavai , and Subtiliolithus microtuberculatus through possessing a greater CL: ML ratio, lesser eggshell thickness, and possible presence of an external zone ( Table 1). Styloolithus sabathi also lacks the small tubercles sometimes present on the surface of S. microtuberculatus eggshell ( Mikhailov 1991). S. sabathi shares the presence of a possible external zone with Parvoolithus tortuosus and unnamed eggs containing enantiornithine embryos from the Late Cretaceous of Mongolia Zelenitsky 2004; Balanoff et al. 2008; Varricchio et al. in press) and Argentina ( Schweitzer et al. 2002; Fernández et al. 2013), as well as a small unnamed egg from the Campanian Bayn Dzak locality, Mongolia ( Grellet-Tinner and Norell 2002). S. sabathi differs from all of these eggs by having a larger CL: ML ratio, greater eggshell thickness (except for the Argentine eggshell, which has a nearly equal thickness), and larger egg size.

Differences between Styloolithus sabathi and the two Gobioolithus oospecies include larger egg size, a greater elongation index, greater CL: ML thickness ratio, and the possible presence of an external zone. S. sabathi additionally possesses thicker eggshell than G. minor ( Table 1, Fig. 4C ).

Styloolithus sabathi has smaller eggs, a greater CL: ML ratio, and a lesser eggshell thickness than the North American troodontid Troodon formosus (oospecies Prismatoolithus levis ). The external zone of Troodon also does not always appear more translucent compared to the prismatic layer in thin section (Jackson et al. 2010), as is potentially the case for S. sabathi , some Eocene avian eggs ( Kohring and Hirsch 1996; Jackson et al. 2013) and extant avian ( Kohring and Hirsch 1996; Jackson et al. 2010) eggshells. In one T. formosus thin section (Jackson et al. 2010: fig. 2F), a transition from a dark- er to a lighter portion of the prismatic layer seen about twothirds of the way towards the exterior eggshell surface does not correspond to the boundary between the prismatic layer and external zone as confirmed by SEM imaging (Jackson et al. 2010: fig. 2D). This may suggest that the possible external zone in S. sabathi is not as thick as inferred from color transitions in thin section alone. Nevertheless, correspondence between color transitions in thin section and the position of the external zone in SEM is found for other Troodon eggshell (though defined by a transition from a lighter prismatic layer to a darker external zone, opposite of the avian examples below) ( Varricchio et al. 2002) and the eggshell of fossil and extant birds ( Kohring and Hirsch 1996; Jackson et al. 2010, 2013). However, the effects of differing thin section thickness and potential diagenetic alteration of eggshell should temper assertions about the thickness of eggshell structural zones or layers inferred solely on the basis of color transitions.

The presence of an external zone in Cretaceous Mongolian avian eggs remains controversial, and questions wheth- er or not the possible presence of an external zone in Styloolithus sabathi is unique among these eggs. Despite his earlier (1991) assignment of Gobioolithus eggshell to the “neognathous morphotype”, Mikhailov (1997b, 2014) considered an external zone to be absent in Gobioolithus minor eggshell, and Mikhailov (2014) proposed all third layers reported from similar eggs (e.g., those in Grellet-Tinner and Norell 2002; Balanoff et al. 2008) to be diagenetic in origin. If Mikhailov’s (1997b, 2014) assertions are correct, the lack of an external zone in G. minor (and presumably G. major ) represents a major difference with Styloolithus sabathi . However, reexamination of an SEM image ( Mikhailov 1991: pl. 35: 1) of G. minor microstructure suggests an alternative hypothesis. If the diagenetically altered upper third of the eggshell is considered to be a recrystallized external zone, then the ratios between the three layers would be similar to those observed for S. sabathi . Additionally, a possible third layer can be observed in some G. minor thin sections ( Fig. 4C ). Similar to the possible third layer in the SEM image, this layer varies greatly in thickness and exhibits blocky crystal morphology. While both of these traits can be indicative of recrystallization, the possibility remains that what is observed is an altered external zone. Mikhailov (2014) noted that well-preserved examples of G. minor lack the “explicit presence” of an external zone, though given the apparent ease with which this zone can separate from the remainder of the eggshell, this may not constitute definitive evidence against the presence of an external zone in G. minor . We consider the presence or absence of an external zone in G. minor eggshell an open question, awaiting the application of analytical techniques such as those utilized by Jackson et al. (2010) to assess the presence of an external zone in Troodon formosus eggshell.

Interestingly, Zelenitsky (2004) described an external zone for Parvoolithus tortuosus Mikhailov 1996b , a possible avian egg ( Zelenitsky and Therrien 2008) from Mongolia that is nearly identical in size to both Gobioolithus minor and an egg with a possible external zone described by Balanoff et al. (2008). Though Mikhailov (1996b) considered Parvoolithus as incertae sedis on the basis of diagenetically altered material, Zelenitsky (2004) examined better preserved eggshell of the oogenus and identified a typical non-avian or avian theropod division of structural layers and zones mammillary layer, squamatic and external zones). The avian identity of P. tortuosus is posited based on the results of a cladistic analysis of oological characters ( Zelenitsky and Therrien 2008). An external zone is also known for a probable enantiornithine egg from Neuquén, Argentina ( Schweitzer et al. 2002; Fig. 4D ). Thus, Mongolian avian eggshells other than Styloolithus sabathi may also possess an external zone, necessitating use of additional diagnostic characters to identify S. sabathi in the absence of whole eggs.

