Aporotus dicyrtus du Bus, 1868
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publication ID |
https://doi.org/ 10.5281/zenodo.5376445 |
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persistent identifier |
https://treatment.plazi.org/id/03FED57F-FFC4-9F59-804A-FBDAFE3EFBEC |
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treatment provided by |
Marcus |
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scientific name |
Aporotus dicyrtus du Bus, 1868 |
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Aporotus dicyrtus du Bus, 1868
Aporotus dicyrtus du Bus, 1868: 627 . — Van Beneden & Gervais 1880: pl. 27bis, fig. 6.
HOLOTYPE AND ONLY REFERRED SPECIMEN. — IRSNB 3808- M.541, partial skull, single specimen of Aporotus dicyrtus du Bus, 1868 , and referred to Mioziphius belgicus by Abel (1905).
TYPE HORIZON. — No data available, probably Miocene or lower Pliocene.
TYPE LOCALITY. — Antwerp, Belgium, exact locality uncertain.
EMENDED DIAGNOSIS. — Smaller than Aporotus recurvirostris , with a skull size close to Choneziphius planirostris . It differs from Aporotus recurvirostris in: the shorter and rectilinear rostrum; the lower premaxillae on the rostrum, with the top of the longitudinal crests more anteriorly located and the posterior slope less steep; the reduced prenarial basin; the lower vertex with transverse premaxillary crests more laterally directed.
DESCRIPTION OF THE HOLOTYPE ( FIG. 31 View FIG )
This eroded specimen includes a nearly complete straight rostrum, the premaxillary sac fossae, the vertex, and parts of the supraorbital processes. The skull has a size close to Choneziphius planirostris , slightly smaller than Ziphirostrum marginatum (see Table 6), with a relatively shorter rostrum.
The aspect of the premaxilla on the rostrum is completely different from the rest of the skull, even from the underlying maxillae: the surfaces are smooth and non-eroded, indicating osteosclerotic bone ( Fig. 31A, D View FIG ). This portion of the premaxilla is dorsally and laterally thickened, with a maximum width and height at mid-length of the rostrum, where the bone overhangs the maxilla. The two premaxillae are roughly medially apposed, but without obvious sutural contact; the internal surface of those bones is hollowed by numerous small vascular sulci, as in Aporotus recurvirostris . This dense part of the premaxilla nearly reaches the premaxillary sac fossa, with a short posterior separation between the premaxillae, opening the mesorostral groove for 40 mm before the premaxillary sac fossae. The dorsal surface of the premaxillary sac fossa is flat, smooth, and higher than the adjacent lateral maxilla. The right premaxilla is wider than the left at that level (with a maximum width of respectively 56 and 42 mm). The ascending process of the premaxilla does not become vertical. The poorly thickened transverse premaxillary crest is anterolaterally directed, with a terminal lateral curvature ( Fig. 31C View FIG ).
The triangular nasals are well developed between the premaxillary crests; the rounded anteromedian tip overhangs the bony nares. The median suture of the nasals is slightly anteriorly deflected on the left side. The nasals do not go farther posteriorly than the premaxillary crests and their suture with the frontals is roughly rectilinear.
The width of the strip of frontals on the vertex is more than two times its length and the median suture is not distinct.
At the base of the rostrum, the maxilla occupies a large and poorly elevated portion of the dorsal surface, progressively anteriorly narrowing, disappearing under the premaxilla before half the length of the rostrum. The main feature in the dorsal view of the supraorbital process is the longitudinal crest starting 40 mm anteromedially to the antorbital notch and ending before the large dorsal infraorbital foramen on the supraorbital process. The inner slope of the crest is more pronounced than the outer, and a large foramen is present between the crest and the premaxillary sac fossa, at the level of the antorbital notch.
There is no trace of alveoli for maxillary teeth on the poorly preserved lateral face of the rostrum. The ventral face is not better preserved; only small fragments of the palatine indicate an anteri- or limit of that bone 130 mm anteriorly to the antorbital notches.
