Borvocarbo tardatus, Göhlich & Mourer-Chauviré, 2010

Göhlich, Ursula B. & Mourer-Chauviré, Cécile, 2010, A New Cormorant-like Bird (Aves: Phalacrocoracoidea) from the Early Miocene of Rauscheröd (Southern Germany), Records of the Australian Museum 62 (1), pp. 61-70 : 63-69

publication ID	https://doi.org/ 10.3853/j.0067-1975.62.2010.1536
persistent identifier	https://treatment.plazi.org/id/03F90677-2608-FF8C-0854-FD9FE9C71183
treatment provided by	Felipe
scientific name	Borvocarbo tardatus
status	sp. nov.

Treatment

? Borvocarbo tardatus n.sp.

Figs 2 View Figure 2 , 3 View Figure 3

Holotype. Nearly complete left tibiotarsus ( BSPG 1990 IV 16), figured also in Pfeil & Werner, 1991, pl. 5, fig. 2.

Paratypes. Proximal half of left ulna ( BSPG 1990 IV 17), proximal third of right radius ( BSPG 1990 IV 18) .

Type locality. Gravel pit of the company “Sand- und Kieswerk Rauscheröd Ulrich Alex GmbH” in the village of Rauscheröd, near Passau (eastern Lower Bavaria, southeastern Germany), (topographic map 7445 Ortenburg, r: 4592970 h: 5380700). Early Miocene, MN4b, transition of upper Brackish Molasse to Upper Freshwater Molasse.

Etymology. From the Latin tardatus , meaning late; used in the sense that this species is the stratigraphically youngest (latest) of the genus Borvocarbo and that it retained primitive osteological characters up to the Early Miocene. All contemporaneous cormorant taxa show more derived osteological features.

Diagnosis. Largest species of Borvocarbo . Size between that of the extant cormorants P. carbo and P. aristotelis . Ulna with processus cotylaris dorsalis lacking hook-like distal end. Tibiotarsus with sulcus extensorius in almost central position along midline, with pons supratendineus proximodistally wide and almost horizontal, and condylus medialis extending only slightly further distally than condylus lateralis. These morphological characters also characterize? B. stoeffelensis , but? B. tardatus is distinctly larger.

Description

The left tibiotarsus ( Fig. 2 View Figure 2 ) is nearly complete, with only the cranial and proximal part of the crista cnemialis cranialis and the proximal ends of the crista patellaris and crista cnemialis lateralis broken off. However on the cranial face, in proximal view, the remaining part of the crista patellaris is roughly aligned mediolaterally and this indicates that the tip of the crista cnemialis lateralis was not offset cranially. In proximal view, the incisura tibialis is deeply marked. Two strong and projecting tubercles are situated on the proximal end, a smaller one on the area interarticularis and a larger one caudal to the facies articularis lateralis. In proximal view, the lateral margin of the facies articularis lateralis is somewhat flattened. The crista fibularis is strong and about 26 mm long. The shaft is oval in cross section and slightly compressed craniocaudally. The distal end is distinctly medially bent. The canalis extensorius is wide and runs obliquely from distomedially to proximolaterally and the sulcus extensorius extends along the midline of the shaft. Situated lateral to the pons supratendineus, the tuberositas retinaculi extensoris is developed as a moderate tuberculum, which is separated from the condylus lateralis by a weak sulcus. The tuberculum retinaculi m. fibularis is long and its proximal end merges smoothly into the shaft. Condyli medialis and lateralis are of almost equal size and are parallel and oriented vertically (in cranial view). The condylus medialis bears a deep indentation on its lateral side. The condylus medialis is only slightly longer in distal direction than the condylus lateralis and also exceeds the latter in craniocaudal direction. In distal view, the condylus medialis is tilted craniolaterally so that the distal trochlea tapers from the cranial end to the caudal one. For measurements, see Table 2.

Only the proximal half of the left ulna ( Fig. 3 View Figure 3 ) is preserved. The cotyla dorsalis is slightly narrower than the cotyla ventralis. The dorsal projection of the cotyla dorsalis (processus cotylaris dorsalis) is blade-like and does not form a distal hook. The olecranon is swollen proximally and caudally and projects ventrally. The impressio m. scapulotricipitis on the dorsal side of the proximal end forms a small but distinctive pit; the impressio m. brachialis is well marked, about 3 cm long and becomes shallow distally. The insertion for the lig. collateralis ventralis on the ventral side of the proximal end is large and flat. The linea intermuscularis along the median cranial surface of the proximal shaft forms a prominent crest. The quill knobs (papillae remigales caudales) are weakly developed.

Only the proximal third of a right radius ( Fig. 3 View Figure 3 ) is preserved. The ventral margin of the cotyla humeralis is broken off. The caput radii on the proximal end is dorsally strongly swollen. The tuberculum bicipitale radii, situated ventrally somewhat distally to the proximal end, forms a short crest-like structure. The linea intermuscularis ventralis becomes more prominent distally.

