Sankofa (oogenus)

Sankofa is an oogenus of prismatoolithid egg. They are fairly small, smooth-shelled, and asymmetrical. Sankofa may represent the fossilized eggs of a transitional species between non-avian theropods and birds.[1]

Sankofa
Temporal range: Late Cretaceous
Egg fossil classification
Basic shell type: †Dinosauroid-prismatic
Oofamily: †Prismatoolithidae
Oogenus: †Sankofa
López-Martínez & Vicens, 2012
Oospecies
  • †S. pyrenaica López-Martínez and Vicens, 2012 (type)

Etymology

The name Sankofa comes from the Ashante word "sankofa", meaning 'learning from the past', symbolized by a bird with an egg in its bill. The oospecific epiphet pyrenaica refers to the Pyrenees, where the eggs were first discovered.[1]

Distribution

Sankofa pyrenaica is known solely from the Aren Formation, in the southern Pyrenees of Catalonia, Spain, dating from the Upper Campanian to Lower Maastrichtian.[1]

Description

Sankofa eggs are uniformly 7 cm long and 4 cm wide, and their eggshell averages 0.27 mm thick. The eggshell consists of two layers, the prismatic (or palisade) layer and the mammillary layer, similar to most other non-avian dinosaur eggshells.[2] In Sankofa, these two layers have a gradual boundary, and the mammillary layer is much thinner than the prismatic. The prismatic has a slightly squamatic microstructure, very similar to the eggs of Troodon and other prismatoolithids, a step towards the fully squamatic texture of bird eggs. The eggshell has a smooth surface with no trace of ornamentation, and highly variable pore density.[1]

The most significant characteristic of Sankofa is its shape: they are asymmetric, with an ovoid shape, like bird eggs. A morphometric analysis by López-Martínez and Vicens in 2012 found that it was an intermediate shape between avian and non-avian theropods, and also very similar to an enantiornithine egg from the Bajo de la Carpa Formation in Argentina, though their microstructures are quite different.[1]

Classification

Parataxonomically, Sankofa is classified in the oofamily Prismatoolithidae, because of its microstructure. Cladistic analysis found Sankofa to be in a polytomy with Protoceratopsidovum, Troodon eggs, and birds. The mosaic of avian and non-avian characteristics makes it uncertain whether S. pyrenaica was laid by a bird or a non-avian theropod, and provides further evidence for the theory that birds evolved from dinosaurs.[1] It probably represents a transitional form between the two groups.[1]

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References
  1. López-Martínez, Nieves; Vicens, Enric (2012-03-01). "A new peculiar dinosaur egg, Sankofa pyrenaica oogen. nov. oosp. nov. from the Upper Cretaceous coastal deposits of the Aren Formation, south-central Pyrenees, Lleida, Catalonia, Spain". Palaeontology. 55 (2): 325–339. doi:10.1111/j.1475-4983.2011.01114.x. ISSN 1475-4983.
  2. Laura E. Wilson, Karen Chin, Frankie D. Jackson, and Emily S. Bray. (2012). "Fossil eggshell: Fragments from the past" UCMP's online fossil egg exhibit.
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