Nasidytes ypresianus Mayr & Kitchener 2022, SP. NOV.
Creators: name: Mayr, Gerald
name: Kitchener, Andrew C.
Publisher: name: Zenodo
Year: 2022
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Aves
Gaviiformes
Gaviidae
Nasidytes
Nasidytes ypresianus

description: NASIDYTES YPRESIANUS SP. NOV.
Zoobank registration: urn: lsid: zoobank. org:act: 9DCFC3A6-4AC9-4E89-937D-21A3153ED83C
Holotype: NMS.Z.2021.40.24 (Fig. 1A‒C; partial skeleton including mandible, most major wing, pectoral girdle and leg bones), collected in 1992 by M. Daniels (original collector’s number WN 92720).
Diagnosis: As for genus.
Etymology: The species epithet refers to the geological age of the holotype, which is from Ypresian strata.
Type locality and horizon: Walton-on-the-Naze, Essex, UK; Walton Member of the London Clay Formation (previously Division A2; Jolley, 1996; Rayner et al., 2009; Aldiss, 2012); early Eocene (early Ypresian, 54.6‒55.0 Mya; Collinson et al., 2016).
Referred specimen: NMS.Z.2021.40.25 (Fig. 1D; left carpometacarpus), collected in 1996 by M. Daniels (original collector’s number WN 96898).
Measurements (in millimetres): NMS.Z.2021.40.24: coracoid, 34.3 (right); scapula, 53.5 (right); humerus length, 74.0 (left), 72.4 (right), humerus distal width, 11.1 (left), 11.0 (right); ulna length, 63.8 (left); femur, length, 38.9 (left); tibiotarsus, distal width, 7.5 (right); right tarsometatarsus length, 40.7, distal width, 6.9, proximal width, 8.1. NMS.Z.2021.40.25: carpometacarpus length, 42.1 (left).
Description and comparisons: The mandible (Fig. 2Q) is different from that of extant gaviiforms and indicates a proportionally much shorter beak in N. ypresianus. The symphysis is wide and shallow; its tip is broken and missing (as evidenced by Fig. 1B, the tip of the mandible was complete originally). Overall, the mandible of the new species resembles that of extant coots (Fulica spp., Rallidae; Fig. 2S) in its proportions, whereas the mandibular symphysis is much narrower in crown group Gaviiformes (Fig. 2R). However, unlike in Fulica Linnaeus, 1758 and like in Gavia, the rostral portions of the mandibular rami are near parallel in Nasidytes.
The quadrate (Fig. 2A‒E) closely corresponds to that of crown group Gaviiformes (Fig. 2F‒J) in its shape. The tip of the processus oticus is wider than in crown group Gaviiformes and, unlike in the latter, there is a pit-like fossa on the caudal surface of the processus oticus; as in extant loons, pneumatic foramina are absent. Also as in extant gaviiforms, there is a small tuberculum subcapitulare (sensu Elzanowski & Stidham, 2010) ventral to the capitulum squamosum. The processus orbitalis is dorsoventrally deeper than in crown group Gaviiformes; unlike in crown group Gaviiformes, it does not exhibit a well-delimited articular surface for the pterygoid (Fig. 2G). The condylus medialis is ventrally prominent and, as in extant gaviiforms, the condylus medialis has a concave lateral surface. Also as in extant gaviiforms, the condylus caudalis forms a distinct lip, and the condylus pterygoideus is prominent and ball-shaped.
The pterygoid (Fig. 2K‒M), likewise, resembles that of extant gaviiforms (Fig. 2N‒P). The bone has a curved shaft and a widened caudal end. There is no facies articularis basipterygoidea, which indicates the absence of basipterygoid processes on the cranium.
The corpus of the single thoracic vertebra preserved in the holotype (Fig. 3O) exhibits distinct fossae on its lateral surfaces (pleurocoels), which are absent in extant loons. Pleurocoels occur in many neornithine stem group representatives (Mayr, 2021), and their absence in crown group Gaviiformes is likely to be attributable to the mediolaterally compressed body of the thoracic vertebrae of extant loons. The holotype also includes a caudal vertebra (Fig. 3P), which has somewhat longer processus transversi than extant gaviiforms.
