Arnheimograptus anacanthus

• Rhabdosome up to 10 mm long and widens rather gradually but rapidly from about .7mm at the proximal end to a maximum of about 1.8mm
• Thecae form an angle of 20 deg- 30 deg to the axis of the rhabdosome, number 14-18 in 10 mm, and lack spines
• Three, short, spinelike projections extend from the sicula at the proximal end

• Another stratigraphic interval yielding abundant graptolites, particularly in Butler and adjacent counties, is the Arnheim formation, from which Arhnheimograptus anacanthus can be collected.
• The rhabdosome of Arhnheimograptus anacanthus is up to 10 mm long and widens rather gradually but rapidly from about .7mm at the proximal end to a maximum of about 1.8mm. In some specimens, the distal part of the rhabdosome is narrower than the middle part. The thecae form an angle of 20 deg- 30 deg to the axis of the rhabdosome, number 14-18 in 10 mm, and lack spines. There are three, short, spinelike projections extending from the sicula at the proximal end, but these are visible in only well-preserved specimens. This species was originally described from excellent three-dimensionally preserved specimens isolated from limestone nodules from a now covered outcrop of the Arnheim Formation.

MItchell & Bergstrom (1977):

• Diagnosis A small aseptate subspecies of Glyptograptus lorrainensis without mesial spine on th1^1. Thecae variably sigmoidal, often approaching orthograptid shape, 7-9 in 5 mm, 14-18 in 10 mm, with introverted apertures. Rhabdosomes rarely exceeding 10 mm in length and 1.2 mm in width from an initial width of about 0.66 mm. Th1^2 development prosoblastic.
• Description of Mature Rhabdosomes Several hundred isolated specimens were studied and measurements were taken on 20 specimens. The largest rhabdosomes observed are slightly more than 10 mm long. The specimens widen from an average of 0.66 mm (range: 0.43-0.84 mm) at the level of the th1^1 aperture to an average maximum width of 1.09 mm (range: 0.75-1.45 mm), which is attained in most specimens at the level of the th5^1 or th6^1. Flattened specimens have a maximum width of 1.80 mm. In some specimens, the rhabdosome widens very gradually and does not attain its maximum width until th10^1. When reached, the maximum width is maintained in most specimens but some specimens show a tendency of becoming narrower for a few thecal pairs distally to the point of maximum width. The rhabdosome is aseptate, and ovid to subquadrangular in cross section when uncompressed, with a moderately to sharply rounded proximal end as th1^1 curves upward. A thin nema may extend beyond the distal end of the rhabdosome for a distance of more than 5 mm. The thecae are slightly inclined toward the observe side of the rhabdosome leaving a wider space between the rows on the reverse side than on the obverse side. In many specimens, the periderm in this area on the reverse side has collapsed, which gives an artificial convex-concave appearance to the rhabdosome. The thecae alternate with seven to nine pairs in the proximal 5 mm and 14 to 18 pairs in the proximal 10 mm. The thecal overlap is about 30% proximally and 40% distally, and the thecal inclination is 20 to 30 degrees to the axis of the rhabdosome, the larger inclinations being in the most distal parts of the rhabdosome. The thecae are semicircular slightly quadrangular in cross section and straight to slightly sigmoidal. They generally lack a well-developed geniculum. The infragenicular wall is restricted just above, and at, the aperture of the preceding theca and is concave in almost all specimens studied, resulting in the formation of a nearly circular aperture. In obverse view, the inclination of the apertural margin ranges from 20 to 90 degrees of the axis of the theca, the average value being about 45 degrees. The apertural margin is rimmed by a selvage, which merges with the fussellar tissue of the succeeding theca and which commonly develops into a small lip. The apertural excavation of th1^1 is rather shallow (0.11-0.18 mm) but is two to three times deeper in the distal thecae. The sicula is short (on the average 1.2 mm with a maximum observed length of 1.35 mm) and is partially exposed on the obverse side of the rhabdosome. The sicular aperture is moderately concave and the ventral apertural region extends somewhat beyond the proximal portion of th1^2. The sicula is provided with a virgella, which is about 0.3 mm long, and two slightly shorter antivirgellar spines.

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