Arnheimograptus

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Mitchell (1987):

• Diagnosis. Minute aseptate species with glyptograptid thecae; thecal apertures undulating to cuspate; Th1(*) may or may not possess a mesial spine; primordial astogeny follows Pattern F. The sicula is slender and bears a prominent pair of antivirgellar spines in addition to the usual virgella. The metatheca of th1 1 is tightly recurved and grows upward along its protheca. Thl 2 is prosoblastic and the hood which covers the foramen of th 12 is generally short and largely or entirely enclosed by thl 1 (text-fig. 8o, 1, M). The crossing canal of thl 2 arises as an isolated flange on thl 1 below the foramen (text-fig. 8E, 1), as in Pattern E. It subsequently grows diagonally upward across the sicula. Near the mid-line of the sicula, this upward-growing flange bifurcates to produce both the metatheca of thl 2 and the protheca of th 21 (text-fig. 8H, N). Th22 arises by a pattern of differentiation that is like that of all subsequent thecae (text-fig. 8H). There are three primordial thecae and one crossing canal (thl 2). Pattern F rhabdosomes are aseptate. The proximal end is strongly asymmetrical and generally quite narrow compared to the distal width of the colony. In obverse view the sicula is exposed for a large part of its length, nearly to the level of the aperture of th21 • Post-primordial thecae range from climacograptid with prominent genicular flanges to glyptograptid or nearly orthograptid.

(*- The abbreviation Th-x/y refers to Arnheimograptus morphology, explained in Fig. 6 of Mitchell (1987).

• Discussion. The rhabdosomes of these species exhibit a proximal end which is nearly identical to that of Geniculograptus gen. nov. species. The common ancestor of the Arnheimograptus species probably arose from one of these by the loss of the distinctive genicular flanges, converting the amplexograptid thecae of Geniculograptus gen. nov. to the glyptograptid thecae of Arnheimograptus. Also like Geniculograptus gen nov., this taxon differs from similar looking species of Glyptograptus in being aseptate, by exhibiting a long slender sicula that is extensively exposed on the obverse side of the colony, and in possessing antivirgellar spines on the dorsal margin of the sicula.

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