Systematic paleontology.

Aves Linnaeus, 1758

Sphenisciformes Sharpe, 1891

Kumimanu biceae, n. gen. et sp.

Holotype. NMNZ S.45877: partial skeleton of a single individual including cranial end of left scapula, incomplete right coracoid, cranialmost portion of sternum, partial left humerus, incomplete proximal end of left ulna, right femur, right tibiotarsus lacking proximal end, partial synsacrum, three vertebrae, and various bone fragments.

Etymology. From kumi (Maori), a large mythological monster, and manu (Maori), bird. The species epithet honors Beatrice (“Bice”) A. Tennyson, the mother of AJDT, who fostered his interest in natural history (pronounced “bee-chee-ae”).

Type locality and horizon. Hampden Beach, Otago, New Zealand (NZ Fossil Record Number J42/f0956; precise locality information is recorded at NMNZ); Moeraki Formation, late Paleocene (late Teurian, local stratigraphic level NZP522, which has an absolute age of 55.5.-59.5 million years23; a matrix sample taken from the fossil (GNS Science sample L29126) contained a specimen of the dinoflagellate Palaeocystodinium australinum and an unnamed dinoflagellate taxon that support a Teurian age for this sample; C. Clowes, pers. comm.).

Diagnosis. A very large-sized sphenisciform species, which is characterized by proximodistally low and widely spaced condyles of the tibiotarsus. Distinguished from the late Paleocene Crossvallia and all post-Paleocene Sphenisciformes of which humeri are known in the dorsoventrally narrower humerus shaft, with ratio of maximum width of proximal end of humerus to minimum width of shaft being 2.4 (less than this value in Crossvallia and all post-Paleocene Sphenisciformes of which the humerus is known). Distinguished from Waimanu tuatahi in having the bicipital crest of humerus not forming a distally directed bulge. Distinguished from Waimanu manneringi (the humerus of which is unknown) in having the tibiotarsus with proximodistally lower and more widely spaced condyles.

Dimensions (in mm). Coracoid, maximum length as preserved, 141.8; humerus, maximum length as preserved, 149.6; dorsoventral width of proximal end, 75.0; Tibiotarsus, length from distal margin of condylus lateralis to preserved proximal end, 151.3; width of distal end across condyles as preserved, 51.9; estimated actual width of distal end, ~48. Femur, maximum length along longitudinal axis, 161 mm.

Description and comparisons

The bones were densely packed in a very hard concretion (Fig. 1a). The humerus (Fig. 1i‒l) lacks most of the caput humeri and the distal end, with the latter having already been broken and lost before the bone was embedded in sediment. The attachment scar for the supracoracoideus muscle is slightly raised and dorsoventrally narrower than in Pachydyptes (Fig. 1n), Anthropornis, and Icadyptes. The fossa pneumotricipitalis is single and deeply excavated. The humerus shaft is proportionally narrower than in all post-Paleocene Sphenisciformes, and this is especially true if the bone is compared to the humeri of the giant taxa Pachydyptes, Anthropornis, and Icadyptes. An estimation of the total length of the humerus of Kumimanu biceae depends on the assumed proportions. If the proximal end had a similar relative size to that of Waimanu tuatahi, less than two thirds of the bone would be preserved and the estimated total length would have been about 228 mm (Fig. 1l). Our most conservative length estimate, by contrast, is based on comparisons with the short (albeit much stouter) humerus of Anthropornis—in this case only a small portion of the distal end would be missing and the humerus of Kumimanu biceae would have had a length of about 185 mm (Fig. 1k). If the bone was indeed that short, it would have had a proportionally much wider proximal end than the humeri of other Paleocene penguins (Table 1) and would have had an unusually slender shaft relative to the massive proximal end of the bone, so we consider our longer length estimate to be more likely.

