Bactritida

Taxobox
name = Bactritida
regnum = Animalia
phylum = Mollusca
classis = Cephalopoda
subclassis = Bactritoidea
ordo = Bactritida
ordo_authority = Shimanskiy 1951

The Bactritida form a small order of more or less straight-shelled (orthoconic) cephalopods that first appeared during the Emsian Stage of the Devonian Period (390 million years ago) and persisted until the Carnian Stage of the Triassic Period (235 million years ago). They are considered ancestors of the ammonites as well as of the coleoids (octopus, squid, cuttlefish, and the extinct belemnites).

Bactritids are distinguished from the more primitive nautiloids by the small size and globular shape of the protoconch, the so-called embryonic shell. Nautiloids have relatively large embryonic shells, and living species lay a few large eggs. In contrast, bactritids and ammonoids, produced large numbers of small eggs, each housing a small embryonic shell .

Classification and description

Bactritida (Erben 1964) are characterized by orthoconic to cyrtochonic shells that may be long or short with a narrow siphuncle invariably in contact with the ventral wall and sutures uniformly with V-shaped ventral lobes. Septal necks are orthochoanitic to cyrtochoanitic, the apical angle may be small or large, and the protoconch is globular to egg-shaped. The Bactritida comprise two families, the Bactritidae and the Parabactritidae.

Bactritidae

The Bactritidae (Erben 1964) are characterized by long orthoconic to cyrtochonic shells with a small apical angle (less than 10 deg.) and septal necks that are orthochoanitic. Chamber length is variable. The Bactritidae contain 8 recognized genera. "Bactrites" has the longest range, from the Lower Devonian to the Upper Permian, and even possibly from the Silurian. The Bactritidae gave rise to the Ammonoidea in the Early Devonian starting with an early Bactrites and going with increasingly tight curvature from "Lobobactrites" to "Cyrobactrites", leading to the gyroconic Anetoceras of the Anarcistida (Ammonoidea) (Erben 1964, Klug and Korn 2004).

Parabactritidae

The Parabactritidae (Erben 1964) are characterized by orthoconic and breviconic shells with a large apical angle (greater than about 10 deg) and septal necks that are vary from orthochoanitic or suborthochoanitic to cyrtochoanitic. The Parabactritidae contain some five described genera and are thought to have given rise to the Belemnoidea (Coleoidea).

Derivation

The bactritida have their origin in the early Orthocerida, in the Ordovician "Eobactrites" considered by Erben (1964) and Holland (2003) to be a true bactritid. "Eobactrities" is found in central Europe and Scandanavia differs from Bactrities in being more ciircular in cross sections and having deeper V-shaped ventral lobes.

Paleoecology and lifestyle

Bactritids, especially the Bactritidae, seem to have lived in a vertical orientation, head down, as probably did many of the Orthocerida such as "Michelinoceras" and "Buttsoceras". Or these forms without extensive internal ballast may simply have been more flexible in their vertical orientation than the well ballasted bottom dwellers like the actinocerids and endocerids. The ventral siphuncle of the Bactritida would have facilitated chamber dewatering in either horizonatal or vertical orientation or in between. A central siphuncle would have made horizontal or near horizontal dewatering more difficult on the other hand. (Holland 2003, Klug and Korn 2004) Bactritids, like cephalopods in general, were probably active preditors with tentacles of some sort. As with other externally shelled forms they were probably not active swimmers in the sense of fish or squid. Rather, it is likely they were stalkers and ambush preditors hiding among the sea weeds snatching prey found below.

References

* Erben H.K. 1964; Bactritoidea , in the Treatise on Invertebrate Paleontology Vol K Nautiloidea, K491 – , Teichert, C and Moore R.C. Eds, GSA and Univ Kansas Press.
* Holland C.H. 2003. Some Observations on Bactritid Cephalopods; Bulletin of Geosciences, Vol 78, no4; Czech Geological Survey.
* Klug, C. and Korn, D. 2004. The origin of ammonoid locomotion. Acta Palaeontologica Polonica 49 (2): 235–242