A revision of “Climacograptus” caudatus (Lapworth) based on isolated three–dimensional material from the Viola Springs Formation of Central Oklahoma, USA  

Daniel Goldman¹ and Shirley J. Wright²

¹ Department of Geology, University of Dayton, 300 College Park, Dayton, OH 45469.   E–mail: dan.goldman@notes.udayton.edu

² Department of Biology, University of Dayton, 300 College Park, Dayton, OH 45469.

Keywords: Graptolites. Climacograptus. Diplacanthograptus. Viola Springs. Ordovician.

The Late Ordovician Viola Group of the Arbuckle Mountains in Oklahoma is well known for containing three–dimensionally preserved specimens of graptolites. One horizon, 5 meters above the base of the Viola Springs Formation at the Mountain Lake section (Alberstadt, 1973, section I) yielded numerous well–preserved specimens of Diplacanthograptus (previously Climacograptus) caudatus (Lapworth). Diplacanthograptus caudatus is an abundant and widespread taxon that has always been recognized by a characteristic long virgella and well developed parasicula. Its distinctive morphology and global distribution have made it an excellent index fossil for early Late Ordovician (post–Climacograptus bicornis Zone) strata. Although it has been recorded from many important graptolite successions around the world, no specimens have ever been described from isolated material, and hence, the details of the proximal end remained unknown.

The new isolated material includes numerous growth stages that allow for a new and surprising revision of the morphology and taxonomy of this species. The most conspicuous feature of D. caudatus, which has always been assumed to be an elongate virgella is actually a theca 11 spine. A very short, dorsally deflected virgella is also present, hidden by the large parasicula. The overall proximal end morphology is extremely similar to D. lanceolatus and D. spiniferus indicating a close phylogenetic relationship with these two taxa. In this paper we provide a revised systematic description and scanning electron microscope photographs of D. caudatus.

Subfamily Climacograptinae Frech, 1897

Genus Diplacanthograptus Mitchell, 1987

Diplacanthograptus caudatus (Lapworth, 1876)

Figure 1, 1–5; Plate 1, Figures 1–7

1876 Climacograptus caudatus Lapworth, Plate 2, Figure 48.

1877 Climacograptus caudatus Lapworth, p. 138, pl. 6, Figure 34.

1906 Climacograptus caudatus Elles and Wood, pp. 202–203, pl. 27, Figure 7a–e, Figures 33a–d.

1908 Climacograptus caudatus Ruedemann, pp. 438–439, pl. 28, Figure 17, Figure 406.

non 1987 Climacograptus caudatus Mitchell, p. 362, Figures 5 a–t.

1989 Ensigraptus caudatus Riva and Ketner, p. 89.

1992 Ensigraptus caudatus Vandenberg and Cooper, p.47, Figure 9a.

1995 Climacograptus caudatus Williams, pp. 48–49, pl. 3, Figures 1–4, Figures 11a–m.

See for full synonymy.

1996 Climacograptus caudatus Finney, p.444, Figure 9h.

Approximately 30 isolated, three–dimensionally preserved specimens, 1 silicified specimen, and numerous flattened specimens from the lower Viola Springs Formation at the Mountain Lake Section (Alberstadt, 1973, Section I).

Diplacanthograptus with a very short, dorsally deflected virgella, a long, straight mesial theca 11 spine, and well developed parasicula.

Diplacanthograptus caudatus has a long tapered rhabdosome that widens gradually from 0.60 – 0.75 mm at the Th11 aperture (mean= 0.64 mm) to 1.10–1.2 mm at Th51, and 1.3–1.5 mm at the tenth thecal pair. The proximal end has a dorsally deflected sicula with a very short virgella that extends across the sicula aperture. The virgella is completely enclosed within a well–developed parasicula in mature specimens (Plate 1, Figure 7). The prosicula is absent, replaced by two rods that merge with the nema.