Fig. 1 Wing and pectoral girdle bones of the new giant penguin. a K. biceae n. gen. et sp. (holotype, NMNZ S.45877) from the Paleocene of New Zealand, partially prepared concretion with all bones in situ. b K. biceae (holotype), right coracoid in dorsal view (dotted lines indicate reconstructed outline of bone). c Left coracoid of Waimanu tuatahi from the late Paleocene of New Zealand (CM zfa 34; specimen mirrored to ease comparisons). d–f Fragmentary proximal end of the left ulna of K. biceae in (d) dorsal, (e) ventral, and (f) proximal view. g, h Left ulna of an undescribed new sphenisciform from the Waipara Greensand (CM 2016.6.1) in (g) ventral and (h) proximal view; the dashed line in g indicates the portion of the bone preserved in the K. biceae holotype. i CT image of cranial surface of partial left humerus. j Exposed caudal surface of the bone, surrounding bones and matrix were digitally brightened. k, l CT images of caudal humerus surface with (k) minimum and (l) maximum length estimates based on the reconstructed outline of the bone (dotted lines). m Left humerus of Crossvallia unienwillia from the late Paleocene of Antarctica (holotype, MLP 00-I-10-1), which is one of the largest previously known Paleocene penguin species. n Left humerus of Pachydyptes ponderous from the late Eocene of New Zealand (holotype, NMNZ OR.1450), which was previously considered one of the largest fossil penguins. Abbreviations: cor, coracoid; dcp, dorsal cotylar process; fem, femur; fpt, fossa pneumotricipitalis; hum, humerus; olc, olecranon; ppc, procoracoid process; scc, scapular cotyla; sup, attachment scar for supracoracoideus muscle; tbt, tibiotarsus; vct, ventral cotyla. Scale bars equal 50 mm; same scale for b and c, f and h, and i-l, respectively Full size image

Table 1 Selected bone dimensions (in mm) of the new species and other Sphenisciformes Full size table

The fragment of the proximal end of the ulna exhibits half of the ventral cotyla, next to which there is a distinct, flange-like projection (Fig. 1d‒f). The ulna of W. tuatahi and that of a phylogenetically basal penguin from the Waipara Greensand (CM 2016.6.1), whose description is pending24, has two such flanges on the caudal and cranial surfaces of the bones, respectively: the olecranon and the dorsal cotylar process (Fig. 1g, h). Based on comparisons with the ulnae of the Sphenisciformes from the Waipara Greensand, we consider the flange on the ulna of the K. biceae holotype to be the dorsal cotylar process, because the olecranon has a more convex margin in Paleocene Sphenisciformes and is much more prominent in post-Paleocene taxa. A well-developed dorsal cotylar process is a plesiomorphic feature24, which is absent in post-Paleocene Sphenisciformes.

The coracoid lacks the sternal end (Fig. 1b) and the acrocoracoid process is damaged, with its medial portion being broken. The scapular cotyla is of circular outline and more deeply excavated than in post-Paleocene Sphenisciformes including Pachydyptes ponderosus. The well-developed procoracoid process projects perpendicular to the longitudinal axis of the bone and has a ventrally deflected tip. As in W. tuatahi (Fig. 1c), there is no coracoidal fenestra, with this fenestra being present in all Eocene and Oligocene Sphenisciformes of which well-preserved coracoids are known (in P. ponderosus, the corresponding area is damaged, although the remaining medial section of the shaft suggests the former presence of a fenestra).

The cranial extremity of the scapula (Fig. 2a, b) resembles that of Waimanu tuatahi (Fig. 2c) and differs from the known scapulae of post-Paleocene penguins in that the humeral articulation facet is craniocaudally more extensive. Unlike in W. tuatahi, however, the scapular blade widens caudally, as it does in phylogenetically more crownward Sphenisciformes, and the articulation facet of the humerus is craniocaudally narrower, with a more pointed ventral tip.

