6
1.2.1.4 Order Coelolepida
Thelodonts (from Greek: “nipple teeth”, formerly coelolepids) are an ensemble of
fossil jawless vertebrates, distinguished from other jawless vertebrate groups by the
structure of their minute scales-based exoskeleton (Wilson and Caldwell 1993 ).
These scales superfi cially resemble the placoid scales of sharks. Thelodonts lived in
shallow-water marine environments from the Lowermost Silurian (and possibly the
Late Ordovician) to the Late Devonian (430–370 million years ago) (Turner 1992 ).
It is suggested that some thelodonts migrated into fresh water, perhaps to spawn
(Van der Brugghen and Janvier 1993 ).
Probably, thelodonts were closely related and morphologically very similar to
fi sh of the taxa Heterostraci and Anaspida, differing mainly in their covering of
distinctive small, spiny scales. The small size and resilience of these scales makes
them the most common vertebrate fossil of their time.
The bony scales of the thelodont group were formed and shed throughout the
organisms’ lifetimes, and quickly separated after their death. Correspondingly, they
are the most abundant form of fossil as most resistant materials to the process of
fossilisation and thus most useful for analysis because of exceptional preservation
of internal details (Piepenbrink 1989 ). The scales contain an aspidine base and
comprise a non-growing “crown” composed of dentine, with a specifi cally ornamented
enameloid upper surface. The cell-free bone is the main element of its growing base,
including anchorage structures which fi x it to the side of the fi sh.
It is established that fi ve types of bone-growth, which may represent fi ve natural
groupings within the thelodonts exist. Moreover, each of these scale morphs appears
to resemble the scales of more derived groupings of fi sh. Therefore, it can be hypothesized that thelodont groups may have been stem groups to succeeding clades of fi sh.
The taxa of thelodonts have traditionally been defi ned on the basis of histological
and morphological investigations of their scales. However, because a wide range
of scale morphologies can occur in the same individual, some recent studies on
articulated thelodonts show that scale morphology can be also misleading. On the
basis of their scale morphology and histology, thelodonts are currently classifi ed
into following groups: Achanolepida, Loganiida, Turiniida, and Katoporida (see for
review Janvier 1997a ).
1.2.1.5 Order Cyclostomata
Class Myxini (Myxinoidea)
Hagfi shes or Hyperotreti. A taxon of ocean-dwelling fi sh, which are small and
jawless, as well as scavenging their food from both invertebrates and dead and
dying fi sh. They habituate in cold ocean waters of both hemispheres. Myxinikela
siroka is the only fossil hagfi sh, that remnants are localized in the Francis Creek
Shale of northeastern Illinois (Bardack 1991 ). Fragments of the head and jaws,
paired tentacles, internal organs were found within an iron carbonate (siderite)
1 Introduction
1.2.1.4 Order Coelolepida
Thelodonts (from Greek: “nipple teeth”, formerly coelolepids) are an ensemble of
fossil jawless vertebrates, distinguished from other jawless vertebrate groups by the
structure of their minute scales-based exoskeleton (Wilson and Caldwell 1993 ).
These scales superfi cially resemble the placoid scales of sharks. Thelodonts lived in
shallow-water marine environments from the Lowermost Silurian (and possibly the
Late Ordovician) to the Late Devonian (430–370 million years ago) (Turner 1992 ).
It is suggested that some thelodonts migrated into fresh water, perhaps to spawn
(Van der Brugghen and Janvier 1993 ).
Probably, thelodonts were closely related and morphologically very similar to
fi sh of the taxa Heterostraci and Anaspida, differing mainly in their covering of
distinctive small, spiny scales. The small size and resilience of these scales makes
them the most common vertebrate fossil of their time.
The bony scales of the thelodont group were formed and shed throughout the
organisms’ lifetimes, and quickly separated after their death. Correspondingly, they
are the most abundant form of fossil as most resistant materials to the process of
fossilisation and thus most useful for analysis because of exceptional preservation
of internal details (Piepenbrink 1989 ). The scales contain an aspidine base and
comprise a non-growing “crown” composed of dentine, with a specifi cally ornamented
enameloid upper surface. The cell-free bone is the main element of its growing base,
including anchorage structures which fi x it to the side of the fi sh.
It is established that fi ve types of bone-growth, which may represent fi ve natural
groupings within the thelodonts exist. Moreover, each of these scale morphs appears
to resemble the scales of more derived groupings of fi sh. Therefore, it can be hypothesized that thelodont groups may have been stem groups to succeeding clades of fi sh.
The taxa of thelodonts have traditionally been defi ned on the basis of histological
and morphological investigations of their scales. However, because a wide range
of scale morphologies can occur in the same individual, some recent studies on
articulated thelodonts show that scale morphology can be also misleading. On the
basis of their scale morphology and histology, thelodonts are currently classifi ed
into following groups: Achanolepida, Loganiida, Turiniida, and Katoporida (see for
review Janvier 1997a ).
1.2.1.5 Order Cyclostomata
Class Myxini (Myxinoidea)
Hagfi shes or Hyperotreti. A taxon of ocean-dwelling fi sh, which are small and
jawless, as well as scavenging their food from both invertebrates and dead and
dying fi sh. They habituate in cold ocean waters of both hemispheres. Myxinikela
siroka is the only fossil hagfi sh, that remnants are localized in the Francis Creek
Shale of northeastern Illinois (Bardack 1991 ). Fragments of the head and jaws,
paired tentacles, internal organs were found within an iron carbonate (siderite)
1 Introduction