Basslerocerida

Content sourced from Wikipedia, licensed under CC BY-SA 3.0.

Basslerocerida is an order of nautiloid cephalopods that lived during the Ordovician period. It was introduced by Flower and Kummel in 1950 to describe forms that seem to be transitional between the earlier Ellesmerocerida and the later Tarphycerida and Oncocerida. Today, the group is not widely used in modern classifications.

Taxonomy
- The order was originally defined to include two families: Bassleroceratidae, which have thick-walled siphuncles and are thought to give rise to the Tarphycerida; and Graciloceratidae, derived from Bassleroceratidae, with thin-walled siphuncles and giving rise to the Oncocerida.
- Flower and colleagues even suggested that Basslerocerida might connect to Barrandeocerina and could extend into the Devonian in some classifications. However, the idea that barrandeoceroids derived from Tarphycerida is now well established.
- Over time, Basslerocerida fell from common use. Many taxa once placed in it are now assigned to Ellesmerocerida, Tarphycerida, or Oncocerida. Some workers (e.g., Shevyrev 2006) continued to recognize the order, while others (Furnish & Glenister 1964; Sweet 1964; Flower 1976) moved the families into other groups or placed Basslerocerida within Tarphycerida.

Morphology
- Soft-part anatomy is unknown, but these animals were probably small with a somewhat squid-like body and perhaps 8–10 arms.
- Shells are small, about 12–15 cm long, and elongate with an upward, exogastric curvature (a rocker-like shape). The cross-section is near circular to slightly compressed. The ventral side on the outer curve is more sharply rounded, giving a keel-like appearance, while the dorsum on the inner curve is less pronounced. Septa are closely spaced and the siphuncle is ventral.
- The siphuncle in Bassleroceratidae has thick connecting rings (like the ancestral Ellesmerocerida and primitive Tarphycerida), whereas in Graciloceratidae the rings are thin (as seen in Oncocerida).

Temporal range
- Ordovician.

In short, Basslerocerida was once seen as a bridge group among early nautiloid cephalopods, but it is now largely considered obsolete as a formal order, with its members usually reassigned to other cephalopod groups.