The Bactritida are a small order of more or less straight-shelled (orthoconic)
cephalopods that first appeared during the
Emsian
The Emsian is one of three faunal stages in the Early Devonian Epoch. It lasted from 407.6 ± 2.6 million years ago to 393.3 ± 1.2 million years ago. It was preceded by the Pragian Stage and followed by the Eifelian Stage. It is named after the ...
stage of the
Devonian period (407 million years ago) with questionable origins in
Pragian stage before 409 million years ago, and persisted until
Carnian pluvial event in the upper middle
Carnian
The Carnian (less commonly, Karnian) is the lowermost stage (stratigraphy), stage of the Upper Triassic series (stratigraphy), Series (or earliest age (geology), age of the Late Triassic Epoch (reference date), Epoch). It lasted from 237 to 227 m ...
stage of the
Triassic
The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...
period (231 million years ago). They are considered ancestors of the
ammonoids, as well as of the
coleoids (
octopus,
squid,
cuttlefish, and the extinct
belemnite
Belemnitida (or the belemnite) is an extinct order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous. Unlike squid, belemnites had an internal skeleton that made up the cone. The parts are, from the arms-most ...
s).
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 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. 1964)] are characterized by long orthoconic to cyrtochonic shells with a small apical angle (less than 10°) and septal necks that are orthochoanitic. Chamber length is variable. The Bactritidae contain eight 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).
[Klug, C. and Korn, D. 2004. The origin of ammonoid locomotion. Acta Palaeontologica Polonica 49 (2): 235–242
]
Parabactritidae
The Parabactritidae
are characterized by orthoconic and breviconic shells with a large apical angle (greater than about 10°) 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
Orthocerida, nautiloid cephalopods that first appeared in the
Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya.
T ...
.
''
Bactroceras
''Bactroceras'' is a genus of orthoceratoid cephalopods that lived during the early Middle Ordovician, from about 472—464 mya, existing for approximately 8 million years.Nautiloidea – Ellesmerocerida by W.M Furnish and Brian F. Glensister; ...
'' (''Eobactrites''), an early middle-Ordovician cephalopod, is considered as a true bactritid by some paleontologists,
because of its spherical
apex
The apex is the highest point of something. The word may also refer to:
Arts and media Fictional entities
* Apex (comics), a teenaged super villainess in the Marvel Universe
* Ape-X, a super-intelligent ape in the Squadron Supreme universe
*Apex, ...
and
ventral
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
siphuncle.
This view is challenged by more recent research. The shell of ''Bactroceras'' has important differences from those of true bactritids. For instance, the first chamber of ''Bactroceras'' resembles that of other
Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya.
T ...
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, a large stratigraphic gap of nearly 50 million years occurs between ''Bactroceras'' and the next
orthocones 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 ''
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 such as 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, though.
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, they were likely stalkers and ambush predators hiding among the seaweeds, snatching prey found below.
References
{{Taxonbar, from=Q26704395
Prehistoric nautiloids
Prehistoric cephalopod orders
Paleozoic cephalopods
Paleozoic cephalopods of Europe
Transitional fossils
Carboniferous cephalopods
Devonian cephalopods
Permian cephalopods
Triassic cephalopods
Early Devonian first appearances
Carnian extinctions
Paraphyletic groups