Thursday, January 7, 2016

[PaleoOrnithology • 2016] Is the “Genyornis” Egg of A Mihirung or Another Extinct Bird from the Australian Dreamtime?

Fig. 1. A, Comparison of SAM P.42421, the ‘Spooner Egg’, left, attributed to Genyornis newtoni, and an emu egg (Dromaius novaehollandiae) SAM B.9899. B, Comparison of femora of Genyornis newtoni SAM P13864 and a 22 cm long Dromaius novaehollandiae femur (FUR 058). C, The Spooner Egg as partly excavated revealing its intact nature; photograph by Gifford Miller, INSTAAR, Colorado. D, The Spooner Egg in situ as found by NS on 23rd July 2000, photograph by Gifford Miller using a reflex camera, and is the best image taken of the egg prior to excavation.
Scale bars in A and B = 10 cm. DOI:  10.1016/j.quascirev.2015.12.011  

• Eggshell previously identified as from Genyornis newtoni is reassessed.
• Egg size and microstructure is not conducive with an identity as an dromornithid.
• We suggest that this eggshell is from one of the extinct megapodes in Progura.
• Previous assessments of the timing of Genyornis extinction relate to Progura species.

The iconic Australian Genyornis newtoni (Dromornithidae, Aves) is the sole Pleistocene member of an avian clade now hypothesized to be alternatively in Anseriformes or the sister group of crown Galloanseres. A distinctive type of fossil eggshell commonly found in eroding sand dunes, has been referred to Genyornis newtoni since the 1980s. The 126 by 97 mm Spooner Egg, dated at 54.7 ± 3.1 ka by optical dating of its enclosing sediments, is a complete specimen of this eggshell type that was reconstructed from fragments of a broken egg. We show that the size of the eggs from which this ‘Genyornis’ eggshell derives, either as predicted from measurements of fragments, or as indicated by the Spooner Egg, is unexpectedly small given the size of G. newtoni, which has an estimated mass of 275 kg, or about seven times the mass of the emu that has a similar sized egg. We compared the microstructure of the putative Genyornis eggshell to that of other dromornithids and a range of galloanseriform taxa using several microcharacterisation techniques. The ‘Genyornis’ eggshell displays a mosaic of oological characters that do not unambiguously support referral to any known modern bird. Its shell structure, coupled with chemical compounds in the accessory layer, makes it unlikely to have been laid by a dromornithid, whereas several characters support a megapode origin. A potential candidate for the bird that laid the putative ‘Genyornis’ eggs in the Pleistocene fossil avifaunal record has been ignored: Progura, a genus of extinct giant megapodes, whose species were widespread in Australia. Regression of egg size of megapodes and body mass shows that the Spooner Egg approximates the expected size for eggs laid by species of Progura. We advance the suggestion that the fossil eggshell hitherto referred to Genyornis newtoni, is more likely to have been laid by species of the giant extinct Progura. As megapodes, the species of Progura were obligate ectothermic incubators, which we suggest laid their eggs into a hole dug in sand like the modern megapode Macrocephalon maleo, thus explaining the abundant ‘Genyornis’ eggshell in sand dunes. Referral of this eggshell to Progura means that the fossil record of Genyornis newtoni is limited to bones and the timing of the extinction of this last dromornithid is unknown. In addition, structural similarities of eggshell in megapodes, the putative Genyornis eggshell and dromornithids, raise the possibility that these taxa are phylogenetically more closely related to each other than any is to anseriforms. Specifically, this means that dromornithids might be a sister group to galliforms rather than to or within anseriforms.

Keywords: Eggs; Eggshells; Paleoenvironments; Genyornis newtoni; Dromornithids; Megapodes; Progura; Micro-CT; EBSD; Quaternary; Australia

We have described in detail the structure of putative Genyornis eggshell and raised several obstacles to the hypothesis first advanced by Williams (1981) and accepted thereafter ( Miller et al., 1999 and Miller et al., 2005), that this ootype was laid by the giant dromornithid G. newtoni. Rather, we think it more likely that it was laid by one of the several species of giant megapodes in the genus Progura that were widespread in the Pleistocene in Australia.

Gerald Grellet-Tinner, Nigel A. Spooner and Trevor H. Worthy. 2016. Is the “Genyornis” Egg of A Mihirung or Another Extinct Bird from the Australian Dreamtime? QUATERNARY SCIENCE REVIEWS. 133:147-164   DOI:  10.1016/j.quascirev.2015.12.011