Prophaethon shrubsolei, , Harrison & Walker, 1976

Milner, Angela C. & Walsh, Stig A., 2009, Avian brain evolution: new data from Palaeogene birds (Lower Eocene) from England, Zoological Journal of the Linnean Society 155 (1), pp. 198-219 : 206-209

PROPHAETHON SHRUBSOLEI

As with Odontopteryx it has been possible to reconstruct nearly all of the cranial endocast of Prophaethon , with the exception of the tractus opticus, part of the left bulbus olfactorius, and the anastomosis intercarotica (see Table 2 for list of measurements). Both osseous labyrinths are complete, but no evidence of the columella was detected within the recessus antivestibularis. The in vivo alert head posture of Prophaethon based on the position of the canalis semicircularis horizontalis of the osseous labyrinth in the skull is shown in Figure 2B . As with Odontopteryx and extant avian taxa, the major cranial bones of the braincase are fused, but unlike Odontopteryx the parietal-frontal suture is fully closed. The shape of the tractus opticus is not possible to reconstruct, although some information about the lateral outline shape can be gained from the dorsal and ventral margins at the midline ( Fig. 6C ).

With a length of 29 mm (measured from the caudal-most extent of the cerebellum to the rostralmost extent of the bulbus olfactorius) and a maximum telencephalic width of 26 mm, the cranial endocast of Prophaethon is slightly more elongate than that of Odontopteryx . Including cranial nerves the total endocranial volume of this specimen of Prophaethon is 5.5 mL. The tectum mesencephali are entirely occluded in dorsal view by the lateral expansion of the telencephalon ( Fig. 6A ). The rostral margins of the telencephalic lobes are slightly concave in dorsal view, unlike Phaethon where the margins are approximately straight. The bulbus olfactorius is proportionately larger than in Odontopteryx and forms a smooth extension of the dorsal and lateral surfaces of the telencephalon, with only a slight expansion in the caudal-most region demarcating the point at which the telencephalon begins (in Pelecanus and Phalacrocorax there is a sharp angle between the bulbus olfactorius and the telencephalon). The bulbus is approximately twice as large as in Phaethon rubricauda ( Fig. 8A ) and, unlike that species, the bulbus is not overstepped dorsally by the eminentia sagittalis ( Fig. 6C ). As with Odontopteryx the bulbus olfactorius occupies a position close to the dorsal surface of the telencephalon, and a rostrocaudally directed sulcus on its dorsal surface is not present ( Fig. 6B ).

The eminentia sagittalis is rostrally positioned and is more prominently developed than in Odontopteryx . In dorsal view ( Fig. 7A ) its shape is very similar to Phaethon in that it is wider caudally (approximately triangular in shape) and is rostrally more dorsally expanded (compare Fig. 7A with Fig. 8B ). It differs from Phaethon in that the dorsal expansion is only around half that of the living taxon ( Fig. 8B ), and that the eminentia does not reach the rostral-most margin of the telencephalon. In Pelecanus the eminentia is similar in shape, but is positioned caudally on the telencephalon. In Phalacrocorax the eminentia is rostrally positioned, but is oval rather than triangular. Also as in Odontopteryx , there is no evidence of a vallecula.

All values were measured at maximum orthogonal points, and are in millimetres. Note that semicircular canal length measurements refer to maximum dorsal (for csr) and lateral (for csh and csc) extent of canal arc; width measurements refer to maximum canal diameter.

The cerebellum is short (44% of the total brain length as measured along the central fissure) and broad (40% of the maximum telencephalic width); it is thus less elongate than that of Phaethon and does not exhibit the marked rostral narrowing in that taxon ( Fig. 6A ). The auricula cerebelli are caudolaterally directed but are shorter, narrower, and much less distally expanded than in Odontopteryx , and are also rostrocaudally compressed rather than approximately round in section. Although less rostrocaudally compressed, the feature is very similar to that seen in Phaethon . It is possible to detect impressions of the fissura cerebelli in Phaethon ( Fig. 8B ), but similar impressions are absent in BMNH A683 ( Fig. 6A ). However, it is possible to discern a trace of the median groove for the sinus occipitalis that narrows caudally. The path of the vena semicircularis rostralis is a wide and well-marked ridge that follows the dorsal curve of the canalis semicircularis rostralis of the labyrinth before extending onto the tectum mesencephali ( Fig. 6C ). Unlike Phaethon rubricauda where the vena semicircularis rostralis arcs ventrally on the tectum mesencephali to reach the base of the n. trigeminus ( Fig. 8A ), the vein only extends to the midpoint of the tectum mesencephali. In Phaethon rubricauda the vein enters an osseous tunnel ventral of the protruberantia tentorialis; this tunnel is absent in Prophaethon .

The shape of the tractus opticus is difficult to estimate, but apparently extended about the same distance rostrally as in Odontopteryx ( Fig. 7C ). Unlike Odontopteryx and Phaethon the rostral surface of the telencephalon between the tractus opticus and bulbus olfactorius bears a slight wishbone-shaped ridge ( Fig. 6B ). The three branches of the wishbone shape terminate in an expansion. We interpret the two lateral expansions to be the exits of the arteria ophthalmica externa. In Phaethon the two projections are smaller and less laterally positioned, and occur more dorsally below the bulbus olfactorius.

The tectum mesencephali is larger relative to the telencephalon than in Odontopteryx , but proportionately similar to Phaethon . The region is a flattened hemisphere because of the expansion of the telencephalon, but unlike Odontopteryx the junction between the two is almost rectimarginate. The rostral-most ventral margin of the tectum mesencephali does, however, exhibit a slightly angular penetration by the telencephalon ( Fig. 6C ); in Phaethon this region is much more rounded.

