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AMPHIBIANS 2

 

    Prehistoric amphibians diversified into more than170 genera.  Some of the large, primitive species would last through the Cretaceous.  Amphibians adapted to exploit terrestrial, aquatic, and semiaquatic environments.  Some became quite large including Eryops (3 m) and Mastodonosaurus (4 m).

aquaticamphibian

MASTODONOSAURUS ERYOPS

Primitive amphibians are divided into a number of groups:    

Labyrinthodonts

Labyrinthodonts, which include Ichthyostega and Acanthostega, still retain rhipidistian characteristics.

ICHTHYOSTEGA

Labyrinthodonts could reach more than 1 m in length (similar to rhipidistian fish were also large).  They were the most primitive amphibians.  The notochord was still a major structural element for the support.   It wasn’t much constricted by the vertebrae and extended from the tail into brain.  The components of vertebrae were not attached to each other  (the pleurocentra and intercentra were still not attached to neural arch; see the illustrations of the next section).   The skulls contained almost all the bones present in rhipidistian skulls, unlike the skulls of modern amphibians. 

     Labyrinthodonts were a successful group through the Triassic Period and most became extinct during the mass extinctions at the end of that Period.  They had continued to thrive after the evolution of reptiles in the Carboniferous and in many areas, they were the most common fossil group in the vertebrate fauna.  Unlike the Paleozoic forms, the Triassic labyrinthodonts were aquatic.  Most were more than 2 meters in length, some with 1 meter long skulls and large fangs in their lower jaws.  Members of five families survived the end-Triassic extinctions and existed in the Jurassic and some into the Cretaceous as well (Milner, 1989; Colbert, 1956). Hynerpeton was a Devonian tetrapod from North America which possessed a mosaic of primitive and derived tetrapod features (Daeschler, 1994).

skull skull
skull comparison

The Paleozoic labyrinthodonts retained vertebrae formed from three separate parts and a larval stage which was quite distinct from that of the adults, possessing external gills. Most were large, although the skull size of adults ranged from 5 to 50 cm (Carroll, 2001). A number of fossils of amphbian larvae are known:

larva

larvae

Early amphibians retained may skull bones which, while present in the skulls of sarcopterygian fish, are absent from the skulls of all modern amphibians.

amphibian skull amphibian skull

The early amphibians possessed skulls which were very similar to those of the fish from which they had descended. In contrast, the skulls of modern amphibians (such as the frog and 2 different salamander species depicted below) were quite different.

frogfrog

salamandersalamander

Diverse lineages of amphibians became secondarily adapted for aquatic life.

amphibiansamphibian

Anthracosaurs:

Anthracosaurs were a group of later amphibians which may have given rise to the reptiles (discussed in future chapters). Anthracosaurs are known from the early Carboniferous, but they were never very common or diverse.   They developed a deep skull, a shorter snout, a short body, large eyes, and strong legs.  This group of amphibians gave rise to the reptiles.  In the labyrinthodonts and anthracosaurs, the back of the skull is not solid and therefore the stapes (a middle ear bone in humans) was still large to function in the support of the skull and would not have functioned well in hearing.

 

anthracosaur
anthracosaur
     Proterogyrinus (above) is an early anthracosaur while Seymoria was very advanced and close to the ancestry of reptiles.  Many Paleozoic amphibian groups became extinct in the end-Permian extinction. No anthracosaurs survived into the Triassic.

seymouria

amphibian

amphibian

Eogyrinus (an anthracosaur) possessed tusk-like teeth on its palatine and ectopterygoid bones and small teeth on its ectopterygoid bones. In some anthracosaurs, the lacrimal bone lost contact with the nostril. Eogyrinus and Archeria possessed scales, presumably on the ventral surface. These dermal scales could be 6 cm long and 2 cm wide (Panchen, 1972).

