24
Subclass Sarcopterygii (Lobe-Finned Fishes)
The fl eshy pelvic and pectoral fi ns with well-developed muscles and bones are
characteristic morphological features of lobe-fi ns. Their humerus joins to the
shoulder or pectoral girdle, and femur to the pelvis. Sarcopterygians possess a
hinged braincase and a corresponding intracranial joint in the skull roof. This determines corresponding fl exibility within the head that provides additional bite strength
(see for review Thomson 1969 ).
Fossil remnants of lobe-fi ns are reported in the Lower Devonian rocks. For
example, such genera as Powichthys and Youngolepis , those show some affi nity to
the Dipnomorpha. The diversity of lobe-fi ns, however, remained high during the
Upper Devonian, Carboniferous and Lower Permian. Most Sarcopterygians belong
to one of three major groups: the Actinistia (coelacanths), the Dipnomorpha
(lungfi shes and Porolepiformes), and the Tetrapodomorpha (Rhizodontoformes,
Osteolepiformes and Elpistostegalia).
Intriguingly, the Actinistians which fi rst appear in the Middle Devonian were
thought to have become extinct until the well-known discovery of Latimeria chalumnae
in 1938 (see for review Fricke and Plante 1988 ).
Both Porolepiformes and the Dipnoi (lungfi shes) are related to Dipnomorpha
(Clément 2004 ). Porolepiforms with the size up to 2.5 m are known as predators in
Middle and Late Devonian. These species habituate nearshore and in freshwater
niches (Ahlberg 1989 ) and possess following morphological features: “a broad
head, small eyes, in the cheek they had a prespiracular bone, and the large fangs had
a special style of infolding of the enamel and dentine – called a dendrodont tooth
structure,” ( Monroe MH “Australia: The Land Where Time Began”). The large
whorl of stabbing teeth was located at the front of the jaw.
Lungfi shes are among the fi rst Sarcopterygians to appear in the fossil record with
two diversity maxima in the Upper Devonian, and in the Triassic, respectively (see
for review Graham 1997 ). Lungfi shes and the Actinsitia are the only representatives
of lobe-fi ns to have survived the Permian; both also have living species (Bemis
and Northcutt 1992 ). New discoveries of the lungfi sh Rhinodipterus from marine
limestones in Australia confi rms that these animals habituate in marine environments. It is suggested that they developed specializations to breathe air about
375 Ma ago (Clement and Long 2010 ). It is believed that global decline in oxygen
levels during the Middle Devonian together with higher metabolic costs is a more
likely driver of air-breathing ability in lungfi sh. This happened in both marine and
freshwater lungfi sh, and in tetrapodomorph fi sh.
Rhizodonts, osteolepiforms, elpistostegalids and tetrapods are typical representatives of Tetrapodomorpha, a clade of lobe-fi ns (see for review Ruta et al. 2003 ). Some
of the Rhizodonts, which fi rst appeared in the Middle Devonian, were up to 7 m large
and habituate in freshwater habitats. Osteolepiformes, which fi rst appeared in the late
Lower Devonian, and reached a maximum diversity in the Late Devonian, is the group
of Sarcopterygians that gave rise to the tetrapods. This order contains two monophyletic families, the Megalichthyidae and the Tristichopteridae (see for review Ahlberg
and Johanson 1998 ). The Megalichthyids are known as the only osteolepiform group
to survive past the end of the Devonian. Tristichopterids with Eusthenopteron foordi
1 Introduction
Subclass Sarcopterygii (Lobe-Finned Fishes)
The fl eshy pelvic and pectoral fi ns with well-developed muscles and bones are
characteristic morphological features of lobe-fi ns. Their humerus joins to the
shoulder or pectoral girdle, and femur to the pelvis. Sarcopterygians possess a
hinged braincase and a corresponding intracranial joint in the skull roof. This determines corresponding fl exibility within the head that provides additional bite strength
(see for review Thomson 1969 ).
Fossil remnants of lobe-fi ns are reported in the Lower Devonian rocks. For
example, such genera as Powichthys and Youngolepis , those show some affi nity to
the Dipnomorpha. The diversity of lobe-fi ns, however, remained high during the
Upper Devonian, Carboniferous and Lower Permian. Most Sarcopterygians belong
to one of three major groups: the Actinistia (coelacanths), the Dipnomorpha
(lungfi shes and Porolepiformes), and the Tetrapodomorpha (Rhizodontoformes,
Osteolepiformes and Elpistostegalia).
Intriguingly, the Actinistians which fi rst appear in the Middle Devonian were
thought to have become extinct until the well-known discovery of Latimeria chalumnae
in 1938 (see for review Fricke and Plante 1988 ).
Both Porolepiformes and the Dipnoi (lungfi shes) are related to Dipnomorpha
(Clément 2004 ). Porolepiforms with the size up to 2.5 m are known as predators in
Middle and Late Devonian. These species habituate nearshore and in freshwater
niches (Ahlberg 1989 ) and possess following morphological features: “a broad
head, small eyes, in the cheek they had a prespiracular bone, and the large fangs had
a special style of infolding of the enamel and dentine – called a dendrodont tooth
structure,” ( Monroe MH “Australia: The Land Where Time Began”). The large
whorl of stabbing teeth was located at the front of the jaw.
Lungfi shes are among the fi rst Sarcopterygians to appear in the fossil record with
two diversity maxima in the Upper Devonian, and in the Triassic, respectively (see
for review Graham 1997 ). Lungfi shes and the Actinsitia are the only representatives
of lobe-fi ns to have survived the Permian; both also have living species (Bemis
and Northcutt 1992 ). New discoveries of the lungfi sh Rhinodipterus from marine
limestones in Australia confi rms that these animals habituate in marine environments. It is suggested that they developed specializations to breathe air about
375 Ma ago (Clement and Long 2010 ). It is believed that global decline in oxygen
levels during the Middle Devonian together with higher metabolic costs is a more
likely driver of air-breathing ability in lungfi sh. This happened in both marine and
freshwater lungfi sh, and in tetrapodomorph fi sh.
Rhizodonts, osteolepiforms, elpistostegalids and tetrapods are typical representatives of Tetrapodomorpha, a clade of lobe-fi ns (see for review Ruta et al. 2003 ). Some
of the Rhizodonts, which fi rst appeared in the Middle Devonian, were up to 7 m large
and habituate in freshwater habitats. Osteolepiformes, which fi rst appeared in the late
Lower Devonian, and reached a maximum diversity in the Late Devonian, is the group
of Sarcopterygians that gave rise to the tetrapods. This order contains two monophyletic families, the Megalichthyidae and the Tristichopteridae (see for review Ahlberg
and Johanson 1998 ). The Megalichthyids are known as the only osteolepiform group
to survive past the end of the Devonian. Tristichopterids with Eusthenopteron foordi
1 Introduction