SYSTEMATIC DISCUSSION
This partial skull IRSNB 3808-M.541, holotype and only specimen of the species Aporotus dicyrtus , possesses a set of characters that makes it difficult to clearly identify the genus. The attribution by du Bus (1868) to the same genus as A. recurvirostris is justified by the non-fusion of the premaxillae above the mesorostral groove, a character that might be considered as primitive, even if it is also present in the extant Ziphius – for which it was considered as a reversion by Bianucci et al. (1994). Owen (1870) criticized the separation of Aporotus from Ziphirostrum , thinking that the unfused premaxillae constitute a feature not admissible as a generic character. In my opinion, this character might be valid because of functional consequences at the level of the vascularization: the strong vascularization described on the median surface of the premaxillae in the two species of Aporotus is a feature impossible to retain if the two premaxillae are fused, as in Ziphirostrum or Choneziphius . This surface of vascularization might help to develop larger prominences of the premaxillae. The lack of fusion of the premaxillae might therefore be related to the higher premaxillary prominences of Aporotus , which are likely derived.
IRSNB 3808-M.541 also shares with A. recurvirostris a longitudinal maxillary crest on the preorbital process and the valley between this crest and the elevated dense premaxilla on the rostrum. However, that morphology, separating the two species of Aporotus from Ziphirostrum , is also observed in Ziphius ; rostro-facial structure might group Ziphius and Aporotus in a monophyletic taxon, with Ziphius more derived because of the more dorsoanteriorly elongated nasals and the more asymmetrical premaxillary sac fossae.
REVISION OF ZIPHIROSTRUM TUMIDUM DU BUS, 1868 The species Ziphirostrum tumidum was established by du Bus (1868) for the single rostrum IRSNB 3807-M.1889, stressing the spectacular dorsolateral development of the premaxillae. The partial fusion of the premaxillae above the mesorostral groove probably lead du Bus (1868) to include the species in Ziphirostrum . This specimen was later included in the species Mioziphius belgicus by Abel (1905).
Description of the rostral fragment ( Fig. 32 View FIG )
This rostrum, 410 mm long, lacks the base and fragments of the apex. The premaxillae are extremely developed to form a massive elongated dome, hiding the maxillae from the dorsal view. The maximum height of the rostrum, roughly at mid-length, is 92 mm, with a maximum width of 82 mm at the same level. The premaxillae are o n l y p a r t l y f u s e d a l o n g t h e i r d o r s o m e d i a n contact above the reduced mesorostral groove; a clear break surface is only present on the ventral part of the contact surface. The surface of the premaxillae is smooth, hollowed by numerous small vascularization sulci, in a way similar to Aporotus dicyrtus . A main longitudinal sulcus on the lateral surface, progressively dividing from the apex of the rostrum, is reminiscent of Ziphirotrum turniense . The maxillae have a more erod- ed surface, only partially preserved.
Discussion
The general morphology of the premaxillae is similar to A. dicyrtus ; even the position of the top of the dome is roughly at the same level relatively to the anterior margin of the palatine (only 20 mm more anterior in IRSNB 3807-M.1889). The two main differences are the much more important development of the dome and the partial fusion of the premaxillae above the mesorostral groove on IRSNB 3807-M.1889. This fragmentary specimen probably belongs in or is close to A. dicyrtus ; if the former, then individual variation is marked, and it might correspond to an important sexual dimorphism and/or ontogenetic development. If it was the case, the definition of the genus Aporotus , including A. recurvirostris and A. dicyrtus , would be weakened, as IRSNB 3807-M.1889 does not share with those two species the character “absence of fusion of the premaxillae above the mesorostral groove”. IRSNB 3807-M.1889 is provisionally referred to Ziphiidae aff. A. dicyrtus .
| IRSNB |
Institut Royal des Sciences Naturelles de Belgique |
No known copyright restrictions apply. See Agosti, D., Egloff, W., 2009. Taxonomic information exchange and copyright: the Plazi approach. BMC Research Notes 2009, 2:53 for further explanation.
