AMMONITES AND NAUTILOIDS OF WADI HAJAR 



59 



bifurcating near the ventro-lateral border; secondary ribs form for- 

 wardly pointing chevrons on the venter, where there is a marked 

 mid-ventral interruption. 



Description. The holotype is the phragmocone of a macroconch, 

 still septate at its aperture at 1 25 mm diameter; it must have been at 

 least 200 mm diameter when complete with 270° of adult body- 

 chamber. The paratype is a microconch, septate up to 47 mm 

 diameter, and then has 270° of body-chamber up to its aperture at 72 

 mm diameter; it has no adult features. SM F. 12 164 is an immature 

 microconch; it consists of inner whorls, half of which are missing on 

 one side; the smaller end of the outer whorl is septate, the larger end 

 is not septate, but the position of the last septum cannot be seen; in 

 addition a short portion of the final whorl up to an apparently 

 complete plain mouth-border at 46 mm diameter is attached to these 

 inner whorls. CA837 is a short fragment of a whorl, of about 50 mm 

 whorl height, that closely matches the largest part of the holotype. 

 The whorls are involute, flat sided and compressed, and the 

 microconchs have a slightly larger umbilicus than the macroconch 

 (see Measurements). The ribs are straight, radial and approximately 

 half bifurcate near the ventro-lateral edge, though the proportion 

 that bifurcate increases with size. The holotype has 53 ribs per whorl 

 at 120 mm diameter, and 50 at 80 mm; the paratype has 5 1 ribs at 68 

 mm. The number of primary /secondary ribs per half whorl is 26/44 

 at 117 mm diameter in the holotype, and 27/40 at 68 mm in the 

 paratype. The secondary ribs swing forwards to form chevrons on 

 the venter and are interrupted along the mid- ventral line. One or two 

 poorly developed constrictions per whorl occur up to about 35 mm 

 diameter. 



Measurements 



D 

 C.71097 120.0 



C.71097 70.0 



C.71098 66.7 



SMF.12164 31.1 



Wh Wb U 



47.2(0.39) 28.3(0.29) 41.2(0.34) 



25.3(0.36) 17.7(0.25) 24.3(0.35) 



23.9(0.36) 16.3(0.24) 26.8(0.40) 



11.1(0.36) 9.5(0.31) 11.8(0.38) 



Remarks. This new species belongs to the younger, involute, true 

 Idoceras rather than to the older, evolute, often more strongly ribbed 

 Subnebrodites. One of the closest species morphologically isldoceras 

 farquharsoni Spath ( 1 935: 2 1 3, pi. 24, fig. 1 ) from the Kimmeridgian 

 of Somalia, which also retains its ribs up to the largest size seen ( 104 

 mm diameter), but it has much finer (ca. 70 ribs/whorl at 100 mm 

 diameter), prorsiradiate, curved primary ribs, which bifurcate lower 

 on the side of the whorl than in /. ahwarense. Neither species shows 

 the fading of the ribs on the side of the whorls that is characteristic 

 of/, baldenim (Oppel).The numerous species of Idoceras in Mexico 

 described by Burckhardt (1906: 38-66; 1912: 101-127) and Imlay 

 (1939: 37^1) all differ in whorl proportions or ornament, many 

 being more evolute or tending to become smooth on the side of the 

 whorl. Many species of Idoceras are dimorphic, the microconchs 

 being noticeably more evolute, with slight uncoiling of the umbilical 

 seam towards the end of the adult body-chamber, but only a few (and 

 none of the Yemen or east African specimens) are preserved with rib 

 modifications and a deep terminal constriction followed by a blunt 

 lappet at the mouth-border, as in/, mexicanum Burckhardt (1906: pi. 



1 1 . fig. 1 1 ) and better developed in Subnebrodites (Zeigler, 1 959: pi. 

 l.fig. 6). 



The small Wadi Arus microconch is from the Breadloaf Concre- 

 tions and shows that the age of /. ahwarense is Beckeri Zone. The 

 type specimens from Wadi Ahwar are well-preserved and mostly 

 uncrushed, but are from an unrecorded horizon; it can only be 

 surmised that latter is also in the Kilya Member of the Naifa Forma- 

 tion. They were obtained by an amateur collector in October 1962. 