The hypophysis is larger than in Odontopteryx but is still smaller than the tractus opticus ( Fig. 7B ). Traces of a dorsum sellae are very faint, and it appears that the feature may only have been fully ossified dorsally as in Numenius and Pelecanoides ( Wingstrand, 1951) . In Phaethon the dorsum sellae is thick ( Fig. 8A ). The body of the hypophysis is more laterally compressed than in Odontopteryx , and the rostral-most margin of the pars distalis lacks the conical development seen in Odontopteryx . Like Hesperornis , Enaliornis , Procellariiformes , and all Pelecaniformes except Anhinga ( Saiff, 1978; Elzanowski & Galton, 1991), Prophaethon did not possess bony tunnels for the two ascending rami of the carotid artery. The preservation of their entry into the sella turcica indicates that the rami were narrow compared with those of Odontopteryx , and that the anastomosis intercarotica occurred at or above the level of the hypophysis, but it is not possible to determine the anastomosis pattern from our reconstruction.

The rhombencephalon exhibits a slightly flattened globe shape very similar to that of Phaethon ( Fig. 7B ). There is a slight suggestion of the presence of a ventral sulcus at its base, although this is difficult to determine. The foramen magnum is around twice the size of the same feature in Phaethon rubricauda , and is shaped like a rounded rectangle. The n. trochlearis is proportionately wider than that of Odontopteryx , and extends rostrally from the base of the n. trigeminus, but no trace of the n. occulomotorius appears to have been preserved. The n. trigeminus is proportionately as wide as in Odontopteryx , and its exit from the rhombencephalon partially involves the ventral margin of the tectum mesencephali ( Fig. 6C ). On the left-hand side it is possible to detect two nerve bundles which, based on their position and relative width, probably correspond to the V 2 (maxillary) and V 3 (mandibular) branches. A narrow n. facialis is preserved on both sides of the rhombencephalon, and clearly bifurcates to produce two rami that are directed dorsolaterally and ventrolaterally. The n. cochlearis is the same thickness as the n. facialis, and also bifurcates to produce two dorsolaterally and ventrolaterally directed rami that are directed caudally. The n. glossopharyngeus and n. vagus exit the rhombencephalon as a single broad bundle which widens distally to produce one narrower dorsally directed ramus (n. glossopharyngeus) and a broader ventrally directed ramus (n. vagus). As in Odontopteryx the n. glossopharyngeus exits the skull via the recessus scalae tympani. The combined n. glossopharyngeus and n. vagus nerve bundle is around four times as broad as that of Phaethon rubricauda . The n. hypoglossus is as wide as the n. facialis, and originates at the base of the n. vagus, extending ventrocaudally for 1.9 mm. The n. accessorius exits caudal of the n. glossopharyngeus at the base of the foramen magnum. The ramus of the nerve extends laterally for 0.4 mm.

The canals of the bony labyrinth have well-defined ampullae, and are narrower than in Odontopteryx ( Fig. 9A–D ). The rostral limb of the canalis semicircularis rostralis strongly slopes caudally as it rises from the ampulla ossea rostralis, curving round and back with two relatively sharp changes in direction, until it is directed rostrally before its junction with the canalis semicircularis caudalis. As in Odontopteryx the canalis semicircularis rostralis represents 50% of the total dorsoventral height of the labyrinth. Also like Odontopteryx the caudal region of the canalis semicircularis rostralis frames the foramen magnum, but in contrast to Odontopteryx the long axis is angled medially at 96°, and the sigmoidal curvature seen in dorsal view in that taxon is negligible in Prophaethon . In dorsal view the canalis semicircularis rostralis et caudalis cross at an angle of 62° to each other; the canalis semicircularis caudalis et horizontalis meet at 90°. The ductus cochlearis represents only 38% of the total height of the labyrinth (in the right-hand side labyrinth the duct is shorter than the left and represents only 32% of the total height). The duct is directed rostrally and medially, and exhibits slight curvature toward the midline. The dorsal region of the duct is weakly laterally compressed but becomes more rounded ventrally. The caudally facing fenestra cochlearis is larger and more elongate than in Odontopteryx , the oval window of the columella is also larger, and the recessus antivestibularis extends further laterally. A well-developed dorsoventrally compressed sacculus is present on the lateral surface of the cochlear duct.

Figures

Figure 2. Transparent computed tomographic (CT) segmentation of two Lower Eocene skulls in right lateral views, revealing the virtual endocasts of the brain and osseous labyrinth. Compare positions of the canalis semicircularis horizontali indicating the in vivo alert head positions. A, Odontopteryx toliapica; all sutures are fully obliterated except that between the frontals and parietals. B, Prophaethon shrubsolei.

Figure 6. Virtual endocast of Prophaethon shrubsolei in A, dorsal; B, rostral, and C, left lateral views. See text for list of anatomical abbreviations.

Figure 7. Virtual endocranial cast of Prophaethon shrubsolei. A, expanded view of the dorsal surface of the telencephalon, showing the shape and extent of the eminentia sagittalis. The eminentia sagittalis of this species is similar to living birds, but is not well developed dorsally. B, ventral view of virtual endocast. See text for list of anatomical abbreviations.

Figure 8. Virtual endocranial cast of Phaethon rubricauda showing close morphological similarity to Prophaethon shrubsolei. A, left lateral and B, dorsal aspects. Note the absence of a vallecula and presence of well-marked impressions of the fissura cerebelli. Virtual endocranial cast reconstructed from publicly available data at http://www.digimorph.org/ specimens/Phaethon_rubricauda_melanorhynchos/ using MIMICS 8.13. See text for list of anatomical abbreviations.

Figure 9. Reconstruction of the left osseous labyrinth of Prophaethon shrubsolei in A, rostral; B, lateral; C, caudal, and D, medial views. See text for list of anatomical abbreviations.