Spheniscidae, Bonaparte, 1831

Hospitaleche, Carolina Acosta & Soto-Acuña, Sergio, 2023, A small penguin cranium (Aves, Spheniscidae) from the Late Miocene of Bahía Inglesa Formation, Atacama Desert, Northern Chile, Comptes Rendus Palevol 22 (13), pp. 233-244 : 238-239

Spheniscidae gen. et sp. indet.

( Figs 3; 4)

MATERIAL. — SGO.PV.22245 , partial cranium lacking the rostrum, the palate and most of the occipital portion ( Figs 3; 4 A). LOCALITY AND HORIZON. — Mina Fosforita locality, Caldera, Atacama Region, Northern Chile. Mina Fosforita Member, Bahía Inglesa Formation, Tortonian-Messinian age ( Fig. 2).

MEASUREMENTS. — Postorbital width 48.46 mm in SGO.PV.22245 (50.4 mm in S. demersus UF 21341, 64.3 mm in S. magellanicus MLP 643).

DESCRIPTION

According to the configuration and relative development of the crista nuchalis, the fossa glandulae nasalis, and the morphology of the cranial roof, features described below in detail, SGO.PV.22245 is assigned to Spheniscidae . The attribution of SGO.PV.22245 to any Paleogene penguin species (e.g. Icadyptes , Perudyptes , Anthropornis , Waimanu ) can easily be ruled out based on differences in morphology. In addition to the Paleogene penguins having skulls notably larger than that of the specimen described here, and other morphological differences separate them. Paleogene penguins have a more elongated crania, stronger nuchal crests (and a larger crista sagittalis), and broader fossae temporalis. Differences with the Neogene species and particularly the extant genera are subtler and will be treated in more detail.

The fossa glandulae nasalis is narrow ( Fig.3A, B) like in Eudyptula , Megadyptes , and Paraptenodytes whereas in Eudyptes , some species of Spheniscus , Pygoscelis , Aptenodytes , and Madrynornis is significantly broader ( Fig. 4). This fossa narrows caudally and is not laterally delimited, like in Eudyptula , Spheniscus , Aptenodytes , and Paraptenodytes , whereas a supraorbital edge appears in Eudyptes , Megadyptes , and Pygoscelis ( Fig. 4). The left and right fossa glandulae nasalis leave a narrow interorbital region in between ( Fig. 3A, B), like in Eudyptula , Eudyptes , Pygoscelis , Spheniscus and Madrynornis . This area is wider in Aptenodytes , Megadyptes , and Paraptenodytes ( Fig. 4).

The crista nuchalis transversa and the crista temporalis ( Fig. 3A, E) are expanded like short wings like in most of the modern penguins with the single exception of Spheniscus , in which these cristae are more extensively projected. A short crista nuchalis sagittalis (shorter than in Paraptenodytes ) connects the crista nuchalis transversa with the eminentia cerebellaris, which is rounded and caudally projected ( Fig. 3A, C, E).

The fossae temporales are subtriangular and deeper caudally ( Fig. 3C), completely distinguishable from Paraptenodytes ( Fig. 4B), in which the fossae are quadrangular and dorsally sub-rounded. The dorsal extension of each fossa is intermediate between Eudyptula (in which the fossae are mostly laterally developed) and most of the Spheniscus species (in which the left and right fossae meet dorsally near the sagittal line), approaching the condition of the latter ( Fig. 4 I-Q). This feature, however, can vary intra-specifically ( Ksepka & Bertelli 2006).

The cranial roof is more expanded than in most penguin species ( Fig. 3A, C, D), and it only matches Eudyptula in its morphology. The depressio frontalis is barely marked like in Eudyptula , whereas it differentially develops in the Spheniscus species ( Fig. 4 I-Q).

The processus postorbitales (only the right one is preserved) are ventrolaterally projected ( Fig. 3 A-C), like in the fossil species of Spheniscus , whereas in living Spheniscus and Eudyptula , the process is mostly ventrally projected ( Fig. 4). Although the processus postorbitales observed in Eudyptula varies among individuals (slightly inclined cranially in E. novaehollandiae , and more caudally in E. minor ), it never projects laterally like in SGO.PV.22245 and is rounded and barely projected in Eudyptes , Pygoscelis , Aptenodytes , Megadyptes , and Madrynornis , rostrally and slightly laterally in Paraptenodytes ( Fig. 4).

The prominentia cerebellaris is caudally extended ( Fig. 3C), and its tip is rounded like in Eudyptula , S. megaramphus , and Eudyptes , whereas it is more acute in S. urbinai and all the remaining modern species ( Fig. 4). The prominentia cerebellaris is ventrally broken and the whole occipital region is crushed. Two shallow and symmetrical depressions are dorsally located on the prominentia cerebellaris, a similar condition is observed in some specimens like Spheniscus demersus and Eudyptula novaehollandiae . Unfortunately, the cranial foramina are not visible because of the poor preservation.