The coracoid (Fig. 3I, J) resembles the corresponding bone of Colymboides (Fig. 3K). The processus procoracoideus, which is not completely preserved in any other Eocene loon fossil, is proportionally longer than in Petralca and crown group Gaviiformes (Fig. 3L). The foramen nervi supracoracoidei is proportionally smaller than in Colymboides and extant gaviiforms, in which the foramen is usually also positioned more medially. The medial margin of the sternal extremity forms a convexity (Fig. 3I), which is absent in Colymboides and extant gaviiforms. The processus lateralis is proportionally longer than in Colymboides minutus and crown group Gaviiformes, whereas it has a similar relative length in the holotype of the early Oligocene ‘? Colymboides metzleri ’.
Unlike in Colymboides, Petralca and extant gaviiforms, the scapula (Fig. 3M) has a long and narrow acromion. Furthermore, the facies articularis humeralis is not as elongated as in extant loons (Fig. 3N).
The furcula (Fig. 3E, F) is U-shaped, with wide shafts and simple omal extremities; the shafts are flattened in a craniocaudal (frontal) plane. As such, it differs from the furcula of crown group Gaviiformes (Fig. 3G, H), in which the omal extremity is elongated and tapering, the sternal extremity is markedly caudally bent, and the shafts are flattened in a mediolateral (parasagittal) plane.
The caudal portion of the sternum (Fig. 3A‒D) is damaged, but the bone appears to have been proportionally shorter than the long sternum of crown group Gaviiformes. Unlike in crown group gaviiforms, it exhibits a short spina externa. Four processus costales can be counted, whereas there are seven in crown group Gaviiformes. The carina sterni is low, as it is in crown group Gaviiformes.
The left and right humeri are preserved in the holotype (Fig. 4A, B). The tuberculum dorsale, on the proximal end of the bone, is well developed (Fig. 4A). Both humeri show a sigmoidally curved shaft, which appears to be a real (rather than taphonomic) feature. The proximal end is proportionally wider than in other gaviiforms except for Colymbiculus (Fig. 4C), and the crista bicipitalis is less prominent than in extant loons. As in crown group Gaviiformes, the crista deltopectoralis is low and long, measuring about one-third of the length of the humerus (the crista deltopectoralis of most other fossil loons has a similar shape, but in Colymbiculus it is more strongly dorsally protruding; Mayr & Zvonok, 2012). The distal end of the bone resembles the distal humerus of Colymbiculus; its ventral portion is broadly rounded and, unlike in crown group Gaviiformes, it does not form a marked, ventrally projected processus flexorius. The tuberculum supracondylare ventrale is not as elongated as in crown group gaviiforms (Fig. 4D); in N. ypresianus, the tuberculum supracondylare ventrales reaches only slightly farther proximally than the condylus dorsalis, whereas it reaches much farther proximally in Colymboides, Gavia and Petralca (the tuberculum supracondylare ventrale of Colymbiculus is similar to that of Nasidytes in size). The condylus ventralis is not as large and globose as in extant loons. With regard to its proportions and most morphological features, the humerus of Nasidytes resembles that of Australornis lovei Mayr & Scofield, 2014 from the late early Palaeocene of New Zealand (Fig. 4E; Mayr & Scofield, 2014), in which, however, the shaft is craniocaudally more flattened and has thicker bone walls (the distal end of the humerus of Australornis lovei is unknown).
The holotype includes seven non-ungual pedal phalanges (Fig. 6). The pedal phalanges are elongated as they are in extant loons and many other aquatic birds, and this is particularly true for those of the second toe. Only one of the ungual phalanges is preserved in the fossil and has a shape characteristic for many birds with webbed feet, in which the ungual phalanges are elongated and have a distally located tuberculum flexorium. However, this ungual phalanx does not show the highly derived shape found in crown group Gaviiformes, in which the unguals are extremely flattened and form an elongated, undifferentiated plate-like element.
description: Published as part of Mayr, Gerald & Kitchener, Andrew C., 2022, Oldest fossil loon documents a pronounced ecomorphological shift in the evolution of gaviiform birds, pp. 1431-1450 in Zoological Journal of the Linnean Society (Zool. J. Linn. Soc.) (Zool. J. Linn. Soc.) 196 (4) on pages 1433-1440, DOI: 10.1093/zoolinnean/zlac045, http://zenodo.org/record/7390620