26
often with a specifi c joint in the dorsal armor between the head and neck regions.
Because of this special joint construct, the head of the animal can move upwards as
the jaw dropped downwards, creating a larger gape. Placoderms body was mostly
naked or, partially covered with small scales.
It is suggested that placoderms share a common ancestor with sharks. In both
placoderms and sharks the males have external clasping organs for internal
fertilisation (Ahlberg et al. 2009 ). But only the ptyctodontids, one group of placoderms, have these organs. Ptyctodontids are examples of, probably, oldest defi nite
evidence for vertebrate copulation. According to modern point of view, placoderms
had a remarkably advanced reproductive biology (Long et al. 2008 ) with respect to
the fi rst internal fertilization and viviparity in vertebrates (Long et al. 2009 ).
In contrast to other jawed vertebrates, placoderms did not descend from toothed
ancestors, and, correspondingly, never had teeth. Razor-like, literally self- sharpening
edges on bony plates associated with the jaws performed the function of teeth.
Placoderms differ from all other jawed vertebrates because their nasal capsules
were not fused to the rest of the braincase. These early vertebrates with more than
250 genera became dominant in most brackish and near-shore ecosystems by the
start of the Devonian period (see for review Denison 1975 ). The Placodermi
includes six clades, they are: the Acanthoraci, Rhenanida, Antiarchi, Petalichthyida,
Ptychodontida and Arthrodira. Of these the Arthrodires are the largest group. Class
Placoderm included rhenanids (fl attened, stingray-like forms), petalichthyids (forms
with long spines), and ptyctodontids (slender, streamlined forms with crushing
tooth plates).
1.2.3 Tetrapoda
Terrestrial vertebrates are typical representatives of Tetrapods which, however, also
include numerous aquatic, amphibious, and fl ying groups. These animals occupy
the highest levels of the food chain on land and in both freshwater and marine
aquatic environments (Laurin 2010 ). Under Superclass Tetrapoda, important marine
classes include Reptilia (the reptiles) which contains at least 3,082 species, Aves
(the birds) containing at least 9,842 species, and Mammalia (all mammals) with at
least 4,835 species. These taxa have species common in both marine as well as
terrestrial environments.
The fi sh-tetrapod transition is defi ned as “the greatest step in vertebrate history”
(see for review Long and Gordon 2004 ). Nowadays, the transition between fi sh
with fi ns and tetrapods with limbs and digits is in focus of many scientifi c
groups, especially because of some intriguing fi nds of new material (Ahlberg 1991 ,
1993 ; Boisvert 2005 ; Boisvert et al. 2008 ; Clack 2009 ; Long et al. 2010 ). For example,
discoveries of new tetrapod-like fi sh and very primitive tetrapods stimulated the
resolution of questions with regard to sequence of acquisition of tetrapod characters
during the time and geographic location.
1 Introduction
often with a specifi c joint in the dorsal armor between the head and neck regions.
Because of this special joint construct, the head of the animal can move upwards as
the jaw dropped downwards, creating a larger gape. Placoderms body was mostly
naked or, partially covered with small scales.
It is suggested that placoderms share a common ancestor with sharks. In both
placoderms and sharks the males have external clasping organs for internal
fertilisation (Ahlberg et al. 2009 ). But only the ptyctodontids, one group of placoderms, have these organs. Ptyctodontids are examples of, probably, oldest defi nite
evidence for vertebrate copulation. According to modern point of view, placoderms
had a remarkably advanced reproductive biology (Long et al. 2008 ) with respect to
the fi rst internal fertilization and viviparity in vertebrates (Long et al. 2009 ).
In contrast to other jawed vertebrates, placoderms did not descend from toothed
ancestors, and, correspondingly, never had teeth. Razor-like, literally self- sharpening
edges on bony plates associated with the jaws performed the function of teeth.
Placoderms differ from all other jawed vertebrates because their nasal capsules
were not fused to the rest of the braincase. These early vertebrates with more than
250 genera became dominant in most brackish and near-shore ecosystems by the
start of the Devonian period (see for review Denison 1975 ). The Placodermi
includes six clades, they are: the Acanthoraci, Rhenanida, Antiarchi, Petalichthyida,
Ptychodontida and Arthrodira. Of these the Arthrodires are the largest group. Class
Placoderm included rhenanids (fl attened, stingray-like forms), petalichthyids (forms
with long spines), and ptyctodontids (slender, streamlined forms with crushing
tooth plates).
1.2.3 Tetrapoda
Terrestrial vertebrates are typical representatives of Tetrapods which, however, also
include numerous aquatic, amphibious, and fl ying groups. These animals occupy
the highest levels of the food chain on land and in both freshwater and marine
aquatic environments (Laurin 2010 ). Under Superclass Tetrapoda, important marine
classes include Reptilia (the reptiles) which contains at least 3,082 species, Aves
(the birds) containing at least 9,842 species, and Mammalia (all mammals) with at
least 4,835 species. These taxa have species common in both marine as well as
terrestrial environments.
The fi sh-tetrapod transition is defi ned as “the greatest step in vertebrate history”
(see for review Long and Gordon 2004 ). Nowadays, the transition between fi sh
with fi ns and tetrapods with limbs and digits is in focus of many scientifi c
groups, especially because of some intriguing fi nds of new material (Ahlberg 1991 ,
1993 ; Boisvert 2005 ; Boisvert et al. 2008 ; Clack 2009 ; Long et al. 2010 ). For example,
discoveries of new tetrapod-like fi sh and very primitive tetrapods stimulated the
resolution of questions with regard to sequence of acquisition of tetrapod characters
during the time and geographic location.
1 Introduction