Bactritida
Encyclopedia
The Bactritida form a small order of more or less straight-shelled (orthoconic) cephalopod
s 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 ammonoids 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 .
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 Cyrtobactrites, leading to the gyroconic Anetoceras of the Anarcestida (Ammonoidea) (Erben 1964, Klug and Korn 2004).
. Some paleontologists, such as Erben (1964) and Holland (2003) regard Bactroceras
(Eobactrites) as a true bactritid because of its spherical apex
and ventral siphuncle
.
Recent research suggests otherwise, however. The shell of Bactroceras has important differences from those of true bactritids.
For instance, the first chamber of Bactroceras resembles that of other Ordovician
orthocerids, such as Archigeisonoceras and Hedstroemoceras: it is about 10 mm in diameter and is short, forming a spherical cap. True bactritids more strongly resemble late Silurian and Devonian orthocerids, whose first chamber is only about 5 mm across. Moreover, there is a large stratigraphic gap of nearly 50 million years between Bactroceras and the next orthocone
s with a spherical apex and ventral siphuncle.
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 horizontal 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 predators 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 predators hiding among the sea weeds snatching prey found below.
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
s that first appeared during the Emsian Stage of the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
Period (390 million years ago) and persisted until the Carnian
Carnian
The Carnian is the lowermost stage of the Upper Triassic series . It lasted from about 228.7 till 216.5 million years ago . The Carnian is preceded by the Ladinian and is followed by the Norian...
Stage of the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
Period (235 million years ago). They are considered ancestors of the ammonoids as well as of the coleoids
Coleoidea
Subclass Coleoidea, or Dibranchiata, is the grouping of cephalopods containing all the primarily soft-bodied creatures. Unlike its sister group Nautiloidea, whose members have a rigid outer shell for protection, the coleoids have at most an internal bone or shell that is used for buoyancy or support...
(octopus
Octopus
The octopus is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms...
, squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
, cuttlefish
Cuttlefish
Cuttlefish are marine animals of the order Sepiida. They belong to the class Cephalopoda . Despite their name, cuttlefish are not fish but molluscs....
, 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 Cyrtobactrites, leading to the gyroconic Anetoceras of the Anarcestida (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 OrthoceridaOrthocerida
Orthocerida is an order of extinct nautiloid cephalopods also known as the Michelinocerda that lived from the Early Ordovician possibly to the Late Triassic . A fossil found in the Caucasus suggests they may even have survived until the Early Cretaceous...
. Some paleontologists, such as Erben (1964) and Holland (2003) regard Bactroceras
Bactroceras
Bactroceras is a genus of the family Baltoceratidae, orthoceroid cephalapods that lived during the early Middle Ordovician, from about 472—464 mya, existing for approximately 8 million years.-Taxonomy:...
(Eobactrites) as a true bactritid because of its spherical apex
Apex (mollusc)
Apex is an anatomical term for the tip of the mollusc shell of a gastropod, scaphopod, or cephalopod mollusc.-Gastropods:The word "apex" is most often used to mean the tip of the spire of the shell of a gastropod...
and ventral siphuncle
Siphuncle
The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...
.
Recent research suggests otherwise, however. The shell of Bactroceras has important differences from those of true bactritids.
For instance, the first chamber of Bactroceras resembles that of other Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
orthocerids, such as Archigeisonoceras and Hedstroemoceras: it is about 10 mm in diameter and is short, forming a spherical cap. True bactritids more strongly resemble late Silurian and Devonian orthocerids, whose first chamber is only about 5 mm across. Moreover, there is a large stratigraphic gap of nearly 50 million years between Bactroceras and the next orthocone
Orthocone
An orthocone is a usually long straight shell of a nautiloid cephalopod. During the 18th and 19th centuries, all shells of this type were named Orthoceras, but it is now known that many groups of nautiloids developed or retained this type of shell....
s with a spherical apex and ventral siphuncle.
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 MichelinocerasMichelinoceras
Michelinoceras is the oldest known genus of the Michelinocerida, more commonly known as the Orthocerida, characterized by long, slender, nearly cylindrical orthocones with a circular cross section, long camerae, very long body chambers, and a central or near central tubular siphuncle free of...
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 horizontal 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 predators 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 predators hiding among the sea weeds snatching prey found below.