Who Are Actually Placodermi:
If we talk about the fossil evidence for the primitive Vertebrate we do not have sufficient evidence yet which can give us an exact timeframe when actually Vertebrate arrived on our earth. Now what is the reason behind the missing fossil evidence for the primitive Vertebrate, there are different types of opinions on why the primitive Vertebrate did not leave any fossil evidence. But some workers try to solve this question, some workers believe that the primitive Vertebrate was freshwater form so they do not have the chance to form fossils as that of marine vertebrate.
Some group of workers believe that the primitive Vertebrate before Ostracoderms may be soft-bodied so they don’t leave anycast for us in the sediments. Now whatever the reason, we only have the fossil evidence of Ostracoderm, before then we do not have any fossil evidence for the primitive Vertebrate. So we can assume that Ostracoderms are the primitive Vertebrate.
We placed those fossilised primitive vertebrates underclass Ostracodermi under the group Agnatha along with class Cyclostomata. Ostracoderms are jawless primitive Vertebrates, they are also known as armoured bony fishes. Ostracoderms have a bony endoskeleton, the heavy bony exoskeleton gives them an armoured appearance.
The bony armour give them protection against some arthropods, some arthropods were present in large numbers during their time period and the Arthropods were predators of Placodermi. The heavy armour protects them against the predator arthropods but when the predator arthropods become extinct the bony armour also disappear with time.
Some workers believe that the Ostracoderm in their late time period loosely the heavy bony armour and eventually evolved into exoskeleton less Cyclostomata. Most of the workers believe that the Ostracoderm now extinct was actually evolved into present-day fish.
Most of the workers believe that the Ostracoderm evolved into two groups, Placodermi and Acanthodii. Acanthosis are shark-like fish and their body was covered by diamond-shaped scales, Climatius is an example of this group. They have a large pectoral and pelvic fin, between the pectoral and pelvic fin they have small paired fins which is present in the form of the stout spine, for this reason, they are also known as Spiny Shark.
Timeframe of Placodermi:
Around 450 million years ago when Ostracoderms become extinct then Placodermi appear on our earth. Placodermi is the first jawed Vertebrate who arrived on our earth in the Silurian period, they reach in peak level in Devonian and Carboniferous and leave our earth in Permian, they are now extinct, we only have their fossil evidence at present days.
But we don’t have any evidence of who was present before Placodermi because the Ostracoderm becomes fail to establish the intermediate link between the jawed Vertebrate and jawless Vertebrate. We have some reports which show that some Placodermi is present in their full evolved form even before the presence of Ostracoderm. So how Ostracoderms can be the ancestors of the Placodermi, so we have no intermediate link for now.
So we can say that we are unknown about the actual ancestors of Placodermi, the first jawed Vertebrate. In Placodermi different advanced Vertebrate features arrived which separates them from other primitive Vertebrates, for example, the presence of jaw and teeth, generally on lower jaw Placodermi have actual teeth.
Their external appearance was more or less like present-day fishes, body length was small, members of genus Climatius was around 8 cm in length.
Reference Class Placodermi General Characteristics Classification Phylogeny
Detailed Study On
Classification of Cyclostomata
Characteristics Features of Cyclostomata
Economic Importance of Petromyzon
Nervous System and Excretory System of Petromyzon
Respiratory System and Circulatory System of Petromyzon
Digestive System and Feeding Mechanism of Petromyzon
Anatomy of Petromyzon (Lamprey)
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