Treptoceras duseri

• Siphuncle segments one-sixth to one-fourth the diameter of the shell
• Medium sized, moderately expanding orthoconic longicone that is circular in cross-section with straight transverse sutures
• Camerae short, lengths initially one-fourth the diameter of the shell
• Aperture straight, transverse, lacking sinus
• Shell exterior essentially smooth, marked by fine transverse growth lines and fine longitudinal lirae

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Davis (1998):

• (Under Orthonybyoceras duseri) Cephalopod. One of numerous cephalopods of the Richmondian. (See plate 1 fig 25). Especially abundant in Waynesville

Fossils of Ohio (1996) :

• In fossil specimens in which the shell wall is preserved, some individuals show darker markings on the lighter background of the shell exterior. For example, in specimens of the Ordovician species Treptoceras duseri, there may be longitudinal dark lines. These lines are the remains of color bands on the living animal. It is not known what the actual colors were, but the pattern of those colors is clear. Such color patterns commonly are interpreted as having served as camouflage, although they may have had a function in mating.
• Conch longiconic
• Specimens of Treptoceras are very abundant in certain rock layers. For example, Frey (1989) referred to the “Treptoceras duseri shale” layer within the Waynesville Formation of southwestern Ohio. This layer includes three species of the genus, one of which is a new species, as yet unnamed.

Frey (1995):

• Diagnosis – Large, moderately expanding species of Treptoceras having siphuncle segments one-sixth to one-fourth the diameter of the shell that change shape with ontogeny from equiaxial to compressed and position from central to subcentral; becoming equiaxial or depressed and midventral in the adoralmost segments of large specimens.
• Description– Medium sized (up to 46 cm in length and 5.5cm in diameter), moderately expanding (apical angle 7.5-8.5 degrees) orthoconic longicone, circular in cross-section with straight transverse sutures. Camerae short, lengths initially one-fourth the diameter of the shell, decreasing with ontogeny to one sixth or one-seventh the diameter of the shell in adoral portions 20-35 mm in diameter. Body chamber at least one-fourth the total length of the shell; aperture straight, transverse, lacking sinus. Periphract dorsomyarian. Shell exterior essentially smooth, marked by fine transverse growth lines and fine longitudinal lirae.
• Siphuncle segments central in position (SPR =.50) at a diameter of 15mm, migrating with ontogeny to a subcentral position (SPR= .29-.33) at a diameter of 27mm, and midventral in position (SPR=.25) in adoral portions 40mm in diameter. Segments initially equiaxial at diameters less than 15mm, becoming compressed (SCR=1.06-1.42) between diameters of 15 and 25mm, equiaxial between diameters 25 and 30mm, then depressed for the remaining length of the shell (SCR<1.0). Segment diameter from one-fourth to one-sixth the diameter of the shell, decreasing with ontogeny. Septal necks cyrtochoanitic; dorsal brims recumbent, ventral brims recurved but free. Dorsal brims suborthochoanitic at shell diameters greater than 35mm. Endosiphuncular deposits consisting of dorsal annuli and ventral parietal deposits, restricted to adapical half of large shells. With ontogeny, ventral paretial deposits fuse to form a continuous ventral endosiphuncular lining adapically. Adormost cameral deposits mural-episeptal, the deposits completely filling camerae in adapical half of large shells.
• Remarks – Treptoceras duseri is the types species for the genus Treptoceras Flower. The species is known primarily from the T.duseri shale unit within the “Waynesville biofacies” of the Bull Fork Formation (Upper Ord., lower Richmondian) in Warren and Clinton counties in Sw Ohio. In this claystone unit the species is represented by abundant, largely complete, well-preserved, calcite-replaced internal molds. The distinctive preservation of nautiloids from the T. duseri shale unit indicates that this claystone is also the source of the holotype.
• Treptoceras duseri is most similar to the associated species, T.fosteri (Miller). It differs from T.fosteri in its more rapidly expanding shell (apical angle 7.5-8.5 degrees compared with apical angle 5.5-6 degrees in T.fosteri), the more central position of the siphuncle segment in Treptoceras duseri at comparable diameters , and the shape of the siphuncle segments , the compressed segments in Treptoceras duseri lacking a well developed zone of adnation compared with a well developed dorsal zone of adnation in T.fosteri. The ranges of these two species overlap stratigraphically and geographically.

McFarlan (1931):

• A rather rapidly expanding shell, circular in section, typically about 20 cm in length, expanding from 5 to 35 mm. Septa moderately concave, closely spaced, less so toward the living chamber where 10 chambers in the above specimen occur in 40mm. Siphuncle eccentric somewhat nearer center than margin, very small where passing through the septa, expanding to about 4 times that diameter within the chamber. Surface smooth except for the raised edges of the septa which form narrow rings and the presence opposite the siphuncle of a narrow raised longitudinal line extending the length of the shell, slightly interrupted just above each of the annular rings.
• An important horizon marker at the top of the Fort Ancient member of the Waynesville, known as the O.”fosteri” horizon. It is listed by Cimings from the McMillan and lower Arnheim of Indiana.

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