Fig. 2 Further bones of the giant Paleocene penguin from New Zealand. a K. biceae n. gen. et sp. (holotype, NMNZ S.45877), cranial portion of left scapula; in b the surrounding matrix and bones were digitally removed; the dotted line demarks an overlying bone fragment. c Left scapula of Waimanu tuatahi from the late Paleocene of New Zealand (CM zfa 34). d, e Thoracic vertebra of K. biceae in (d) caudal and (e) right lateral view. f Right femur of K. biceae in craniomedial view. g K. biceae, sternum in cranial view. h Right tibiotarsus in cranial view. i Digitally reconstructed distal end of tibiotarsus, in which the medial condyle was brought into its presumed original position and a piece of adhering bone fragment and matrix were removed. j Distal end of right tibiotarsus of Waimanu manneringi (holotype, CM zfa 35). afh, articulation facet of humerus; cas, coracoidal articulation sulcus; cdf, caudal articulation facet; crf, cranial articulation facet; ext, extensor sulcus; fem, femur; fib, fibular crest; lcd, lateral condyle; mcd, medial condyle; stk, sternal keel; vtp, ventral process. Scale bars equal 50 mm Full size image

Of the sternum, the cranial margins of the carina and the corpus are preserved (Fig. 2g), but only a few osteological details can be discerned. As in Platydyptes marplesi 8 but unlike in extant penguins, the articular sulci for the coracoid meet each other at the midline of the bone. Compared to extant Sphenisciformes, the external lip of the articulation sulcus for the coracoid (labrum externum) is furthermore situated in a very lateral position. The sternal keel appears to have been unusually thick, but is too incompletely preserved to determine its original shape.

A thoracic vertebra (Fig. 2d, e) has a slightly concave caudal articulation facet and an essentially flat cranial one, whereas in extant Sphenisciformes, the vertebrae are opisthocoelous, that is, with a distinctly convex cranial articulation facet and a distinctly concave caudal one. Unlike in W. tuatahi, the lateral surfaces of the corpus do not exhibit well-delimited fossae.

The new species has a longer femur than all other fossil penguins, for which this skeletal element is known (Table 1). The bone is partially embedded in matrix (Fig. 2f) and is stouter than the femur of W. tuatahi, but less stout than the femora of Kairuku and Inkayacu. On the distal end, the fibular trochlea is strongly developed and there is a projecting tubercle for the gastrocnemialis lateralis muscle.

The tibiotarsus lacks the proximal end, but preserves the distalmost portion of the fibular crest (Fig. 2h). The distal end exhibits a crack, which disrupted the medial part of the bone, so that the condylus medialis appears to be unusually splayed. However, even when the bone is digitally restored (Fig. 2i), the shape of the distal end is highly characteristic in that the condyles are more widely spaced and proximodistally lower than in all other stem group Sphenisciformes, of which the tibiotarsus is known. The distal opening of the extensor sulcus is more medially positioned than in post-Paleocene Sphenisciformes, in which it is in a central position along the midline of the long axis of the tibiotarsus, but it is situated less medially than in Waimanu manneringi (Fig. 2j). The width of the distal end of the tibiotarsus is only exceeded by the very large Antarctic P. klekowskii specimens14 (these consist of a humerus fragment and a tarsometatarsus, but an estimate of the distal width of the tibiotarsus is possible, because this value corresponds to the proximal width of the tarsometatarsus; Table 1).

Results of the phylogenetic analysis

Our initial phylogenetic analysis based on the complete data set of ref. 8 resulted in a placement of Kumimanu in a clade including all other Sphenisciformes except W. manneringi and W. tuatahi, where it was recovered in a polytomy together with the much smaller Eocene taxa Perudyptes, Delphinornis, Marambiornis, and Mesetaornis (Supplementary Fig. 1). Analysis of the reduced and emended data set (Methods section) supports a position of K. biceae outside a clade including all post-Paleocene Sphenisciformes, and recovers the new species as sister taxon of a clade including all other Sphenisciformes except W. manneringi and W. tuatahi (Fig. 3).