Comparisons. Most of the Miocene cormorant taxa are created on, and are represented by, only scanty material, often a single bone, which makes comparisons, systematic determinations and phylogenetic discussions with these taxa difficult. For only a few species are a tibiotarsus, ulna and/or radius available for metrical and morphological comparisons ( Table 3) with the material from Rauscheröd described here.

For comparison with extant taxa we consulted the only two European taxa, the Great Cormorant ( Phalacrocorax carbo ) and the European Shag ( P. aristotelis , genus “ Stictocarbo ” after Siegel-Causey, 1988), and a representative of the microcormorants and the darters. The last are usually accepted to be a sister taxon of cormorants; for our comparisons, we selected an American Darter ( Anhinga anhinga , North to South America).

The relationship of microcormorants (genus Microcarbo after Siegel-Causey, 1988) and other Phalacrocoracidae is still controversial. The osteological investigations of Siegel-Causey (1988) resulted in a position of the Microcarbo species within the taxon “ Phalacrocoracinae ” (“cormorants”), which itself is considered as a sister taxon of “ Leucocarboninae ” (shags). On the other hand, the analyses of mitochondrial sequence data by Kennedy et al. (2000) found that the microcormorants are the sister group of all other Phalacrocoracidae . Owing to the probable basal phylogenetic position of the microcormorants, we considered also the Little Pied Cormorant ( P. (M.) melanoleucos , Australia, New Zealand, Tasmania, Indonesia, and some adjacent islands) for comparisons (hereafter Microcarbo melanoleucos ).

Numbers in the following descriptions refer to features in Figs 4 View Figure 4 and 5 View Figure 5 .

Comparison of tibiotarsus ( Fig. 4 View Figure 4 ). In comparison with the tibiotarsi of the extant taxa Phalacrocorax carbo and P. aristotelis , that of? B. tardatus differs by having (1) higher and stronger protuberances proximal on the facies articularis. (2) thickening at the tip of the crista cnemialis lateralis, slightly more developed than in Anhinga and in? B. stoeffelensis ( Mayr, 2007, fig. 6G), but less developed than in Microcarbo . Crista patellaris roughly aligned mediolaterally indicating that the tip of the crista cnemialis lateralis was not offset cranially. (3) incisura tibialis deeply expressed, as for example in Anhinga , while in P. carbo , P. aristotelis and Microcarbo it makes a shallow curve. (4) a less oblique (almost horizontal) and proximodistally wide pons supratendineus and (5) a sulcus extensorius that runs almost in the midline of the shaft (more laterally in Phalacrocorax , meaning in P. carbo and P. aristotelis also in the following text). (6) The condylus medialis is stronger (about same size as condylus lateralis) than in Phalacrocorax and, in distal view, (7) is tilted slightly obliquely in the mediocranial direction so that the distal trochlea tapers slightly caudally, whereas in Phalacrocorax the condyles and their medial and lateral margins are almost parallel. In cranial view, (8) the condylus medialis is distally shorter than in Phalacrocorax , only slightly distally surpassing the lateral one. (9) The tuberculum retinaculi m. fibularis is less prominent and proximally ends less abruptly than in extant Phalacrocorax . (10) The epicondyli medialis and lateralis are stronger than in extant Phalacrocorax . (9) The apophysis externa ligamenti obliqui ( Ballmann, 1969) is stronger and more individualised.

The microcormorant Microcarbo melanoleucos is almost half of the size of? B. tardatus and furthermore differs in its tibiotarsus in the aforementioned mentioned features (3) (4), (5), (6), (7), (10), and (11). In M. melanoleucos , (2) the crista patellaris is roughly flat and aligned lateromedially and joins the caudal side of the thickened tip of the crista cnemialis lateralis, (4) the pons supratendineus is more oblique, (5) the sulcus extensorius runs more laterally, (6) the condylus medialis is more slender (in cranial and distal views) than the lateral one and, in cranial view, is tilted distomedially (almost vertical in? B. tardatus ), and (10) the epicondyli medialis and lateralis and (11) the apophysis externa ligamenti obliqui are weaker. Furthermore, in proximal view, the lateral prominence for the contact with the caput fibulae is more pointed in M. melanoleucos .

No tibiotarsus of P. intermedius is known so far. Metrical comparisons of other skeletal elements suggest that P. intermedius and? B. tardatus might have been of similar size. On the other hand, N. miocaenus , N. anatolicus and O. littoralis are all distinctly smaller than the new Rauscheröd cormorant.

Unlike the tibiotarsus of O. littoralis from Saint-Gérandle-Puy (MN2, France) (Milne-Edwards 1867–1871b, pl. 42, figs 9–12) that of? B. tardatus differs by the aforementioned features (4) and (5): (4) the pons supratendineus is less oblique in transversal direction in comparison to that of O. littoralis . In distal view, (7) the distal condyles of both taxa are similar in tapering slightly caudally, but in P. littoralis , the condyles are oriented more parallel, whereas in? B. tardatus the condylus medialis is slightly oblique.

