Ganoid

Ganoid refers to a type of fish scale that is characteristic of certain primitive bony fishes. These scales are known for their distinctive structure and composition, which sets them apart from other types of fish scales such as cycloid and ctenoid scales.

Structure

Ganoid scales are typically rhomboid in shape and are composed of a bony base layer covered with a layer of dentine-like material called ganoin. The ganoin layer is often shiny and enamel-like, giving the scales a hard, glossy appearance. This structure provides the fish with a durable and protective outer layer.

Types of Ganoid Scales

There are two main types of ganoid scales:

• Palaeoniscoid scales: Found in extinct fish of the order Palaeonisciformes, these scales are characterized by their thick bony base and heavy ganoin layer.
• Lepidosteoid scales: Found in modern-day gars (family Lepisosteidae), these scales are thinner and more flexible than palaeoniscoid scales but still retain the characteristic ganoin layer.

Distribution

Ganoid scales are found in several groups of primitive bony fishes, including:

• Gars (family Lepisosteidae)
• Bichirs (family Polypteridae)
• Extinct groups such as Palaeonisciformes

Function

The primary function of ganoid scales is to provide protection. The hard, enamel-like ganoin layer helps to shield the fish from predators and environmental hazards. Additionally, the interlocking nature of the scales provides a flexible yet sturdy armor.

Evolutionary Significance

Ganoid scales are considered to be an ancient feature, indicative of the early evolutionary stages of bony fishes. Their presence in both extinct and extant species provides valuable insights into the evolutionary history of fish.

Related Pages

• Fish scale
• Lepisosteidae
• Polypteridae
• Palaeonisciformes
• Ganoin