Centropus colossus

Shute, Elen, Prideaux, Gavin J. & Worthy, Trevor H., 2016, Three terrestrial Pleistocene coucals (Centropus: Cuculidae) from southern Australia: biogeographical and ecological significance, Zoological Journal of the Linnean Society (Zool. J. Linn. Soc.) 177 (4), pp. 964-1002 : 972-975

CENTROPUS COLOSSUS BAIRD, 1985

Holotype

SAM P24240 (L humerus with minor damage to the proximal and distal ends).

Referred material

SAM P42065 (L humerus, missing proximal end and with damage to distal end) ; SAM P42027 (R femur, nearly complete but with some damage to caput femoralis and condyles, and with dendritic etching to the shaft surface). Including the holotype humerus, number of individual specimens = 3, minimum number of individuals = 2 .

Locality

All specimens come from the type locality, Green Waterhole Cave (=Fossil Cave, 5L81), near Tantanoola, South Australia ( Fig. 1 ). The fossil deposit was below the water table, and the bones were recovered by divers in 1979 along with the remains of numerous other vertebrates ( Baird, 1985; Rich, van Tets & Knight, 1985b). Centropus colossus was described from a left humerus ( Baird, 1985), but we later identified the two additional referred bones, which are from the same site, within the Palaeontology collection of the South Australian Museum.

Age and fauna

The Green Waterhole Cave fossil deposit has not been directly dated, but the species composition of the assemblage suggests a Middle or Late Pleistocene age ( Pledge, 1980; Baird, 1985; Prideaux, 2004, 2007). This falls within the Naracoortean land mammal age ( Megirian et al., 2010).

Revised diagnosis

A species of Centropus distinguished from all extant species of Centropus by its much greater size and the following combination of humeral features ( Fig. 2): a stout, dorsally arched shaft; a caput humeri that is rotated cranioventrally; an incisura capitis that is wide and orientated more proximodistally than dorsoventrally; a tuberculum ventrale that is large and placed quite proximally; a small fossa pneumotricipitalis (the degree of pneumatization is difficult to assess because the fossa of the holotype is filled with putty, and this region is missing in the referred humerus); a crista deltopectoralis that is robust and very short, and in dorsal aspect ( Fig. 2A) its proximal extreme curves outwards dorsocranially (crista of the holotype is damaged and thus the extent of dorsocranial projection more distally is difficult to assess – see Fig. 2K); an impressio m. pectoralis that is reduced to a prominent, proximodistally foreshortened scar ventrad of, and linking to, the distal end of the crista deltopectoralis; a processus supracondylaris dorsalis that is low and rounded; a corpus that is swollen around the fossa m. brachialis; and a processus flexorius that is dorsoventrally wide in caudal aspect.

The humerus is very similar in size to that of S. novaehollandiae , but differs as noted in the generic diagnosis above. It differs from the humerus of Ce. phasianinus by its much larger size, having a relatively stouter shaft that is less arched dorsally, a caput humeri that is offset cranially from the longitudinal axis of the shaft in caudal aspect, a tuberculum ventrale that is placed more proximally relative to the caput humeri, an incisura capitis that is more proximodistally aligned (diagonally traverses the longitudinal aspect of the shaft in Ce. phasianinus ), a smoothly curved rather than angular profile of the crista deltopectoralis in dorsal aspect, and a dorsoventrally wider processus flexorius.

The humerus of Ce. colossus is distinguished from those of the two new extinct species of Centropus described in this paper as follows. It is much larger than the humerus of Centropus bairdi sp. nov. ( Fig. 2), and is further differentiated from that new species by the following features: the shaft is straighter in dorsal aspect and is proportionally wider ( Fig. 7 ); the caput humeri is orientated more cranially in dorsal aspect; the tuberculum ventrale is less caudally projected; the fossa pneumotricipitalis is deeper; the fossa m. brachialis is deeper, and the ventral shaft adjacent to the fossa is swollen (not so in Ce. bairdi sp. nov.); the impressio m. brachialis is proportionally much deeper and wider; and the condylus dorsalis is less cranially projected in dorsal aspect.

The humerus of Ce. colossus approaches the size of that of Centropus maximus sp. nov. ( Fig. 2), but is distinguished from this new species by the following features: the shaft is proportionally wider; the proximal end is proportionally narrower and not as robust; the caput humeri is more inflated, and extends further proximally than the tuberculum ventrale in ventral aspect; it lacks a prominent scar for the insertion of the m. supracoracoideus between the caput humeri and the crista deltopectoralis (marked in Ce. maximus sp. nov.); the intumescentia humeri is less inflated; the impressio coracobrachialis is shallower; the impressio m. pectoralis is less prominent; the shaft adjacent to the fossa m. brachialis is ventrally swollen; the processus supracondylaris is less prominent; the processus flexorius is narrower and less robust; and the condylus ventralis and condylus dorsalis project less far cranially.

