Placodermi is a very important group of animals for study in the field of evolution of Gnathostomata. The study on the evolution of Placodermi gives us an idea about the evolutionary history of primitive Vertebrates. Placodermi is intermediate between the primitive extinct Ostracoderms and the modern fish, the fossil evidence of Placodermi gives us knowledge about the evolution of modern fish from the Placodermi.
Table of Contents
Biological Significance of Placodermi:
Most Primitive Jawed Vertebrate:
Before Placodermi we see that Ostracoderms was also primitive Vertebrate, we have fossil evidence for the Ostracoderm. The fossil evidence of Ostracoderm and their study reveals some very much important aspects of evolution. But they all are jawless, Placodermi were the first primitive Vertebrate in which we can see jawed mouth, so we can say that the most primitive jawed Vertebrate was Placodermi.
The fossil evidence of Placodermi in the Devonian strata shows us the picture of the primitive animals during Devonian. The appearance of the jaw was a very successful event in the history of evolution because the jaw makes them very powerful and helps them to become dominant.
Intermediate Between Ostracodermi and Modern Fishes:
The bony armor on the body of Placodermi shows that they evolved from the Ostracoderm in which the body is covered by the bony exoskeleton. So we can find the connection between the Ostracoderm and the present-day modern fish.
Development of Paired Fin:
Before Placodermi all the primitive Vertebrate have an only unpaired fin, the unpaired fin are not as effective in navigation as paired fin. The development of paired fin make them swim more swiftly, the paired fin and strong jaw make them very powerful in their habitat and they become dominant soon.
Development of Modern Fish:
The method of evolution of the development of modern fish is not clear yet, it is controversial due to insufficient evidence. In Devonian, Placodermi become dominant due to their powerful jaw and paired fin but they also become extinct very soon, but the reason why such a powerful and successful group of animals become extinct suddenly.
Some workers believe that some fish developed during the timeframe of Placodermi who have sharp teeth. Due to their teeth, the primitive fish become more powerful than Placodermi and become dominant over Placodermi soon. Some workers believe that maybe those fish who had powerful teeth pushed the Placodermi in the path of extinction.
However why Placodermi become extinct so quickly is not clear yet, the arrival of Placodermi and their extinction so quickly are famous to the evolutionary scientists as “unsuccessful ancient experiments“.
How modern fish developed, is yet a matter of debate but some workers put forward Placodermi as ancestors of modern fish. Some workers believe that a group of Placodermi evolved into a fish group known as Acanthodii which are much resemblance to the shark, according to their opinion the Acanthodians were the first primitive fish that later give rise to modern fish.
Some workers believe that Placodermi gives rise to two separate groups of animals which evolved into Osteicthys and Chondricthys with the process of evolution. But some workers do not believe that modern fish evolved from Placodermi. There is some evidence that shows that some primitive fish was present much earlier than the timeframe of Placodermi, so there is a strong doubt that modern fish evolved from the Placodermi.
Some workers previously believed that cartilaginous fish give rise to bony fishes and the bony fishes are more modern than the cartilaginous fish. But it is not true because we have evidence of bony exoskeleton much earlier than cartilaginous fish, so the cartilaginous fish did not evolve into bony fishes.
It may be possible that separate ancestors give rise to bony fishes and cartilaginous fishes separately and there is no connection between modern fish and Placodermi, but until we get strong evidence we can not say anything surely.
Reference Class Placodermi General Characteristics Classification Phylogeny
Detailed Study On
Characteristics Features of Placodermi
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