Occurrence. Lower part of the Kilya Member, Naifa Formation, 

 Wadi Arus and Wadi Ahwar; Beckeri Zone, Upper Kimmeridgian. 



Idoceras cf. balderum (Oppel, 1863) 



PI. 7, fig. 4 



1863 Ammonites balderus Oppel: 242, pi. 67, fig. 2. 



1878 Ammonites (Perisphinctes) balderus Oppel; de Loriol; 94, 



pi. 15, fig. 7 (non fig. 8). 

 1929 Idoceras balderum (Oppel); Wegele: 78, pi. 9, fig. 7. 

 1959 Idoceras balderum (Oppel); Ziegler: 25, fig. la; pi. 1, figs 



3.4. 



Material. One specimen, CA735, from the Breadloaf Concre- 

 tions in the Kilya Member in the east cliff of Wadi Arus. 



Description. This fragment is a quarter of a whorl of body- 

 chamber, preserved solid, but mainly on one side only, though the 

 whole of the width of the venter is exposed. The whorl height at the 

 larger end is 56 mm, indicating a diameter of approximately 150 

 mm. The radial ribs have faded over much of the side of the whorl, 

 leaving large undulations at the umbilical edge, and secondary ribs 

 near the ventro-lateral edge, which are projected forwards to form 

 chevrons on the venter, then inteiTupted at mid-venter. 



Remarks. Although it compares closely with the largest part of 

 the example figured by Ziegler (1959: pi. 1, fig. 3), this specimen is 

 only compared with / balderum on account of its fragmentary 

 nature. In southern France and Germany / balderum characterizes a 

 subzone of the Divisum Zone at the top of the Lower Kimmeridgian. 



Occurrence. Kilya Member, Naifa Formation, Wadi Arus; 

 Beckeri Zone. Upper Kimmeridgian. 



Idoceras cf. hararinum Venzo, 1942 Fig. 3; PI. 9, fig. 1 



1942 Idoceras hararinum Venzo: 48, pi. 6, fig. 8; pi. 7, figs 2-4; 



pi. 13, fig. 3. 

 71943 Ringsteadia africana Scott: 64, pi. 10, figs 1, 2. 

 1954 Idoceras hararinum Venzo: Valduga: 2 1 , fig. 7; pi. 5, fig. 3. 

 1959 Idoceras hararinum Venzo; Venzo: 151, pi. 6, fig. 8; pi. 7, 



figs 2^; pi. 12, fig. 3. 



Material. One specimen. CA1034. from the base of the middle 

 limestone part of the Kilya Member, Naifa Cliff. Another specimen 

 of 450-500 mm diameter was .seen at the same horizon and locality, 

 but could not be collected. 



Description. The single ammonite is a very large complete body- 

 chamber, 320° long, 475 mm diameter at the mouth-border, and 

 approximately 270 mm diameter at the end of the phragmocone. It is 



PLATE 8 



Figs 1, 3 Idoceras ahwarense sp. nov. la, lb, paratype, Ghanam al Kuffar (I3''45'N, 46°4rE), Wadi Ahwar. 30 km north of Ahwar and .^6 km north of 



the mouth of Wadi Ahwar on the coast of the Gulf of Aden, C.7I098. 3a, 3b, Breadloaf concretions. Kilya Member (fauna 7). east cliff. Wadi Arus. SM 



F12164. 

 Fig. 2 Pachysphinctes major Spath, upper part of middle limestone part of Kilya Member (fauna 7). Wadi Kilya. CAi 170. xO.36. 

 Fig. 4 Nebrodites hospes (Neumayr). Breadloaf Concretions, Kilya Member (fauna 7). east cliff. Wadi Arus. 4a-c, CA736. 

 Fig. 5 Pachysphinctes mahokondobeyrichi (Dietrich), base of middle limestone part of Kilya Member (fauna 7), Naifa Cliff. SM F 12192. 



