COLIIFORMES (MOUSEBIRDS)

Ksepka, DT & Clarke, JA, 2015, Phylogenetically vetted and stratigraphically constrained fossil calibrations within Aves, Palaeontologia Electronica (Miami, Fla.) 18, pp. 1-25 : 12-13

COLIIFORMES (MOUSEBIRDS)

Node Calibrated. Divergence between Coliiformes and their extant sister taxon (see below).

Fossil Taxon. Sandcoleus copiosus ( Houde and Olson, 1992)

Specimen. USNM 433912 , holotype of Sandcoleus copiosus : nearly complete skeleton collected in articulation from a calcareous nodule, now disarticulated by preparation .

Phylogenetic Justification. Phylogenetic analysis of morphological data ( Mayr and Mourer-Chauviré, 2004; Zelenkov and Dyke, 2008; Ksepka and Clarke, 2009, 2010c). Analyses utilizing a composite Sandcoleidae with scorings from Sandcoleus and several related taxa also support placing Sandcoleus copiosus along the mousebird stem lineage ( Mayr and Mourer-Chauviré, 2004; Mayr, 2005d). No alternative hypothesis for a close relationship with an extant group other than mousebirds has been forwarded for Sandcoleus .

Minimum Age. 56.22 Ma.

Age Justification. USNM 433912 was collected from the Sand Coulee beds of the Willwood Formation in Clark Quadrangle, Wyoming ( USA). These beds fall within the Plesiadapis cookei zone (biozone Cf-2) of the Clarkfordian North American Land Mammal Age ( Houde and Olson, 1992). Biozone Cf-2 is located above the base of Chron C24r. Although Chron C24r spans the Paleocene-Eocene boundary, biozone Cf-2 occurs entirely within the Paleocene. Secord et al. (2006) obtained a weighted mean 40 Ar/ 39 Ar age of 59.00±0.30 Ma for the Belt Ash, which occurs within the older Chron C26r. These authors used a cubic spline function to calibrate biozone boundaries to numerical dates, estimating the age of Cf-2 as between 56.60 Ma and 56.22 Ma. Together, the radiometric, paleomagnetic, and biostratigraphic data provide a tightly constrained estimate for the appearance of Sandcoleus copiosus .

Phylogenetic Position of Coliiformes . Higher level relationships of mousebirds remain uncertain, and they have been considered a "rogue" taxon (Wang et al., 2012). Coliiformes were recovered as the sister taxon of Strigiformes by the phylogenomic analysis of Hackett et al. (2008) ( Figure 9 ). However, many alternate results have been reported, including sister group relationships between Coliiformes and Psittaciformes ( Sorenson et al., 2003; Mayr and Clarke, 2003; Chubb, 2004), Trogoniformes (Espinosa de los Monteros, 2000; Livezey and Zusi, 2006, 2007), and Piciformes ( Mayr et al., 2003) . Other analyses did not fully resolve the position of Coliiformes but nonetheless resolved Strigiformes as distantly related to Coliiformes ( Ericson et al., 2006; Brown et al., 2008). Placement of Sandcoleus copiosus is secure despite these conflicts, as each of the first three candidate sister taxa have been included as single outgroups in three separate analyses testing the placement of the fossil ( Ksepka and Clarke, 2009) as have all four simultaneously ( Ksepka and Clarke, 2010c). All of these analyses placed the fossil along the mousebird stem lineage, a position that is supported by multiple apomorphies ( Mayr and Peters, 1999). Given the lack of consensus regarding the higher level affinities of mousebirds, we suggest the mousebird calibration is most appropriately utilized in analyses that sample a wide range of "higher land bird" taxa.

Fossil Record of Coliiformes . A relatively continuous temporal record is now recognized following the first appearance of Coliiformes , with fossils recovered from Early Eocene horizons in the Willwood Formation (early Wasatchian ~54 Ma; Houde and Olson, 1992), the Early Eocene Green River Formation (~51.5 Ma; Houde and Olson, 1992; Ksepka and Clarke, 2010c), Middle Eocene Bridger Formation (~48 Ma; Houde and Olson, 1992), Middle Eocene Messel Formation (~47 Ma; Mayr and Peters, 1998; Peters, 1999; Mayr, 2000a), multiple Late Eocene horizons at Quercy ( Mourer-Chauviré, 1988b) the latest Eocene Florissant Formation ( Ksepka and Clarke, 2009), and the Early Oligocene Frauenweilerand Late Oligocene Enspel deposits of Germany ( Mayr, 2000b, 2013b) ( Figure 10 ). Mousebirds are also represented by a suite of fossil taxa from younger Neogene deposits.

Notably, the fossil record of mousebirds is concentrated to the Northern Hemisphere, whereas all six extant mousebird species are restricted to Africa. Although it is possible that older Southern Hemisphere fossils remain undiscovered, the placement of all phylogenetically constrained North American and European species outside the crown clade and the early Pliocene first appearances of mousebirds in Africa ( Rich and Haarhoff, 1985) are consistent with origination in the Northern Hemisphere followed by a later Neogene dispersal into Africa. This scenario does not require substantial Southern Hemisphere gaps in the fossil record of the group.

Fossil Record of Related Clades. Due to the unresolved position of Coliiformes within Aves, it is premature to discuss in detail whether concordance exists between the fossil record of this group and its sister taxon. Most of the proposed candidates do show significant agreement, however, as Strigiformes also appear in the Paleocene ( Rich and Bohaska, 1976) and Psittaciformes and Trogoniformes appear in the Early Eocene ( Dyke and Cooper, 2000; Kristoffersen, 2002).

Figures

FIGURE 9. Phylogenetic tree showing position of Sandcoleus copiosus, illustrating the divergence calibrated by USNM 433912.

FIGURE 10. Stratigraphic distribution of fossil Coliiformes in the Paleogene. The minimum possible age of the calibrating specimen of Sandcoleus copiosus is presented, and midpoints of age ranges are shown for the ages of other Paleogene mousebird specimens.