Description and comparisons

Humerus: See revised diagnosis. The second, less complete, humerus (not illustrated) is referred to Ce. colossus on the basis of its similar size, robusticity, and morphology compared to the holotype. Measurements: Table 1.

Femur: The femur of Ce. colossus has not previously been described, but SAM P42027 ( Fig. 3) is referred to this species for the following reasons: it has the generic features of Centropus listed above; comes from the same site as the holotype humerus and is of congruent size; is differentiable from all extant species of Centropus , including the only extant Australian species Ce. phasianinus , on the basis of its much larger size and robusticity; and is differentiable from the femora of the two new extinct Australian Pleistocene species of Centropus described later in this paper. It is around one third longer and has more than double the shaft diameter of the femur of Ce. phasianinus , and is further distinguished this species by the following features: the shaft is straighter in cranial aspect, and is proportionally thicker; the caput femoris is proportionally larger; the crista trochanteris and the cranial margin of the facies articularis antitrochanterica merge at approximately a 90° angle to form a continuous, evenly curved crest that is cranially prominent and overhangs the proximocranial surface of the bone (in Ce. phasianinus only the crista trochanteris is cranially prominent); the condylus lateralis and condylus medialis are proportionally deeper and wider; the fovea tendineus m. tibialis cranialis is much deeper; the insertion for the m. gastrocnemialis lateralis is proportionally larger, and forms a circular scar proximal to the trochlea fibularis on the caudolateral margin; the impressio ligamentum collateralis lateralis is very large and well marked in a distinct fossa centred laterally on the trochlea fibularis; the impressio ansa m. iliofibularis caudalis distally abuts the insertion for the m. gastrocnemialis lateralis, and the impressio ansa m. iliofibularis cranialis is an elongate scar further proximally on the lateral facies; the fossa poplitea is deeper; and the scar for the origin of the ligamentum cruciati caudalis is large and prominent on the lateral wall of the fossa poplitea, directly proximal to the crista tibiofibularis.

The femur of Ce. colossus has similar proportions to that of the extinct Ce. bairdi sp. nov., which is described later in this paper, but is distinguished by its considerably larger size. The femur of Ce. colossus approaches the size of the femur of the extinct L, left; R, right; TL, total length; PW, proximal width; SW, mid-shaft width; DW, distal width; *, minimum measurement owing to damage.

Ce. maximus sp. nov., which is described later in this paper, but it is more gracile at the proximal and distal ends and has a smaller minimum shaft circumference (see body mass estimates). For further distinguishing features, see the description of Ce. maximus sp. nov. below. Measurements (mm): for TL, PW, SW, and DW see Table 1; minimum shaft circumference = 25.8.

Remarks

This species was much larger than any extant species of Centropus , which as a group are poor flyers. We concur with Baird (1985) that features of the humerus, including the stout, curved shaft, reduced crista deltopectoralis, and small muscle scar for the m. pectoralis are indicative of reduced volancy. To these humeral features, we add a cranially placed caput humeri relative to the longitudinal axis of the shaft in caudal aspect, a more proximodistally aligned incisura capitis, and more proximal placement of the tuberculum ventrale, as well as the large size and robustness of the femur and enhanced development of the crista trochanteris ( Rich, McEvey & Baird, 1985a), as indicative of reduced flying ability and greater use of the legs in locomotion.

Figures

Figure 1. Map of key localities mentioned in the text, showing the range of extant Centropus phasianinus, and showing landbridge connections between Australia and Melanesia at different Pleistocene sea level low-stands (–40 and –130 m) relative to modern sea level; (modified from Sahul and Sunda Shelf Basemap, CartoGIS CAP, 2013, Map 00-101; downloaded from http://asiapacific.anu.edu.au/mapsonline/base-maps/sunda-and-sahul-shelf, 08-05-2015).

Figure 7. Simpson log ratio diagram of extinct and extant Centropus bones; zero line represents the standard species channel-billed cuckoo Scythrops novaehollandiae against which all values are calculated. Abbreviations: HL, humerus length; HP, humerus proximal width; HM, humerus midshaft width; HD, humerus distal width; UL, ulna length; CMCL, carpometacarpus length; KD, keel depth; CL, coracoid medial length; FL, femur length; FP, femur proximal width; FM, femur midshaft width; FD, femur distal width; TTL, tibiotarsus length; TTP, tibiotarsus proximal width; TTM, tibiotarsus midshaft width; TTD, tibiotarsus depth; TML, tarsometatarsus length; TMP, tarsometatarsus proximal width; TMD, tarsometatarsus distal width. Gaps in lines reflect absent data owing to incomplete specimens.