Node Calibrated (1): Charadriiformes (shorebirds and allies). Divergence of the clade Charadriiformes (shorebirds and allies) from its nearest outgroup (see below for discussion of outgroup; Figure 1).

Fossil Taxon. Charadriiformes incertae

sedis Mayr, 2000.

Specimen. SMF-ME 2458A+B (Forschungsinstitut Senckenberg, Frankfurt, Germany) partial postcranial skeleton.

Phylogenetic Justification. SMF-ME 2458A+B was referred to Charadriiformes

by Mayr (2000) based on apomorphies of

the coracoid, humerus and carpometacarpus. The following apomorphies that support referral of SMF-ME 2458A+B were cited by Mayr (2000:626): (1) coracoid with protruding tuberculum brachiale and broad facies articularis clavicularis; (2) furcula with long and slender processus acromialis (except Burhinidae); (3) humerus with more or less well developed processus supracondylaris dorsalis (except Alcidae, Burhinidae, Jacanidae); (4) synsacrum perforated by many foramina intertransversaria; (5) crista cnemialis cranialis of tibiotarsus greatly enlarged; (6) fourth phalanx of fourth toe shorter than third phalanx (except Alcidae); (7) hallux strongly reduced or absent (except Jacanidae). However, the affinities of SMF-ME 2458A+B within Charadriiformes (i.e., stem or crown) are uncertain because it has not been included in a phylogenetic analysis. No alternative hypothesis has been proposed linking this specimen to any clade other than Charadriiformes.

Minimum Age. Middle Eocene (Lutetian) 46.5 Ma

Soft Maximum Age. Not specified

Age Justification. Igneous rocks from Messel were dated using 40 Ar/ 39 Ar methods to 47.8 ± 0.2 Ma and correspond with biostratigraphic reference level MP 11 (i.e., the base of the Geiseltalian European Land Mammal Age; Mertz and Renne, 2005). The span of the Geiseltalian is estimated at 47.5-46.5 Ma (Mertz and Renne, 2005).

Discussion. The systematic position of Charadriiformes among Aves is not unambiguously established because the results of analyses of higher-level phylogenetic relationships of Aves are incongruent (Mayr and Clarke, 2003; Hackett et al., 2008; Livezey and Zusi, 2006, 2007; Brown et al., 2007; Ericson et al., 2006), thus complicating the choice of a fossil calibration to date the split of Charadriiformes from its nearest outgroup. For example, if Charadriiformes are the sister taxon to the 'waterbird' clade as hypothesized by Brown et al., 2007, then the earliest known penguins, Waimanu manneringi and Waimanu tuatahi (~61 Ma; Slack et al., 2006) might provide a calibration for that split. However, if Charadriiformes are the sister taxon to the 'landbird' clade as proposed by Hackett et al. (2008), the earliest 'landbird' fossil, that of the Late Paleocene (Thanetian, ~56 Ma) owl Ogygoptynx wetmorei Rich and Bohaska 1976 might provide a calibration for the split between Charadriiformes and the ‘landbird’ clade. Because of the uncertainty associated with the establishment of the outgroup to Charadriiformes, a conservative strategy of using the earliest record of Charadriiformes is justified.

All Mesozoic and Paleocene fossils referred to Charadriiformes (e.g., Harrison and Walker, 1977; Olson and Paris, 1987) consist of unassociated and undiagnostic fragments (Hope, 2002; Clarke and Norell, 2002; Mayr, 2005). Records of charadriiforms from the Eocene include the German specimen listed above (Mayr, 2000), a Pan-Alcidae fossil from the Late Eocene (~35 Ma) of eastern North America (Chandler and Parmley, 2002; see below), an isolated humerus from the Middle Eocene of China (Hou and Ericson, 2002; see below), and a putative charadriiform specimen from eastern North America (Olson, 1999). Early Eocene (~53 Ma) material referred to Charadriiformes by Olson (1999) is considered too fragmentary for referral to Charadriiformes (Mayr, 2005). Therefore, SMF-ME 2458A+B represents the earliest definitive record of Charadriiformes. However, the possibility that SMF-ME 2458A+B represents the stem lineage of Charadriiformes cannot be excluded, and awaits its inclusion in a phylogenetic analysis with sufficiently sampled outgroup taxa to Charadriiformes.

Despite the uncertain affinities of putative charadriiforms from the Mesozoic and Paleocene, these fossils have been used as calibrations in previous divergence time analyses. For example, the list of fossil taxa used to constrain the age of the split of Charadriiformes from its nearest outgroup by Baker et al. (2007) and Pereira and Baker (2008) includes three species of Paleotringa, three species of Cimolopteryx, and Ceramornis major from the Late Cretaceous or Early Paleocene (see Baker et al., 2007, table S3). Ceramornis is considered to be Neornithes incertae sedis (Hope, 2002), and is therefore inappropriate for use as a charadriiform fossil calibration. The referral of Cimolopteryx to Charadriiformes is not certain (Hope, 2002). Accordingly, Cimolopteryx should not be used as a calibration point to date the basal divergence among charadriiforms. Baker et al. (2007) and Pereira and Baker (2008) used specimens referred to the Late Cretaceous or Early Paleocene taxon Paleotringa to date the minimum age of the Lari-Scolopaci split and the split of Charadriiformes from their outgroup respectively. Specimens referred to Paleotringa include isolated partial tibiotarsi, tarsometatarsi, and humeri (Brodkorb, 1967) and do not preserve any apomorphies that would allow their referral to Charadriiformes (Hope, 2002). Accordingly, continued use of Paleotringa to date the divergence of Charadriiformes from its nearest sister group, or to date the split between scolopacid and larid lineages is inadvisable.