Gobioolithus major Mikhailov, 1996a

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

Gobioolithus major Mikhailov, 1996a

Fig. 1B .

Holotype: PIN 4478-1, complete egg.

Type locality: Khulsan locality, South Gobi Aimak, Mongolia.

Type horizon: Barun Goyot Formation, Upper Cretaceous.

Material.—Five complete eggs from localities Khulsan and Gilbent (PIN 3142-460, 4478-2, 4478-5, 4478-6, 4478-7).

Diagnosis.—Egg size 50–53.5 by 25–32 mm; eggshell thickness 0.2–0.4 mm.

Description.— Gobioolithus major is here redefined to include only the small sample of eggs collected by Soviet-Mongolian teams from Khulsan and Gilbent. These eggs show a limited size range averaging 52 mm long with a diameter of 29 mm ( Fig. 2A ). Average EI of the available specimens was 1.75, the lowest value among the Gobioolithus oospecies and the larger avian eggs of Sabath (1991), here recognized as Styloolithus sabathi oogen. et oosp. nov. ( Fig. 2B ). Although prior publications lack specific microstructural descriptions, photomicrographs, and details of the field occurrence for G. major, Mikhailov (1996a , 1997b) confirms their similarity to the microstructure of G. minor .

Remarks.—Sediments within and surrounding the eggs consist of fine sand. Further, two small “hills” at the Khulsan locality preserved a distribution and orientation of the G. major eggs similar to that of the G. minor at Khermeen Tsav, suggesting the same breeding and nesting ecology in the two ootaxa (Konstantin Mikhailov personal communication). The PIN specimens show some lateral distortion as well as telescoping of shell parallel to the long axis, consistent with a sub vertical posture during burial.

Stratigraphic and geographic range.—Upper Cretaceous, Barun Goyot Formation, Mongolia.

Incertae sedis

Oogenus Styloolithus nov.

Figs. 1A , 3 , 4A .

Etymology: From Greek styl, pillar or column, referring to the columnar arrangement of the elongate eggs within the clutch; oo, ova; and lithos, stone.

Type oospecies: Styloolithus sabathi nov., monotypic; see below.

Diagnosis.—As for type and only known oospecies.

Stratigraphic and geographic range.—As for the only oospecies.

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. 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.

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.