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Plannapus hornibrooki O'Connor, 1999

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Plannapus hornibrooki
Description: Cephalis internally spheroidal, externally hemispheroidal to inflated truncate-conical because lower part enveloped by upper thorax; surface dimpled or with low, irregular, discontinuous ridges; bearing anteriorly offset, tiny, three-bladed apical horn; pores sparse, circular to subcircular, irregular in size and distribution: vertical tube at posterior base of cephalis, generally circular in transverse section, often externally expressed as very low truncate-conical protrusion (pl. 1, fig. 7a, b, 8a, b); collar stricture generally externally visible as slight change in shell contour.
Thorax truncate-ovoid; approximately same width distal ly as proximally, greatest width generally at mid-thorax; generally circular in transverse section but some specimens may be slightly laterally compressed; uppermost part envelopes lower cephalis; surface slightly rough due to dimples or low, discontinuous, irregular ridges; pores circular to subcircular, small, widely spaced, generally roughly quincuncially arranged, flush with thoracic surface; three internally distinct, externally indistinct ribs corresponding to D, LI and Lr may extend outside as tiny wings on mid to upper part of thorax; generally no marked external contour change between thorax and peristome; peristome short, often smooth (or less rough than thorax), internally cylindrical, externally inverted truncate-conical; aperture constricted; termination smooth.
Internal skeletal elements consist of bars A, D, V, M, LI, Lr, ll, lr, Vbl, Vbr, arches A-Vbl, A-Vbr. Ll-Vbl, Lr-Vbr, Vbl-Vbr, ll-Ll, lr-Lr, D-ll, D-lr and spines Ax, Vs (text-fig. 2; pl. 1, fig. 10); A extends freely upwards from M to wall of cephalis and extends outside as apical horn; V extends obliquely upwards from M to bars Vbl and Vbr; short Vs extends into vertical tube from intersection of V, Vbl and Vbr; Ax reduced to a node; D extends obliquely downwards from M to join thoracic wall, becomes rib, may extend outside as small wing; Ll and Lr extend laterally to arches then continue obliquely downwards to join thoracic wall, become ribs, may extend outside as small wings; ll and lr extend laterally to join cephalic wall; arches A-Vbl and A-Vbr fused to cephalic wall, curve downwards from intersection of A with cephalis to join bars Vbl and Vbr at cephalic wall; bars Vbl and Vbr and arches Ll-Vbl, Lr-Vbr, ll-Ll, lr-Lr, D-ll, D-lr form ring at base of cephalis; arch Vbl-Vbr forms lower base of vertical tube.
Dimensions: Range of 43 specimens (Holotype measurement given in parentheses): length of apical horn: 4-12pm (8pm); length of cephalis: 15-20pm (18pm); maximum width of cephalis: 26-37pm (35pm); length of thorax: 70-87pm (82pm); maximum width of thorax: 65-85pm (76pm); maximum number of pores on half equator of thorax: 8-11 (10); maximum number of pores in longitudinal pore row on thorax: 5-7 (7).
Etymology: In honour of Dr N. de B. Hornibrook for his vast contribution to micropaleontology and biostratigraphy, both in New Zealand and abroad, and in particular for his work on the Oamaru Diatomite.
Holotype and Type Locality: R401, Flume Gully (J41/f8003), Oamaru.
Discussion: Plannapus hornibrooki is placed in Plannapus because it is a two-segmented artostrobiid (see O'Connor 1997a). It differs from other members of the genus in the following ways (see Genus Plannapus for synonymies): from P. australis (Haeckel) by having a vertical tube that is larger in transverse section and not as prominent, pores that are flush with the surface of the thorax, smaller and more thoracic pores, and a shorter and less constricted peristome; from P. mediterraneus (Haeckel) by being more inflated and by having smaller, more and wider-spaced pores, no marked contour change between thorax and peristome, and dimples or ridges on the surface; from P. microcephalus (Haeckel) by having a smaller apical horn, larger thoracic pores, a less prominent vertical tube that is larger in transverse section, a dimpled or ridged thoracic surface, and by not being markedly laterally compressed in the sagittal plane; from P. papillosus (Ehrenberg) by having thoracic pores which are flush with the surface and more numerous and smaller, dimples or ridges on the thorax, markedly smaller wings, a shorter peristome with a smooth termination, and by not having a marked contour change between thorax and peristome; from Plannapus sp. (Nishimura) by being more inflated and by having dimples or discontinuous irregular ridges on the thorax, thoracic pores which are flush with the thoracic surface and more numerous, a less prominent vertical tube that is larger in transverse section, and a shorter peristome; from Plannapus sp. (Petrushevskaya) by being more inflated, and by having more and smaller pores, pores which are flush with the thoracic surface, dimples or ridges on the thorax, and a shorter peristome with a smooth termination. Petrushevskaya (1969. fig. 8.9) illustrated as Dictyocephalus mediterraneus ? a specimen that appears very similar to P. hornihrooki but has a longer peristome and is less inflated. It is also much younger, being dated in the figure caption as Recent.
O'Connor 1999


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