Node Calibrated (11): Cepphus (guillemots). Divergence of the Spectacled Guillemot Cepphus carbo from the Black Guillemot Cepphus grylle and the Pigeon Guillemot Cepphus columba (Figure 2)

Fossil Taxon. Cepphus olsoni Howard, 1982

Specimen. LACM 107032 (Los Angeles County Museum of Natural History, Los Angeles, California, USA) holotype specimen of Cepphus olsoni, humerus.

Phylogenetic Justification. The only phylogenetic assessment of the systematic position of Cepphus olsoni recovered it as the sister taxon of the extant species Cepphus carbo, with Cepphus grylle and Cepphus columba in unresolved positions at the base of Cepphus (combined analysis; Smith, 2011a).

Minimum Age. Late Miocene (Messinian) 6.7 Ma Soft Maximum Age. Not specified

Age Justification. The vertebrate assemblages of the San Mateo Formation were reviewed by Barnes et al. (1981), who designated the lower assemblage the San Luis Rey River Local Fauna (SLRRLF), and the upper assemblage the Lawrence Canyon Local Fauna (LCLF). Based on biostratigraphic correlation of marine vertebrates and terrestrial mammals, the age of the younger LCLF has been proposed to be latest Miocene or earliest Pliocene (~5.0 Ma), and correlative with the Late Hemphillian North American Land Mammal Age (Domning and Deméré, 1984). Pan-Alcidae fossils, including the holotype humerus of Cepphus olsoni have been recovered from the older SLRRLF. Age estimates for the SLRRLF based upon terrestrial mammal and marine bird fossils range from approximately 6.7-10.0 Ma (Late Miocene or Tortonian-Messinian equivalent; Barnes et al., 1981; Domning and Deméré, 1984). Therefore, the minimum estimated age of 6.7 Ma is suggested as a minimum age of divergence for the split between Cepphus carbo and other extant species of Cepphus.

Discussion. Although an older putative Cepphini fossil (~14 Ma) was described by Wijnker and Olson (2009) and included in the phylogenetic analysis of Smith (2011a), the position of Pseudocepphus teres as the sister taxon to the rest of Cepphini was not considered strongly supported in the results of that analysis. Furthermore, the affinities of Pseudocepphus teres were originally hypothesized to be closer to Alcini (i.e., Alca and Miocepphus), and morphological characters shared with Cepphus were considered convergent by Wijnker and Olson (2009). Until additional material representing Pseudocepphus teres is recovered and the systematic position of that taxon can be more confidently established, its application as a fossil calibration remains uncertain.