223
– the bony scales of some actinopterygian taxa ( dermal bony scales and scutes ),
– the scales of basal sarcopterygian taxa and most actinopterygian species ( elasmoid scales ),” (Sire and Akimenko 2004 ).
The appendages listed above have a different structure in spite all of them represents
derivatives of a common ancestral type. From the evolutionary point of view, ganoid
and cosmoid scales, the two types of rhombic scales within the osteichthyans, can
be traced back to a primitive scale similar to the scales of Lophosteus (Schultze
1977 ). The primitive rhombic scale did not have a peg-and-socket articulation, it is
composed of lamellar bone superposed by many layers of spongy bone + dentine.
This kind of superposition of layers of spongy bone + dentine (+enamel) has been
retained in the cosmoid scale. In contrast, in the ganoid scale the growth of the
dentine has become restricted to the lateral surface, the growth of ganoin to the
outer surface, and the growth of bone to the inner surface of the scale. Interestingly,
the scales of Andreolepis have a position between the primitive rhombic scale and
the ganoid scale (Schultze 1977 ).
Cosmoid Scales The rhombic scale of basal sarcopterygians is the characteristic
example of so called cosmoid scale. For example, such Devonian (about 410–
415 Ma) fi sh as Meemannia eos, Psarolepis romeri , Achoania jarvikii , and
Styloichthys change are the oldest known examples of cosmine-based scales (see for
review Vickaryous and Sire 2009 ). Also animals like both Actinistia (coelacanths)
and Dipnomorpha (lungfi sh) possess cosmoid-like scales (Meinke 1984 ). On histological level, there are some similarities with the ganoid scale: “a shiny superfi cial
tissue comparable with enamel or enameloid, overlying a stacked sequence of dentine and lamellar bone,” (Vickaryous and Sire 2009 ; see also Goodrich 1907 ;
Meinke 1984 ). However, Vickaryous and Sire ( 2009 ), reported that “cosmoid scales
(and cosmine) are uniquely characterized by an intrinsic, interconnected canal system, with numerous fl ask-shaped cavities and superfi cial pores. The cosmoid scale
(and cosmine tissue) is extinct, and is no longer found in living species,” (Vickaryous
and Sire 2009 ).
The cosmine-free cosmoid-like scales of coelacanth are thinner than true cosmoid
scales. Smith et al. ( 1972 ) described the structure of this scale as follow:
“The greater part of the scale of Latimeria chalumnae is composed of layers of
unmineralized isopedine surmounted by the exposed portion of the scale which is
pigmented and ornamented by a series of denticles of tubular dentine tipped with
enameloid. Between these two parts is a thin ridged bone-like layer. In the electron
micrographs the isopedine was seen to consist of layers of densely packed collagen
fi bres; the orientation of which was uniform in each layer but varied markedly from
layer to layer. Only a few cells were found between the fi bre bundles. The X-rays
revealed numerous concentric annulae and, lying approximately at right angles to
these, a further series of ridges radiating from the centre of the scales. It is suggested
that the basal unmineralized isopedine and the ridged layer of bone-like tissue covering it represents a highly modifi ed cosmoid scale on which the denticles and
pigmented layer have become superimposed,” (Smith et al. 1972 ).
4.3 Fish Scales, Scutes and Denticles: Diversity and Structure
– the bony scales of some actinopterygian taxa ( dermal bony scales and scutes ),
– the scales of basal sarcopterygian taxa and most actinopterygian species ( elasmoid scales ),” (Sire and Akimenko 2004 ).
The appendages listed above have a different structure in spite all of them represents
derivatives of a common ancestral type. From the evolutionary point of view, ganoid
and cosmoid scales, the two types of rhombic scales within the osteichthyans, can
be traced back to a primitive scale similar to the scales of Lophosteus (Schultze
1977 ). The primitive rhombic scale did not have a peg-and-socket articulation, it is
composed of lamellar bone superposed by many layers of spongy bone + dentine.
This kind of superposition of layers of spongy bone + dentine (+enamel) has been
retained in the cosmoid scale. In contrast, in the ganoid scale the growth of the
dentine has become restricted to the lateral surface, the growth of ganoin to the
outer surface, and the growth of bone to the inner surface of the scale. Interestingly,
the scales of Andreolepis have a position between the primitive rhombic scale and
the ganoid scale (Schultze 1977 ).
Cosmoid Scales The rhombic scale of basal sarcopterygians is the characteristic
example of so called cosmoid scale. For example, such Devonian (about 410–
415 Ma) fi sh as Meemannia eos, Psarolepis romeri , Achoania jarvikii , and
Styloichthys change are the oldest known examples of cosmine-based scales (see for
review Vickaryous and Sire 2009 ). Also animals like both Actinistia (coelacanths)
and Dipnomorpha (lungfi sh) possess cosmoid-like scales (Meinke 1984 ). On histological level, there are some similarities with the ganoid scale: “a shiny superfi cial
tissue comparable with enamel or enameloid, overlying a stacked sequence of dentine and lamellar bone,” (Vickaryous and Sire 2009 ; see also Goodrich 1907 ;
Meinke 1984 ). However, Vickaryous and Sire ( 2009 ), reported that “cosmoid scales
(and cosmine) are uniquely characterized by an intrinsic, interconnected canal system, with numerous fl ask-shaped cavities and superfi cial pores. The cosmoid scale
(and cosmine tissue) is extinct, and is no longer found in living species,” (Vickaryous
and Sire 2009 ).
The cosmine-free cosmoid-like scales of coelacanth are thinner than true cosmoid
scales. Smith et al. ( 1972 ) described the structure of this scale as follow:
“The greater part of the scale of Latimeria chalumnae is composed of layers of
unmineralized isopedine surmounted by the exposed portion of the scale which is
pigmented and ornamented by a series of denticles of tubular dentine tipped with
enameloid. Between these two parts is a thin ridged bone-like layer. In the electron
micrographs the isopedine was seen to consist of layers of densely packed collagen
fi bres; the orientation of which was uniform in each layer but varied markedly from
layer to layer. Only a few cells were found between the fi bre bundles. The X-rays
revealed numerous concentric annulae and, lying approximately at right angles to
these, a further series of ridges radiating from the centre of the scales. It is suggested
that the basal unmineralized isopedine and the ridged layer of bone-like tissue covering it represents a highly modifi ed cosmoid scale on which the denticles and
pigmented layer have become superimposed,” (Smith et al. 1972 ).
4.3 Fish Scales, Scutes and Denticles: Diversity and Structure