Though Styloolithus sabathi shares vertically inclined, asymmetrical eggs and some microstructural features with some prismatoolithid eggs (including columnar prisms and, with Prismatoolithus levis , a possible external zone), its overall egg size and shell thickness are less than that of any known prismatoolithids. Furthermore, its prismatic columns are more consistently obscured under plane-polarized light than those of Prismatoolithus levis and the Protoceratopsidovum oospecies. Additionally, the presence of a possible external zone thicker than the mammillary layer and nearly as thick as the second layer is not observed in prismatoolithids and other non-avian theropod eggs. Similarly thick external zones are known, however, for some members of the extant avian orders Procellariiformes ( Procellariidae , Pelecanoididae , Hydrobatidae ), Ciconiiformes ( Ardeidae , Cochlearidae, Ciconiidae ), Falconiformes ( Falconidae ), Gruiformes Eurypygae), Charadriiformes (Charadrii), and Cuculiformes Cuculinae) ( Mikhailov 1997a). When Styloolithus sabathi is plotted on the EZ / SqZ vs. eggshell thickness graphs of Mikhailov (1997a), it plots above most of the regression lines for the various extant avian groups, showing that its EZ / SqZ ratio is at the upper range for that of living birds. The CL: ML ratio of S. sabathi plots within the range of extant avian taxa on the same graphs.

Stratigraphic and geographic range.—Khulsan locality, South Gobi Aimak, Mongolia, Barun Goyot Formation, Upper Cretaceous; Volcano and Ruins localities, Bayn Dzak, Mongolia, Djadokhta Formation, Upper Cretaceous.

Figures

Fig. 1. Representative eggs for the three forms originally included in the oogenus Gobioolithus from the Late Cretaceous of Mongolia. A. The larger avian eggs of Sabath (1991), Styloolithus sabathi oogen. et oosp. nov. (ZPAL MgOv-II/25). B. Gobioolithus major Mikhailov, 1996a (PIN 4478-2). C. Gobioolithus minor Mikhailov, 1996a (ZPAL MgOv-III/10). The former classification included eggs such as ZPAL MgOv-II/25 representing the “larger avian eggs” of Sabath (1991) with Soviet collected specimens as G. major. Here we separate out the former based on size and shape as S. sabathi.

Fig. 3. Egg clutches for Styloolithus sabathi oogen. et oosp. nov. from the Upper Cretaceous Bayn Dzak locality. A. Type specimen, ZPAL MgOv-II/7a in dorso-oblique (A 1), lateral (A 2), and dorsal (A 3) views. A 1 and A 2 show three of the four eggs, whereas A 3 provides a view of the articulated distal femur and proximal tibia and fibula. The cnemial crest and crista fibularis are visible in this anteromedial view of the tibia. B. ZPAL MgOv-II/25, a second partial clutch with associated bone in lateral view showing one nearly intact egg and a small portion of a second (e). A poorly preserved tibia runs nearly horizontally just above and left of the egg.

Fig. 4. Petrographic thin sections of avian and non-avian theropod eggshells from the Upper Cretaceous. A. Styloolithus sabathi oogen. et oosp. nov. (ZPAL MgOv-II-6); horizontal lines at the left side of A1 mark, from bottom to top, the boundaries between the mammillary, second (possible squamatic zone), and third (possible external zone) layers. A2, same as A1, but in polarized light. A3, thin section 9-6 highlighting the same three structural layers as in A1, including the mammillary layer (ML), second layer (2L), and third layer (3L). DL represents the outer diagenetic layer. A4, thin section 9-5, showing properly proportioned ML and 2L, but a loss of the 3L. B. Protoceratopsidovum sincerum Mikhailov, 1994 (PIN 3143/121), horizontal line at left marks the boundary between the mammillary layer and palisade layer. C. Gobioolithus minor Mikhailov, 1996a (ZPAL MgOv-III/11b), horizontal lines at left mark, from bottom to top, the boundaries between the mammillary layer, squamatic zone, and an outer recrystallized zone, possibly representing either diagenetic overgrowth or an altered biological external zone. D. Neuquén enantiornithine eggshell MSU ES 177 (Schweitzer et al. 2002: fig. 1), horizontal lines at left in D 1 mark, from bottom to top, the boundaries between the mammillary layer, squamatic zone, and external zone. D2, same as D1, but in polarized light. Images in plane (A1, A3, A4, B, C, D1) and polarized (A2, D2) light.

Fig. 2. Plots of diameter vs. length (A) and elongation index (EI) vs. length (B) for the three forms originally included in the oogenus Gobioolithus, showing good separation between the three varieties. Gobioolithus minor and Styloolithus sabathi oogen. et oosp. nov. data comes from ZPAL specimens, the data for G. major from PIN specimens. Much of the variation in EI appears to represent variability in the distortion of eggs, with some foreshortened along their long axis, whereas others exhibit modified diameters.