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- Nov 19, 2002
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Evolution of chromatophores? Well, I’ve had a bit of a think about this and I think that these were a development of the earliest coleoids. I’ll tell why I think this is, but please feel free to shoot me down in flames if you wish! Fossil coleoids, excluding belemnoids, are very rare due, of course, to their soft bodied nature. There do not appear to be many researchers working on them at present and doubtless with one or two future discoveries the whole working system will have to be reworked. Anyway…….
I think the key to this lies with Vampyroteuthis. The Vampire squid is the only surviving member of the Vampyromorpha that we know of. The Vampyromorpha were an incredibly ancient lineage, recent cladistic analyses such as the one available here indicates that the Vampyromorphs were at the root of one of the two main branches of the coleoids, the other being the belemnoid families which split from a common ancestor probably in the early Carboniferous sometime around 350mya. Most of the fossil ‘squid’ that one sees depicted from deposits such as the later Jurassic Solnhofen were members of the Vampyromorphs.
It is believed that these Vampyromorphs and the lineage that led to the modern squid, and cuttlefish, split from a branch soon after this initial divide from the ancestral coleoid, with the modern Spirula representing a comparatively unmodified ancestral form. It is believed that this split into the modern squid and cuttlefish probably happened in the early Tertiary, following the massive Cretaceous marine extinctions. The octopus lineage is thought to have split sometime in the Jurassic from the Vampyromorpha, the earliest known example being Proteroctopus ribeti (160mya)
What has this to do with chromatophores? Well, according to cladistic analyses Vampyroteuthis, the modern squid, cuttlefish and octopus all appear to have had a common ancestor that lived in the early Carboniferous and all these groups have modern examples that demonstrate chromatophores. Unless it is conceivable that chromatophores evolved independently in all these groups, which seems unlikely, it seems at least a strong possibility that the common ancestor of all these groups would have possessed chromatophores at a date of around 350mya. This ancestor had only comparatively recently split from the initial branch in the coleoids in the late Devonian. Perhaps one could speculate that possibly the belemnoids would have possessed chromatophores too, if the common ancestor of both groups had possessed them. (Practically impossible to prove though!).
Vampyroteuthis does have chromatophores though they have weak musculature. To quote from the Tree of Life pages:
“These chromatophores, however, have lost the muscles that enable rapid color change in other coleoids and are probably incapable of changing shape. A few normal chromatophores associated with photophores are still present.”
This is probably an adaption to life in the abyss; who needs a colourful display in the dark? Early Vampyromorphs were certainly not all deep water animals. The recent discovery of an Upper Cretaceous animal from Japan that has been named Provampyroteuthis giganteus is believed to have swum in the surface waters and was much bigger than the modern Vampyroteuthis. Living in an off–estuary environment it is quite possible that this animal had developed chromatophores to a higher degree than its modern descendant. Fossils of this animal have been recovered from stomach contents of Elasmosaurid plesiosaurs; these were believed to be surface or shallow water swimmers. (I can print the reference for this if anyone wants to follow it up).
I’m sure that I have managed to make something comparatively simple much too complicated. This stuff is so much easier with a diagram and timeline, you know!
I think the key to this lies with Vampyroteuthis. The Vampire squid is the only surviving member of the Vampyromorpha that we know of. The Vampyromorpha were an incredibly ancient lineage, recent cladistic analyses such as the one available here indicates that the Vampyromorphs were at the root of one of the two main branches of the coleoids, the other being the belemnoid families which split from a common ancestor probably in the early Carboniferous sometime around 350mya. Most of the fossil ‘squid’ that one sees depicted from deposits such as the later Jurassic Solnhofen were members of the Vampyromorphs.
It is believed that these Vampyromorphs and the lineage that led to the modern squid, and cuttlefish, split from a branch soon after this initial divide from the ancestral coleoid, with the modern Spirula representing a comparatively unmodified ancestral form. It is believed that this split into the modern squid and cuttlefish probably happened in the early Tertiary, following the massive Cretaceous marine extinctions. The octopus lineage is thought to have split sometime in the Jurassic from the Vampyromorpha, the earliest known example being Proteroctopus ribeti (160mya)
What has this to do with chromatophores? Well, according to cladistic analyses Vampyroteuthis, the modern squid, cuttlefish and octopus all appear to have had a common ancestor that lived in the early Carboniferous and all these groups have modern examples that demonstrate chromatophores. Unless it is conceivable that chromatophores evolved independently in all these groups, which seems unlikely, it seems at least a strong possibility that the common ancestor of all these groups would have possessed chromatophores at a date of around 350mya. This ancestor had only comparatively recently split from the initial branch in the coleoids in the late Devonian. Perhaps one could speculate that possibly the belemnoids would have possessed chromatophores too, if the common ancestor of both groups had possessed them. (Practically impossible to prove though!).
Vampyroteuthis does have chromatophores though they have weak musculature. To quote from the Tree of Life pages:
“These chromatophores, however, have lost the muscles that enable rapid color change in other coleoids and are probably incapable of changing shape. A few normal chromatophores associated with photophores are still present.”
This is probably an adaption to life in the abyss; who needs a colourful display in the dark? Early Vampyromorphs were certainly not all deep water animals. The recent discovery of an Upper Cretaceous animal from Japan that has been named Provampyroteuthis giganteus is believed to have swum in the surface waters and was much bigger than the modern Vampyroteuthis. Living in an off–estuary environment it is quite possible that this animal had developed chromatophores to a higher degree than its modern descendant. Fossils of this animal have been recovered from stomach contents of Elasmosaurid plesiosaurs; these were believed to be surface or shallow water swimmers. (I can print the reference for this if anyone wants to follow it up).
I’m sure that I have managed to make something comparatively simple much too complicated. This stuff is so much easier with a diagram and timeline, you know!