Rhizodont
Encyclopedia
Rhizodonts are an extinct group of predatory lobe-finned fishes. They are known from many areas of the world from the Givetian through to the Pennsylvanian
- the earliest known species is about 377 Ma, the latest around 310 Ma. Rhizodonts lived in tropical rivers and freshwater lakes and were the dominant predators of their age. They reached huge sizes - the largest known species, Rhizodus hibberti
from Europe and North America, was an estimated 7 metres in length, making it the largest freshwater fish known.
and premaxilla
, medium sized fangs on the ectopterygoid and dermopalatine bones, and large tusks on the vomer
s and premaxillae. On the lower jaw there were marginal teeth on the dentary, with fangs on the three coronoids and a huge tusk at the symphysial
tip of the dentary. Apparently, the left and right mandible
s have rotated inwards towards each other on biting. This may have been a kinetic mechanism to dig the marginal teeth more deeply into the prey, to help grip slippery or struggling items.
Rhizodonts' trunk was elongated, with pelvic, two dorsal
and anal fin
s much reduced and placed posteriorly The anal and second dorsal fins formed a functional part of the tail. The lateral line system was elaborated on the skull and pectoral girdle - in Strepsodus the main trunk lateral line also had several subsidiary lines running parallel to it. This probably helped rhizodonts detect prey in the turbid, swampy environment in which they lived.
Rhizodont pelvic fins are known only from external morphology. They are smaller than the pectoral fins and positioned toward the rear of the body. In contrast to the other fins, the pectoral fins were much enlarged. They had a well-developed internal skeleton surrounded by robust, largely unsegmented lepidotrichia; the whole fin was then covered in deeply overlapping scales. This turned the pectoral fin into a broad paddle.
s, lungfish
and other lobe-finned fishes, and even tetrapod
s, because all tetrapods at this time still had to lay their eggs in water.
Pennsylvanian
The Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain...
- the earliest known species is about 377 Ma, the latest around 310 Ma. Rhizodonts lived in tropical rivers and freshwater lakes and were the dominant predators of their age. They reached huge sizes - the largest known species, Rhizodus hibberti
Rhizodus
Rhizodus is an extinct genus of a rhizodont, a branch of the Sarcopterygii, the bony vertebrate clade that also includes tetrapods....
from Europe and North America, was an estimated 7 metres in length, making it the largest freshwater fish known.
Anatomy
The upper jaw had a marginal row of small teeth on the maxillaMaxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...
and premaxilla
Premaxilla
The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....
, medium sized fangs on the ectopterygoid and dermopalatine bones, and large tusks on the vomer
Vomer
The vomer is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones.-Biology:...
s and premaxillae. On the lower jaw there were marginal teeth on the dentary, with fangs on the three coronoids and a huge tusk at the symphysial
Symphysis
A symphysis is a fibrocartilaginous fusion between two bones. It is a type of cartilaginous joint, specifically a secondary cartilaginous joint.1.A symphysis is an amphiarthrosis, a slightly movable joint.2.A growing together of parts or structures...
tip of the dentary. Apparently, the left and right mandible
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
s have rotated inwards towards each other on biting. This may have been a kinetic mechanism to dig the marginal teeth more deeply into the prey, to help grip slippery or struggling items.
Rhizodonts' trunk was elongated, with pelvic, two dorsal
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
and anal fin
Fin
A fin is a surface used for stability and/or to produce lift and thrust or to steer while traveling in water, air, or other fluid media, . The first use of the word was for the limbs of fish, but has been extended to include other animal limbs and man-made devices...
s much reduced and placed posteriorly The anal and second dorsal fins formed a functional part of the tail. The lateral line system was elaborated on the skull and pectoral girdle - in Strepsodus the main trunk lateral line also had several subsidiary lines running parallel to it. This probably helped rhizodonts detect prey in the turbid, swampy environment in which they lived.
Rhizodont pelvic fins are known only from external morphology. They are smaller than the pectoral fins and positioned toward the rear of the body. In contrast to the other fins, the pectoral fins were much enlarged. They had a well-developed internal skeleton surrounded by robust, largely unsegmented lepidotrichia; the whole fin was then covered in deeply overlapping scales. This turned the pectoral fin into a broad paddle.
Ecology
Judging from their anatomy, rhizodonts had an extremely powerful bite. They probably employed a 'grip and drag' hunting technique, where prey was ambushed, the tusks sunk in to secure it, and then depending on its size, either thrashed on the surface to subdue it, or dragged to where the rhizodont could consume it without being disturbed. Their prey probably included large sharkShark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
s, lungfish
Lungfish
Lungfish are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed...
and other lobe-finned fishes, and even tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
s, because all tetrapods at this time still had to lay their eggs in water.
External links
- http://www.palaeos.com/Vertebrates/Units/140Sarcopterygii/140.700.html#Rhizodontiformes
- http://www.donnasaxby.com/rhizodonts