The tibiotarsus of? B. tardatus differs from that of Nectornis by its more cranially projecting crista cnemialis cranialis (in medial view) and by (12) a much more projecting and craniolaterally oriented crista cnemialis lateralis (in proximal view). (13) The proximal end is mediolaterally wider (in relation to depth) and the lateral margin of the facies articularis lateralis projects more laterally. (5) The sulcus extensorius runs almost along the midline of the shaft (situated more laterally in Nectornis ) and (4) the pons supratendineus is less oblique in transversal and cranial directions than in Nectornis . (6) The condylus medialis is broader and (7) tapers slightly caudally (in distal view) in? B. tardatus , whereas the medial and lateral margins are parallel in Nectornis .

In contrast to the conditions in A. anhinga , in? B. tardatus (14) the fossa synovialis externa ( Ballmann, 1969) on the proximal end of the tibiotarsus is deeper and (12) the crista cnemialis lateralis projects further craniolaterally (in proximal view). (15) The crista cnemialis cranialis is cranially convex at its distal base (in medial view) (concave in Anhinga ). (9) The tuberculum retinaculi m. fibularis is situated more proximally on the distal end and (11) the apophysis externa ligamenti obliqui forms a strong tubercle proximal to the condylus lateralis, whereas in A. anhinga it forms a deep fossa. In distal view, (16) the incisura intercondylaris in? B. tardatus is less asymmetric.? B. tardatus and A. anhinga agree well in the presence of strong protuberances on the proximal articular surface, the shape of the incisura tibialis, the position of the sulcus extensorius and the orientation of the pons supratendineus.

As in? B. stoeffelensis , the condylus medialis of the distal tibiotarsus of? B. tardatus protrudes only slightly further distally than the condylus lateralis ( Mayr 2007). Unfortunately, the second diagnostic feature of the tibiotarsus of? B. stoeffelensis , the crista patellaris without lateral proximal projection, cannot be ascertained because of the incomplete preservation of the Rauscheröd tibiotarsus.

Comparison of ulna ( Fig. 5 View Figure 5 ). The proximal ulna of? B. tardatus is distinguished from those P. carbo and P. aristotelis by having (1) a less prominent and less projecting processus cotylaris dorsalis without a hook-like distal end; (2) the ventral edge of the cotyla ventralis projecting less ventrally; (3) the olecranon more swollen and projecting more proximally (best observed in caudal view, difference in elevation between proximal end of cotyla ventralis and of olecranon); (4) the impressio brachialis shallower proximally; (5) the tuberculum lig. collateralis ventralis situated more proximally, just below the margin of the cotyla ventralis (more distally in P. carbo and P. aristotelis ); (6) the linea intermuscularis forming a more prominent crest along the midline of the cranial surface of the proximal shaft.

Several of these aforementioned distinguishing features between the ulna of? B. tardatus and those P. carbo and P. aristotelis do not distinguish between? B. tardatus and the microcormorant M. melanoleucos . However, beside the distinctly smaller size of the M. melanoleucos , the species also clearly differs from? B. tardatus by having (1) a welldeveloped hook-like projecting processus cotylaris, (3) a less swollen and less proximoventrally projecting olecranon, and a less cranially projecting margin of the cotyla ventralis.

Unlike in O. littoralis (Milne-Edwards, 1867–1871b, pl. 44, figs 6–8), in? B. tardatus (1) the processus cotylaris dorsalis is less prominent and less projecting, without a hook-like distal end (even if a little broken in O. littoralis ); (5) the tuberculum lig. collateralis ventralis is situated more proximally; (2) the ventral edge of the cotyla ventralis projects less ventrally, and (4) the impressio brachialis is shallower (also in its distal extent).

In contrast to that of N. miocaenus , the ulna of? B. tardatus has (1) the processus cotylaris dorsalis without the hook-like distal end, (3) the olecranon more swollen and projecting slightly more proximally, (5) the tuberculum lig. collateralis ventralis projecting slightly less ventrally, (7) the dorsocaudal border of the cotyla dorsalis rounded (in Nectornis it forms an angle) and (6) the crest-like linea intermuscularis along the proximal part of the cranial surface of the shaft more prominent than in Nectornis .

Unlike that of A. anhinga , the tuberculum lig. collateralis ventralis of? B. tardatus is (5) weaker and less projecting ventrally and (6) the crest-like linea intermuscularis is more prominent.

Unlike in members of the compared fossil and extant Phalacrocoracidae (inclusive also M. melanoleucos ), but similar to Borvocarbo , species of the Anhingidae have the distal hook-like process of the cotyla dorsalis less developed.

Comparison of radius. The proximal end of the radius of? B. tardatus is more similar to that of Anhinga than to those of extant cormorants, in which the tubercle-like caput radii is more prominent and projects further dorsally than in the Rauscheröd specimen.