Theca 11 is sharply upturned, with a very long mesial spine that grows straight down from the point where th 11 bends upward. The parasicula grows down along th11. The theca 12 crossing canal originates as a small flange on th 11 below the th 12 foramen and grows diagonally upward across the sicula with no inner (dorsal) wall. Theca 12 grows vertically upward and is very long. The proximal development is Pattern E of Mitchell (1987).

Distal thecae are climacograptid, with a straight geniculum, and semi–oval apertures. Apertures are short, shallow, and horizontal, surrounded by a mildly thickened rim. 2TRDs average 1.47 mm at th 11, 1.66 mm at th 51, and 1.87 mm at th 101. A median septum was not observed in the Viola Springs material.

Diplacanthograptus caudatus (Lapworth) is a globally distributed taxon, usually recognized by its long virgella and parasicula. Our three–dimensional material reveals that the long spine is actually a theca 11 spine and that the parasicula hides a small, deflected virgella. Williams (1995, p. 49) noted that specimens of D. caudatus from the Lawrence Harbour Formation in Newfoundland had a "small antivirgellar spine or projection on the sicula". Without the benefit of three–dimensionally preserved material it is nearly impossible to recognize that this small spine (clearly visible in Williams, 1995, Plate 3, Figure 2) is actually the virgella and that the long, thick downward growing spine is a th 11 mesial spine.

One flattened specimen from the Viola Springs Formation is silicified and clearly shows the relationship of the virgella to the surrounding parasicula. The parasicula has been preserved as a translucent sheath growing down from the sicular aperture (Plate 1, Figure 7). Underneath the parasicula the small, deflected virgella and the much longer, thicker th 11 spine are visible as dense white rods. With the proximal morphology clarified, the correct relationship between the virgella and the th 11 spine can be observed on many flattened specimens (Figures 1b–d).

The dimensions of the isolated material are nearly identical to uncontroversial flattened specimens from other localities. Several flattened specimens from the Utica Shale of New York State and the Big Fork Chert of Oklahoma are illustrated for comparison (Figure 1 a–d).

The Viola Springs specimens of D. caudatus exhibit a Pattern E proximal development, and are morphologically similar to Diplacanthograptus spiniferus and Climacograptus lanceolatus, differing in dimensions and the relative lengths of the virgella and th 11 spine. Michell (1987, p. 362, Figure 5a–t) illustrated three–dimensionally preserved specimens of a Pattern D climacograptinid as "C. caudatus". These specimens were collected from much younger strata in the Viola Springs Formation (G. pygmaeus Zone) and are likely referable to C. tubuliferus.

Figure 1. Flattened specimens of Diplacanthograptus caudatus. 1, from the Lower Utica Shale, Chuctanunda Creek, New York State, NYSM #15185; 2– 5, from the Big Fork Chert, Black Knob Ridge, Oklahoma. Scale bar is 1 mm.

Plate 1. 1-6, Scanning electron microscope photographs of isolated three dimensional specimens of Diplacanthograptus caudatus, 1 x50, 2-4 x40, 5, 6 x30. 7, Silicified specimen from 6 meters above the base of the Viola springs Formation, x20, v = virgella, p = parasicula.

Diplacanthograptus is here raised to genus level and includes the species D. caudatus, D. spiniferus, and D. lanceolatus. The monotypic genus Ensigraptus erected by Riva and Ketner (1989) is synonymized with Diplacanthograptus.

The specimens of Diplacanthograptus caudatus were collected from a horizon 5 meters above the base of the Viola Springs Formation at the Mountain Lake section (Alberstadt, 1973 section I) in central Oklahoma. D. caudatus specimens occurred with specimens of Corynoides americanus, Cryptograptus insectiformis, Dicranograptus hians, Dicranograptus spinifer, Neurograptus margaritatus, Normalograptus brevis, Rectograptus amplexicaulis, and Orthograptus quadrimucronatus. This assemblage is referable to the Corynoides americanus Zone of eastern North America and the Diplacanthograptus lanceolatus Zone (Ea 1) in the Australasian succession.

I would like to thank Stan Finney for showing me the Viola Springs sections and for a very enjoyable Spring week in the field.

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Received: February 15, 2003

Accepted: June 15, 2003