 

OTHER GROUPS OF AMPHIBIANS

Other Paleozoic amphibians diversified into a number of groups that did not survive into the Triassic.  Aistopods were snakelike amphibians with no legs, no limb girdles, and up to 230 vertebrae.   Nectrideans were a group of entirely aquatic amphibians with small limbs. Some retained the lateral line system of the rhipidistians as an aquatic sensory system.  This group includes Diploceraspis with its distinct V-shaped head.  Microsaurs were the most diverse lepospondyls with 11 families.

various fossils

diploceraspis

skull

     Which groups of lepospondyls evolved into modern amphibians?  Unfortunately, there is a surprising gap in the fossil record of amphibians–virtually none are known from the late Permian to the mid-Jurassic. 

The earliest fossils associated with the group of living amphibians, or lissamphibians include the Early Permian Gerobatrachus and the Early Triassic Triadobatrachus and Czatkobatrachus (San Mauro, 2010). 

The Late Triassic Triadobatrachus was a salamander-like animal that had a froglike skull and hip and may have been an ancestor of frogs.  This genus may have evolved from Doleserpeton from the Paleozoic (Carroll, 1988).  Triadobatrachus possessed more vertebrae than modern frogs and a short tail.  The radius and ulna are shorter than in modern frogs.  (O’Connor, 2003; Rage, 1989; Estes, 1973).
triadobatrachus
skulls

Balanerpeton was a Carboniferous amphibian which may be related to the lineage which gave rise to modern amphibian groups (Clack, 2002).

balanerpeton

     In the Jurassic, frogs and salamanders are known which were essentially modern in most anatomical characteristics.  Vieralla, a possible ancestor of modern families, possessed unfused ribs (unlike modern frogs) and its legs were short.  Prosalirus certainly could jump but the primitive positions of muscle attachments indicate that it did not have same the ability seen in modern frogs (Abrams, 1996).  Notobatrachus possessed a number of primitive traits such as a short pelvis and a continuous notochord through its vertebrae.  A few other Jurassic frogs with primitive traits are known as well (Estes, 1973).  Of the 24 families of frogs, only 6 or 7 are represented by fossils in the Jurassic or Cretaceous. These Mesozoic families Discoglossidae, Leiopelmatidae, Pipidae, Pelobatidae, and Rhinophrynidae are all classified in the more primitive of the two suborders of frogs, Archaeobatrachia while the families Paleobatrachidae and Leptodactylidae are classified in the more derived suborder, Neobatrachia. Some fossil frogs belong to Archaeobatrachia, although not to any modern family, and have a number of primitive features not present in modern frogs such as the parahyoid and unfused lower leg bones. (Lower leg bones are not fused in the fossil species Prosalirus, Notobatrachus, and Yizhoubatrachus) (Gao, 2004). Eodiscoglossus is classified with the modern family Discoglossidae, one of the two most primitive families of living frogs but was unlike all Some Jurassic frogs became extinct while some have survived to modern times; more modern frogs evolved in the Paleocene (and the toad Bufo is known from the Miocene).  Frog fossils from the Early Cretaceous of Brazil represent basal members of neobatrachian frogs, the group of advanced frogs which compose more than 95% of modern frog species (Baez, 2009).

cladogram

There were a number of groups of amphibians which resembled salamanders superficially but which were not closely related (like the Mesozoic albanerpetons) (McGowan, 1995).  Below are images of modern amphibians.

wood frog bull frog
salamander
frog skeleton
salamander salamander
salamander skeleton

The first fossil salamanders (such as Sinerpeton and Laccotriton) are known from the Late Jurassic and fossils of a sister group related to salamanders (which include Marmorerpeton and Kokartus) are known the mid Jurassic (and the incomplete fossil Triassurus is known from the Upper Triassic) (Carroll, 2004; Zhang, 2008).

    A third group of modern amphibians are the legless caecilians of Latinamerica.

Modern species have no limbs nor trace of limb girdles and may have more than 200 vertebrae.  The earliest caecilian fossils are known from the Jurassic Period and, unlike modern caecilians, possess legs.  Their skull was also closer to the ancestral condition than that of modern caecilians (Jenkins, 1993).The fossil Eocaecilia possessed anatomical similarities to the Paleozoic lepospondyl microsaur, Rhynchonkos (Carroll, 2001).The three orders of modern amphibians form a clade with salamanders and frogs closer to each other than either group is to caecilians (Zardoya, 2001)

fossil caecilian

skulls