1. Structure and Composition:
- Paleozoic placoid scales:
- Composed primarily of dentine, a hard calcified tissue.
- Have a central pulp cavity surrounded by dentin.
- Possess a basal plate anchoring them to the dermis.
- Lack enamel, the hard outermost layer found in modern shark teeth and some scales.
- Paleoniscoid modern shark scales:
- Consist of a combination of enameloid (a modified enamel-like substance) and dentin.
- Have a layered structure with an outer layer of enameloid and an inner layer of dentin.
- Basal plate may be reduced or absent.
2. Ornamentation:
- Paleozoic placoid scales:
- Often ornamented with ridges, tubercles, or cusps, providing a rough texture.
- Ornamental patterns vary among different shark species.
- Paleoniscoid modern shark scales:
- Typically smooth or have fine striations.
- Lack the pronounced ornamentation seen in Paleozoic placoid scales.
3. Function:
- Paleozoic placoid scales:
- Primarily served a protective function, providing a tough armor-like covering.
- May also have played a hydrodynamic role, influencing swimming efficiency.
- Paleoniscoid modern shark scales:
- Offer protection but also contribute to streamlining and reducing water resistance.
- Involved in buoyancy regulation by controlling the specific gravity of the shark.
4. Evolutionary Context:
- Paleozoic placoid scales:
- Represent the earliest known type of placoid scales in sharks, appearing during the Paleozoic era (approximately 541-252 million years ago).
- Found in extinct shark groups like acanthodians and xenacanthids.
- Paleoniscoid modern shark scales:
- Evolved later, during the Mesozoic era (252-66 million years ago).
- Found in modern shark species belonging to the subclass Elasmobranchii, which includes groups such as dogfish, rays, skates, and modern sharks.
In summary, Paleozoic placoid scales in sharks differ from paleoniscoid modern shark scales in terms of their composition, ornamentation, function, and evolutionary timeline. The structural variations between these scales reflect the diverse adaptations and modifications that have occurred throughout shark evolution.