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Styloolithus, new avian egg genus from Mongolia + Jurassic marine vertebrates from Mexico

Ben Creisler

Two recent papers:

David J. Varricchio and Daniel E. Barta (2014)
Revisiting Sabath's "Larger Avian Eggs" from the Gobi Cretaceous.
Acta Palaeontologica Polonica (in press)

In 1991, Sabath described "larger avian eggs" from the Upper
Cretaceous Barun Goyot and Djadokhta Formations of Mongolia. These
were later included in the ootaxon Gobioolithus major. Here we
recognize the larger avian eggs of Sabath as a distinct ootaxon,
Styloolithus sabathi, oogen. et oosp. nov. These eggs differ from
those of Gobioolithus in being larger (70 by 32 mm) and more elongate.
Microscopically, the shell bears a third layer (possible external
zone) thicker than the mammillary layer and nearly as thick as the
second layer (possible squamatic zone); the continuous layer
(including layers two and three) to mammillary layer thickness ratio
is 3.1:1. Within the clutch, the tightly spaced eggs stand with their
long axes steeply inclined. Adult remains are associated with two
clutches, suggesting an incubation mode similar to that of troodontid
maniraptorans, where adults sat atop largely buried eggs. S. sabathi
provides evidence that relative egg size in Mesozoic
non-ornithuromorph birds had increased markedly from the non-avian
theropod condition in oviraptorids and troodontids, but has not yet
reached the modern egg-adult proportions of Neornithes. Sediment-bound
upright eggs appear common to Enantiornithes and more basal avians,
suggesting that like nonavian theropods, these birds lack chalazae,
the chords of albumen allowing egg rotation in modern birds. Absence
of this simple structure may have restricted these basal birds to
ground nesting in areas with appropriate substrates and not permitted
the type of nesting diversity found in Neornithes. Neornithes are the
only Mesozoic clade of Dinosauria to nest completely free of sediment;
this may have played a crucial role in their surviving the K-Pg mass
extinction event.


Jesús Alvarado-Ortega, Jair Israel Barrientos-Lara, Luis
Espinosa-Arrubarrena, and María del Pilar Melgarejo-Damián (2014)
Late Jurassic marine vertebrates from Tlaxiaco, Oaxaca State, southern Mexico.
Palaeontologia Electronica Vol. 17, Issue 1;24A; 25p;

This paper provides the first geological and paleontological data of
Yosobé and La Lobera, two Late Jurassic vertebrate localities; both
localities are part of the marine deposits of the Tlaxiaco Basin, near
Tlaxiaco, Oaxaca, Mexico. La Lobera may be included in Cerro de
Titania, a historical invertebrate locality inaccurately described,
whose Oxfordian-Early Kimmeridgian limestone and marl strata are
informally known as “Caliza con Cidaris. ” La Lobera contains the
typical “Caliza con Cidaris” fossils and the remains of a single fish
attributable to the genus Scheenstia. Yosobé is a fossiliferous
outcrop belonging in the Late Jurassic (Kimmeridgian-Tithonian)
bituminous shale strata informally known as Sabinal formation. The
fossil assemblage from Yosobé includes microfossils, plants,
invertebrates, and vertebrates, many of which are totally or partially
tridimensionally preserved within nodules. Yosobé is the most
important vertebrate locality along the entire Tlaxiaco Basin due to a
highly abundant and diverse grouping of fishes and reptiles. The
fossil assemblage from Yosobé reported in this paper includes the
first and most inclusive regional record of taxa such as the first
North American Pleuropholidae fish, the first Mexican Jurassic
Planohybodus shark, the most complete specimens of the pycnodont
Gyrodus in Mexico and the Caribbean, ichthyosaurs, thalattosuchian
crocodyliforms, turtles, and pliosaurids in a single locality at the
Western Tethys Sea Domain and the Hispanic Corridor. Present and
future descriptive and systematic studies on some fossils from these
localities may have important paleobiogeographic and phylogenetic
implications worldwide.