Notochen bannockburnensis, Worthy & Scofield & Hand & De Pietri & Archer, 2022

Worthy, Trevor H., Scofield, R. Paul, Hand, Suzanne J., De Pietri, Vanesa L. & Archer, Michael, 2022, A swan-sized fossil anatid (Aves: Anatidae) from the early Miocene St Bathans Fauna of New Zealand, Zootaxa 5168 (1), pp. 39-50 : 45-46

Notochen bannockburnensis gen. et sp. nov.

Holotype: CM 2017.37 .919 (figure 1), a distal right humerus

Diagnosis: A swan-sized anatid in which the distal humerus is little expanded ventrally, the tuberculum supracondylare ventrale is not distinctly ventral to the shaft, is flattened rather than elevated facing somewhat distally, elongate and extends well proximal to the condylus dorsalis, and is widely separated from but proximally surpasses the distal end of a small ovate fossa musculi brachialis.

Type Locality: Croc Site, Layer 1, c. 10 cm thick sand and cobble layer, in a 3 m cliff on the north side of a small hill, west side Mata Creek , near St Bathans, Otago. 44° 53.370’ S 169° 50.26998’ E. NZ Fossil Record File Number H 41/f84 .

Stratigraphy and age: 3.5 m above base of the Bannockburn Formation, Manuherikia Group, 19–16 Ma, early Miocene (see Worthy et al. 2022).

Etymology: After notios southern, Gk, and chen, from Chenopis , the endemic subgenus of goose-like swans of Australasia, feminine, and bannockburnensis for ‘of the Bannockburn, alluding to its source from the Bannockburn Formation’. Suggested common name Bannockburn swan.

Measurements (mm): Preserved length 114, depth dorsal condyle 13.2, shaft width just proximal to fossa brachialis 17.0, shaft where not crushed 27 mm proximal to fossa brachialis 13.5 wide by 11.7 deep, diameter from pits for the origin of musculus flexor carpi ulnaris and origin of musculus supinator to the ventral side of tuberculum supracondylare ventrale at its mid-length 23.3.

Referred specimens: NMNZ S50823 , S50824 , two omal parts left coracoid, Bed HH1b [not Bed HH1a, contra Worthy et al. 2008, wherein the bed was misidentified], Trench Excavation, Manuherikia River Section; NMNZ S53149, omal part R coracoid, Bed HH1b, Trench Excavation, Manuherikia River Section. All are from the basal 25 m of the Bannockburn Formation in the Manuherikia lacustrina biostratigraphic zone ( Worthy et al. 2022) and part of the St Bathans Fauna ( Worthy et al. 2008; Schwarzhans et al. 2012). As described in Worthy et al. (2008), these coracoids are very fragmentary: they feature a short broad facies articularis humeralis, a dorsoventrally broad base to the processus acrocoracoideus where it extends from the facies articularis humeralis, dorsal part sulcus supracoracoideus lacking pneumatic foramina, and a deep cup-like cotyla scapularis. Therefore, the structure of the processus procoracoideus and all parts more sternal are unknown and so these specimens do not afford any insights on relationships.

Tentatively referred is CM Av40461, a proximal part of the right shaft of a humerus, collected 9-Jan-08, from Bed HH1a, Manuherikia River. The width of the shaft at the distalmost edge is 12.4 mm and the width is 14.7 mm at the distal end of the crista deltopectoralis. Of appropriate size for CM 2017.37.919, it shows that the distal end of musculus latissimus dorsi abuts the crista deltopectoralis and that caudally the margo caudalis is compressed forming a rounded acute capital shaft ridge, as it does in species of Cereopsis and Cygnus .

Remarks: Notochen bannockburnensis cannot be certainly referred to one or other of Cygnini or Anserini , as separation of these taxa on the distal humerus is principally by the distal projection of processus flexorius; it extends past the condylus ventralis in the Anserini but it does not in the Cygnini ( Louchart et al. 2005) , and this part of the fossil is missing. However, the large size, with distal width similar to that of Cygnus atratus , but with a stouter shaft, suggests a referral to Cygnini is likely.

No Oligo-Miocene anatids of similar size are known from Australia ( Worthy 2009). Given the age disparity and geographic separation it is not likely that Notochen bannockburnensis could relate to the few European Oligoceneearly Miocene putative anserines, but where taxa are comparable, differences exist. Guguschia nailiae Aslanova & Burczak-Abramowicz, 1968 is a swan-sized anseriform of late Oligocene-early Miocene age that differs by an absence of a facet on the tuberculum supracondylare ventrale and a larger tuberculum supracondylare dorsale ( Aslanova & Burczak-Abramowicz 1968). Both Cygnopterus affinis ( van Beneden, 1883) of middle Oligocene age and Cygnopterus neogradensis Kessler & Hír, 2009 of middle Miocene age, have a proximodistally short facet for the attachment of the anterior articular ligament, see Lambrecht (1931, Taf. II, 1, 2) and Kessler & Hír (2009). The known material of Cygnavus senckenbergi Lambrecht, 1931 , from the early Miocene, lectotype left femur and a distal tibiotarsus and phalanx, and Cygnavus formosus Kurochkin, 1968 , early Oligocene, a tibiotarsus, are incomparable. Cygnopterus alphonsi Cheneval, 1984 , synonymised with Cygnavus senckenbergi Lambrecht, 1931 by Mlíkovský (2002), has a worn humerus referred to it, however the details of the tuberculum supracondylare ventrale are not discernible in the publication ( Cheneval 1984).

All similar-sized anserines of middle–late Miocene are placed in modern genera. The exception is the late Miocene-Pliocene Afrocygnus chauvireae Louchart, Vignaud, Likius, Mackaye & Brunet, 2005 from Africa, which has a similarly distally robust shaft and differs by the scar for the attachment anterior articular ligament being rather circular instead of proximodistally elongate as in the fossil ( Louchart et al. 2005).