Masillastega rectirostris, Mayr, 2002

Mayr, Gerald, 2002, A skull of a new pelecaniform bird from the Middle Eocene of Messel, Germany, Acta Palaeontologica Polonica 47 (3), pp. 507-512 : 508-511

Masillastega rectirostris sp. nov.

Fig. 1 .

Holotype and only known specimen: Isolated skull on two slabs, IPB 140 a+b ( Fig. 1 ; specimen 140a only shows fragmentary remains of the cranium and the mandible).

Type locality: Messel, near Darmstadt (Hessen, Germany).

Type horizon: Lower Middle Eocene.

Etymology: “Straight−billed”, from rectus (Latin), straight and rostrum (Latin), beak.

Diagnosis.—Same as for genus.

Description and comparison.—Measurements (in millimeters): maximum length of skull, 139; length of upper beak from naso−frontal hinge to tip, 99; length of mandible, 135.

In its shape and relative proportions but not in morphological details (see below) the beak of Masillastega rectirostris resembles that of the extant hamerkop, Scopus umbretta ( Ciconiiformes , Scopidae ; Fig. 2 ). Compared to extant pelecaniform birds, it is most similar to the beak of the Sulidae which is, however, relatively shorter ( Fig. 2 ). The bill of the Pelecanidae is much longer and has a completely different shape; that of Fregatidae , Phalacrocoracidae and Anhingidae is not so deep, and the beak of extant Phaethontidae is relatively shorter and lacks a terminal hook. The beaks of the Eocene genera Limnofregata ( Fregatidae , see Olson 1977) and Prophaethon ( Prophaethontidae ) also are relatively shorter; that of the putatively pelecaniform Pelagornithidae exhibits numerous pseudo−teeth along the cristae tomiales (see Harrison and Walker 1976b).

The upper beak is long, measuring more than 2/3 of the complete length of the skull. It is high in its proximal part and gradually becomes narrower towards the tip. The cristae tomiales are straight, the culmen also is hardly curved. The tip of the rostrum is hooked as in all extant pelecaniform birds except the Phaethontidae and Anhingidae , although this hook is only moderately developed as in extant Sulidae and the Eocene frigatebird Limnofregata . The narial openings are greatly reduced as in other extant Pelecaniformes , but whether they are only very small or completely absent as in extant Sulidae remains uncertain; the narial openings are long and narrow in the early Tertiary pelecaniform taxa Limnofregata ( Fregatidae ) and Prophaethon ( Prophaethontidae ). The ventral surface of the upper beak of Masillastega seems to have been greatly ossified as in all extant Pelecaniformes . Due to the crushing of the specimen, a longitudinal furrow along the rostrum, which is characteristic for extant Pelecaniformes , cannot be clearly discerned in Masillastega . As in extant Sulidae and Phaethontidae , the surface of both the upper beak and the mandible is covered with many distinct impressions of vessels.

The dorsal margin of the mandible is straight over its entire length. As in other pelecaniform birds, but in contrast to the Scopidae and other ciconiiform birds, the dorsal surface of the mandibular rami is medio−laterally very wide ( Fig. 3), measuring about 3.5 mm in the mid−section of the mandible. The mandibular rami are deep as in extant Sulidae , whereas in other extant Pelecaniformes they are lower. A fenestra mandibulae seems to be absent. As in extant Pelecaniformes , but also in contrast to the Ciconiiformes , the pars symphysialis appears to have been very short (although the pars symphysialis itself is not visible, in specimen 140a it can be seen that the mandibular rami are separated over most of their length). A characteristic feature of extant Suloidea is a bipartite processus coronoideus (see Lambrecht 1929) but, owing to preservation of the specimen, this feature cannot be discerned in Masillastega rectirostris . The mandible lacks a terminal hook, in lateral view its tip appears truncated as in extant Phalacrocoracidae , whereas it is more pointed in extant Sulidae .

The cranial part of the skull is crushed and only allows the recognition of few details. The os lacrimale is detached from the frontal which opens the view on the articular facet at the

MAYR—NEW PELECANIFORM BIRD FROM THE MIDDLE EOCENE 511

A B

frontal. The interorbital septum is greatly ossified (visible in specimen 140a), as in all extant Pelecaniformes except the Anhingidae and Phalacrocoracidae . The right os palatinum is visible in specimen 140b, but whether the palatina were fused along their midline as in extant Sulidae cannot be discerned. The pterygoids are fairly long, whereas these bones are much more abbreviated in Scopus and other Ciconiiformes (thus in ciconiiform birds the processus mandibularis of the quadrate and the caudal end of the mandible are shifted much more rostrally than in Masillastega and other Pelecaniformes ). The tip of the short processus postorbitalis projects laterally; it appears not to have been bifurcated as in extant Sulidae (I did not have access to skeletons of Sula abbotti which, according to Olson and Warheit (1988), has long, pointed, and ventrally oriented processus postorbitales). As in extant Sulidae , the fossae temporales are well developed and wide (apart from the Pelecanidae , these fossae are well developed in all extant Pelecaniformes ). As in extant Sulidae , Fregatidae , and Phaethontidae , there further is a deeply excavated recessus tympanicus dorsalis (this recess is small or absent in extant Pelecanidae , Phalacrocoracidae , and Anhingidae , and most other neognathous birds). The processus zygomaticus has a similar shape to that of extant Sulidae , Phalacrocoracidae , and Fregatidae ; its tip also projects laterally. Details of the quadrate cannot be discerned and it is not even certain whether this element is preserved at all. A process which is visible at the caudal end of the cranium in specimen 140b probably represents the deformed right processus paroccipitalis.

Figures

Fig. 1. Masillastega rectirostris gen. et sp. nov., holotype (Universität Bonn, Institut für Paläontologie; collection number 140b). A. Coated with ammonium chloride to enhance contrast, note the impressions of vessels on the beak; scale bar equals 10 mm. B. Interpretative drawing; the hatched area marks the recessus tympanicus dorsalis, the asterisks indicate the wide, plane dorsal surface of the left mandibular ramus. C. X−ray photograph.

Fig. 2. Skulls of extant pelecaniform and ciconiiform birds in comparison. A. Sula bassana (Pelecaniformes, Sulidae), SMF 1535; note the impressions of vessels on the mandible and the upper beak, the small arrow indicates the deep recessus tympanicus dorsalis. B. Phalacrocorax aristotelis (Pelecaniformes, Phalacrocoracidae), SMF 2861. C. Scopus umbretta (Ciconiiformes, Scopidae), SMF 1906. Scale bars 10 mm.