THRESKIORNITHIDAE

Smith, ND & Ksepka, DT, 2015, Five well-supported fossil calibrations within the " Waterbird " assemblage (Tetrapoda, Aves), Palaeontologia Electronica 18 (1), pp. 1-21 : 5-7

STEM THRESKIORNITHIDAE

Ibises and Spoonbills

Node Calibrated. This node represents the split between Threskiornithidae (ibises and spoonbills) and their extant sister taxon, which has been variable among molecular ( Ericson et al., 2006; Hackett et al., 2008) and morphological ( Livezey and Zusi, 2007; Smith, 2010; Mayr, 2011b) estimates of phylogeny.

Fossil Taxon. Rhynchaeites sp.

Specimen. MGUH 20288 (caudal vertebrae, partial sternum, partial pelvis, distal femur, nearly complete left and right legs from the tibiotarsi to phalanges).

Phylogenetic Justification. The referral of MGUH 20288 to Rhynchaeites sp. is based on the following combination of characters, distributed throughout the postcranial skeleton (potential autapomorphies of Rhynchaeites indicated with an asterisk): sternum with a wide trabecula mediana bounded by a single pair of caudal incisions (of similar depth to Rhynchaeites messelensis ); a metatarsal I with a shorter and wider articular process for the tarsometatarsus than in extant Threskiornithidae ; *a proximomedially projecting medial portion of the proximal tarsometatarsus (a distinct, but somewhat similar character is present in Gavia , Podiceps , and some procellariiforms; see Smith, 2010, character 411); the fossa for metatarsal I is located close to the medial surface of the shaft of the tarsometatarsus (a fossa in a fully medial position is present in Plotopteridae ; see Smith, 2010:Character 435); *the presence of a short tendinal ossification on the plantar surface of the tarsometatarsus; and near-identical proportions of the hindlimb and pedal segments (which are also distinct from extant Threskiornithidae ), with the exception that MGUH 20288 is slightly larger than some Rhynchaeites messelensis specimens ( Mayr and Bertelli, 2011, table 1).

Phylogenetic placement of Rhynchaeites messelensis ( Figure 1.2 ) as a member of total group Threskiornithidae is based on three apomorphic characters discussed by Peters (1983) and Mayr (2002a): an elongate recurved bill; a schizorhinal bill with a dorsoventrally broad basal segment of the ventral bar, and a notarium consisting of at least three fused thoracic vertebrae ( Livezey and Zusi, 2006, character 892). Within waterbirds, a notarium is present in Podicipedidae , Phoenicopteridae , Threskiornithidae , and Pelecanidae ( Livezey and Zusi, 2006; though in Pelecanidae , only two vertebrae anterior to the synsacrum are included, leading these authors to suggest that this taxon does not have a notarium sensu stricto). Additional potential apomorphies (optimization as apomorphies dependent upon topology of extant waterbird lineages relative to Threskiornithidae ) uniting Rhynchaeites messelensis and Threskiornithidae include: 1) a dorsoventrally deep sternal keel ( Livezey and Zusi, 2006, character 1199; see also Smith, 2010, character 133); 2) a postacetabular blade of the ilium that is as long as or extends further caudally than the caudal end of the ischium ( Livezey and Zusi, 2006, character 1914; Smith, 2010, character 320); 3) a fibula that is at least 3/4 the length of the tibiotarsus, but still does not reach its distal end ( Livezey and Zusi, 2006, character 2191; convergently present in Podiceps within waterbirds); 4) a medially located sulcus extensorius on the distal tibiotarsus ( Livezey and Zusi, 2006, character 2178); and 5) the relative distal extents of the trochleae metatarsals are II <III> IV, and II> IV ( Livezey and Zusi, 2006, character 2361; Smith, 2010, character 445). Of these traits, characters 1, 3, 4, and 5 are present in the calibrating specimen Rhynchaeites sp. MGUH 20288.

Minimum Age. 53.9 Ma

Soft Maximum Age. None specified.

Age Justification. The specimen was found by Peter Franz in the lower Eocene Fur Formation of northwest Denmark ( Lindow and Dyke, 2006; Mayr and Bertelli, 2011). Nearly 180 layers of volcanic ash are interbedded within the Fur Formation ( Egger and Brückl, 2006; Lindow and Dyke, 2006). The upper set of more closely spaced, black basaltic ash layers are numbered ranging from +1 to + 140 in ascending stratigraphic order, whereas the lower set of more widely spaced, light colored layers are numbered from -1 to - 39 in descending order ( BØggild, 1918; Egger and Brückl, 2006). Two 39 Ar/ 40 Ar radioisotopic dates have been reported from these ash layers, including a date of 54.04 +/- 0.14 Ma from layer +19 and a date of 54.52 +/- 0.05 Ma from layer -17 ( Chambers et al., 2003). More recently, a reanalysis of the 39 Ar/ 40 Ar radioisotopic dates from layer -17 produced an age of 55.12 +/- 0.12 Ma (Storey et al., 2007), and adjusting this estimate based on the recalibration of the Fish Canton Tuff (~28.201 Ma) sanidine reference standard ( Kuiper et al., 2008) for 39 Ar/ 40 Ar radioisotopic dates yields a corrected age of 55.48 +/- 0.12 (Westerhold et al., 2009). Within the time period bounding layer -17 and +19, approximately 0.5 m of volcanic ash and 19.5 m of diatomite were deposited ( BØggild, 1918; Egger and Brückl, 2006). However, the precise horizon from which MGUH 20288 was collected has not been reported, so whether these dates provide a minimum age, maximum age, or together bound the age of the fossils cannot be determined. With these considerations in mind, we apply the youngest possible age for MGUH 20288, inclusive of error: 53.9 Ma.

