17
Subclass Holocephali
Of all groups of living fi shes, the Holocephali are the least well known (de Beer and
Moy–Thomas 1935 ). Holocephali (Holocephalomorphi) (“ complete heads ”) is a
subclass of a mostly extinct species of cartilaginous fi sh with big heads and a long
tails, which relationships to other taxa are considered controversial (see for review
Kriwet and Gadzdicki 2003 ). These animals possess only one single gill opening
in each side. Deep-sea species have large eyes. Extant species habituate mostly
deep–water environments. They have continuously growing tooth plates in the
upper and lower jaws. Typical fossilized remnants of holocephalian chimeroids are
exceptionally well preserved tooth plates, fi n spines, and egg cases. The oldest
record of chimeroid fi sh was found in Early Jurassic deposits of Europe (Ward and
Duffi n 1989 ).
Order Chimaeriformes
Chimaeras split off from the rest of the groups earliest and therefore are known as
the most primitive representatives of the cartilaginous fi sh (Patterson 1965 ). They
have no diffi culties to crunch very hard food including mollusc’s shells. For this,
they use permanent bony plates like nutcrackers Echinochimeroidei, Squalorajoidei,
Myriacanthoidei, and Chimeroidei are four suborders of Chimaeriformes. Following
extant taxa – Callorhinchus , Chimaera , Hydrolagus , Rhinochimera , Hariotta ,
Neohariotta (Stahl 1999 ), − are known. These taxa are arranged in three families,
the Callorhynchidae ( Callorhinchus ), Rhinochimaeridae ( Rhinochimaera,
Neoharriotta, Hariotta ), and Chimaeridae ( Chimaera, Hydrolagus ).
The dorsal spine of some recent Chimaera species is positioned anterior to the
fi rst dorsal fi n. The spine is believed to function as a defensive device, particularly
in juveniles and sub-adults when it is capable of infl icting a painful and venomous
wound (Evans 1923 ; Patterson 1965 ). Interestingly, the dorsal spine is also
considered to reduce turbulence and aid the hydrodynamics of the fi rst dorsal fi n. In
addition, it is equipped with a smooth anterior keel, which runs longitudinally along
the centre of the anterior margin of the spine, and is thought to reduce spine erosion
(Maisey 1979 ).
The structure of the holocephalian spines is of interest for materials scientist.
The dorsal spine of Chimaera monstrosa is composed of an outer and an inner layer
of dentine, which collectively form the trunk dentine (Maisey 1979 ). A distinct boundary termed the trunk primordium is present between the two concentric dentine
layers. This consists of collagen fi bres that run longitudinally through the spine.
A single layer of odontoblasts, located on the external side of the trunk primordium,
centrifugally deposits the outer dentine. A similar layer occurs between the inner
dentine and the spine lumen, and centripetally deposits the inner dentine. The
odontoblasts produce an intracellular matrix into which they secrete amorphous
cement materials through their dendritic processes. These processes leave anastomosing dendritic odontoblast canaliculi in their wake as the dentine increases in density
(Calis et al. 2005 ). Moreover, growth increments, apparent as rings generally within
1.2 Part I: Biomaterials of Vertebrate Origin. An Overview
Subclass Holocephali
Of all groups of living fi shes, the Holocephali are the least well known (de Beer and
Moy–Thomas 1935 ). Holocephali (Holocephalomorphi) (“ complete heads ”) is a
subclass of a mostly extinct species of cartilaginous fi sh with big heads and a long
tails, which relationships to other taxa are considered controversial (see for review
Kriwet and Gadzdicki 2003 ). These animals possess only one single gill opening
in each side. Deep-sea species have large eyes. Extant species habituate mostly
deep–water environments. They have continuously growing tooth plates in the
upper and lower jaws. Typical fossilized remnants of holocephalian chimeroids are
exceptionally well preserved tooth plates, fi n spines, and egg cases. The oldest
record of chimeroid fi sh was found in Early Jurassic deposits of Europe (Ward and
Duffi n 1989 ).
Order Chimaeriformes
Chimaeras split off from the rest of the groups earliest and therefore are known as
the most primitive representatives of the cartilaginous fi sh (Patterson 1965 ). They
have no diffi culties to crunch very hard food including mollusc’s shells. For this,
they use permanent bony plates like nutcrackers Echinochimeroidei, Squalorajoidei,
Myriacanthoidei, and Chimeroidei are four suborders of Chimaeriformes. Following
extant taxa – Callorhinchus , Chimaera , Hydrolagus , Rhinochimera , Hariotta ,
Neohariotta (Stahl 1999 ), − are known. These taxa are arranged in three families,
the Callorhynchidae ( Callorhinchus ), Rhinochimaeridae ( Rhinochimaera,
Neoharriotta, Hariotta ), and Chimaeridae ( Chimaera, Hydrolagus ).
The dorsal spine of some recent Chimaera species is positioned anterior to the
fi rst dorsal fi n. The spine is believed to function as a defensive device, particularly
in juveniles and sub-adults when it is capable of infl icting a painful and venomous
wound (Evans 1923 ; Patterson 1965 ). Interestingly, the dorsal spine is also
considered to reduce turbulence and aid the hydrodynamics of the fi rst dorsal fi n. In
addition, it is equipped with a smooth anterior keel, which runs longitudinally along
the centre of the anterior margin of the spine, and is thought to reduce spine erosion
(Maisey 1979 ).
The structure of the holocephalian spines is of interest for materials scientist.
The dorsal spine of Chimaera monstrosa is composed of an outer and an inner layer
of dentine, which collectively form the trunk dentine (Maisey 1979 ). A distinct boundary termed the trunk primordium is present between the two concentric dentine
layers. This consists of collagen fi bres that run longitudinally through the spine.
A single layer of odontoblasts, located on the external side of the trunk primordium,
centrifugally deposits the outer dentine. A similar layer occurs between the inner
dentine and the spine lumen, and centripetally deposits the inner dentine. The
odontoblasts produce an intracellular matrix into which they secrete amorphous
cement materials through their dendritic processes. These processes leave anastomosing dendritic odontoblast canaliculi in their wake as the dentine increases in density
(Calis et al. 2005 ). Moreover, growth increments, apparent as rings generally within
1.2 Part I: Biomaterials of Vertebrate Origin. An Overview