Phylogenetic position of Threskiornithidae and Rhynchaeites . Rhynchaeites messelensis (neotype SMF ME 1045; holotype specimen named by Wittich, 1898 has been lost, a neotype was designated by Peters, 1983) was the first fossil bird described from Messel, and was originally described as a member of Rostratulidae ( Charadriiformes ) by Wittich (1898). Hoch (1980) also supported charadriiform affinities for the taxon. The threskiornithid affinities of Rhynchaeites messelensis were first noted by Peters (1983), and described in more detail by Mayr (2002a). The Fur Formation specimen of Rhynchaeites was previously described as a "pelecaniform" ( Kristoffersen, 2002) and a galliform ( Lindow, 2007) in two independent Ph.D. dissertations before being referred to Rhynchaeites by Mayr and Bertelli (2011). Mayr (2002a, 2009a), and Mayr and Bertelli (2011) discussed the threskiornithid features of Rhynchaeites in detail. The most salient apomorphies uniting Rhynchaeites with Threskiornithidae are an elongate recurved schizorhinal beak and a notarium consisting of at least three fused thoracic vertebrae ( Mayr, 2002a, 2009a).

The extant sister-taxon relationships of Threskiornithidae have fluctuated considerably across morphological and molecular studies, though nearly all are consistent in recovering them as closely allied to various members of the traditional order Ciconiiformes . The main exceptions are the analysis of FGB-int7 sequences by Fain and Houde (2004), which recovered Threskiornithidae as sister taxon to Musophagidae ; and the mitochondrial genome study of Brown et al. (2008), which recovered the aberrant sister-taxon relationship between Threskiornithidae and Musophagiformes . This clade was the sister taxon to a larger group including some Ciconiiformes , some Caprimulgiformes, and Apodiformes ( Brown et al., 2008). Ericson et al. (2006) recovered Threskiornithidae in a basal polytomy with Ardeidae and a Pelecanidae + Balaenicipitidae + Scopidae clade; whereas Hackett et al. (2008) recovered a sister taxon relationship between Threskiornithidae and Ardeidae , with the Pelecanidae + Balaenicipitidae + Scopidae clade forming the sister taxon of this group. As for analyses of morphological data, Livezey and Zusi (2007) recovered the Threskiornithidae as sister taxon to a Ciconiidae + Phoenicopteridae clade, with this larger grouping as sister taxon to Scopidae . Smith (2010) recovered Threskiornithidae as part of a polytomy including Ardeidae and Ciconiidae , with this larger group constituting the sister taxon to Phoenicopteridae , though as noted by Smith (2010), support for various subgroups of Ciconiiformes was relatively low in this analysis. Similarly, Mayr (2011b) recovered a sister-taxon relationship between Threskiornithidae and Ciconiidae , with this group falling into large polytomy comprised of Ciconiiformes and Pelecaniformes .

Fossil record of total group Threskiornithidae . Rhynchaeites messelensis is one of the more abundant medium-sized birds known from the middle Eocene Messel locality, and is represented by more than a dozen partial to nearly complete specimens ( Mayr and Bertelli, 2011; Smith et al., 2013). Other records that may pertain to total group Threskiornithidae of similar age include: 1) a distal tibiotarsus from the late middle Eocene of Myanmar referred to cf. Threskiornithidae by Stidham et al. (2005); 2) Vadaravis brownae , represented by a nearly complete postcranial skeleton from the early Eocene Green River Formation of Wyoming (Smith et al., 2013); 3) Eociconia sangequansis , represented by a distal left tarsometatarsus that may have Ciconiidae affinities, from the middle Eocene Yixibaila Formation of Xinjiang, China ( Hou, 1982); and 4) Sanshuiornis zhangi , consisting of a distal tibiotarsus and complete articulated foot from the middle Eocene black oil shales of the Huayong Formation in Guangdong Province, southern China that shows affinities to Ciconiiformes , as well as Rhynchaeites messelensis (Wang et al., 2012) . In most cases, these specimens are extremely fragmentary and their relationships have not been tested in a phylogenetic analysis; the exception being Vadaravis brownae , which was recovered as the sister taxon of a Ciconiidae + Phoenicopteridae + Threskiornithidae clade in a reanalysis of the Livezey and Zusi (2006, 2007) dataset by Smith et al. (2013). When the topological constraints of the Hackett et al. (2008) results for waterbird relationships were enforced, Vadaravis brownae was recovered as the sister taxon to Threskiornithidae (Smith et al., 2013) . Potential younger members of total group Threskiornithidae (from the late Eocene onwards) are briefly reviewed by Smith et al. (2013), and likewise constitute fragmentary specimens whose relationships are contested and have not been tested in a phylogenetic analysis or justified with apomorphy-based diagnoses.

Figures

FIGURE 1. Specimens representative of two taxa that provide fossil calibrations within the waterbird assemblage. 1, Limnofregata azygosternon (USNM 22753) from the early Eocene Green River Formation represents the sister taxon (with its congener Limnofregata hasegawai) to Fregata, and calibrates the node Fregatidae + Suloidea. 2, Rhynchaeites messelensis (SMF 218) from the middle Eocene of Messel. Together with other Messel Rhynchaeites messelensis specimens and Rhynchaeites sp. (MGUH 20288) from the early Eocene Fur Formation, these represent the sister taxon to Threskiornithidae, and MGUH 20288 calibrates the node uniting Threskiornithidae and its extant sister taxon. Scale bars equal 50 mm.