106 Questions
What is the significance of the appearance of Neoselachii in the Late Triassic?
It marks the first appearance of the modern elasmobranch radiation.
Describe the feeding mechanism of the holocephalan Cladoselache.
Cladoselache fed by trapping, piercing, and cutting its prey with a circular saw-like tooth whorl.
What distinguished the Paleozoic group known as menaspoids?
Menaspoids were heavily armored fishes with plates of dermal bone on their heads and paired spines projecting from the side of their mouths.
What is the fate of the stem members of extant chondrichthyan lineages mentioned in the text?
None of the stem members of extant chondrichthyan lineages survived into the Cenozoic.
What characterized Xenacanths in terms of their anatomy and timeframe of existence?
Xenacanths had anal fins and basals like stem elasmobranchs, surviving until the end of the Triassic period.
What does the term 'Elasmobranchii' refer to?
Elasmobranchii refers to a group of cartilaginous fish with multiple gill openings on each side of the head.
How do Neoselachii differ from Holocephali in terms of gill openings?
Neoselachii have five to seven gill openings on each side of the head, while Holocephali have a single external gill opening.
What is the characteristic feature of Neoselachii's upper jaw?
Neoselachii have an independently mobile upper jaw known as hyostyly.
Describe the appearance of Holocephali's head.
Holocephali have an undivided appearance of the head due to having a single external gill opening.
What are some common names of extant holocephalans?
Some common names of extant holocephalans include chimaeras, rabbitfishes, ratfishes, and ghost sharks.
What do the terms 'Elasmobranchii' and 'Holocephali' refer to in Greek?
Elasmobranchii means 'plate gills' and Holocephali means 'whole head' in Greek.
What are the body forms of fishes mentioned in the text?
Dorsoventrally flattened, fusiform, ribbonlike
When are definitive forms of chondrichthyans known to appear?
Devonian
What distinguishes iniopterygians from holocephalans?
Primitive nature of the braincase
What is a conspicuous difference between stem elasmobranchs and modern sharks?
Position of the mouth
What is the representative example of a stem elasmobranch mentioned in the text?
Cladoselache
When did the first elasmobranchs appear?
Early Devonian
What is a distinctive feature of the upper jaw in chimaeras and their fossil relatives?
Fused with the skull
What unique skeletal characteristic defines chondrichthyans?
Tesserate prismatic calcifications
Which element of the hyoid arch in chondrichthyans is separate and not fused into a hyomandibula?
Epihyal
What is the purpose of pelvic claspers present in male chondrichthyans?
Internal fertilization
Which group demonstrates the ancestral condition of a cartilaginous endoskeleton in vertebrates?
Hagfishes
What is the microstructural feature of the small calcium-containing units in chondrichthyans' skeletons?
Prismatic calcifications
What is a distinguishing characteristic of stethacanthid stem elasmobranchs from the Early Carboniferous?
Eel-like bodies
What is the unique sexual dimorphism feature observed in some stethacanthid males?
Swordlike nuchal spine
How did Xenacanthans differ from other stem elasmobranchs in terms of habitat?
Mainly freshwater bottom-dwellers
What was the probable function of the structure known as the spine-brush complex in stethacanthids?
Used in courtship
What is inferred about the feeding behavior of Cladoselache based on the text?
Engulfing prey whole or slashing them
When did Xenacanthans cease to exist?
Triassic
What is the purpose of the tooth whorl in Helicoprion?
To spiral into a chamber for lifelong tooth production
How did paraselachians differ from extant chimaeras?
They had fused upper jaws to the cranium
What was a notable characteristic of Helicoprion among paraselachians?
Possessing a spiraling tooth whorl
When did paraselachians first appear?
Middle Devonian
What differentiated holocephalans from paraselachians?
The presence of tooth whorls contained within the mouth
Which group of organisms did not appear until the Late Carboniferous according to the text?
Extant chimaeras
What unique type of scales do chondrichthyans possess?
Placoid scales
How do the teeth of chondrichthyans differ from those of bony fishes?
They form in a tooth whorl
Why are fossil teeth the main remnants of extinct chondrichthyans?
Their teeth fall off and disintegrate after death
How do chondrichthyans differ from other jawed fishes in terms of their exoskeleton?
They have scales composed of dentine, enameloid, and traces of bone
What complex structures do some chondrichthyans, especially chimaeras, have associated with their jaws?
Labial cartilages
How do placoid scales develop in chondrichthyans?
From a single dental papilla, similar to teeth
What was a characteristic feeding behavior of the holocephalan Cladoselache?
Trapping prey with a circular saw-like whorl
Which group of Paleozoic fishes resembled placoderms due to heavy armor on their heads and paired spines projecting from their mouths?
Menaspoids
Which group of fishes survived until the end of the Triassic period, showing characteristics similar to stem elasmobranchs?
Xenacanths
When did the modern elasmobranch radiation, Neoselachii, first appear?
Late Triassic
What characteristic feature distinguishes basal Xenacanths from modern elasmobranchs?
50 mm fin rays
Which term refers to the radiation of stem members of extant chondrichthyan lineages that did not survive into the Cenozoic era?
Mesozoic elasmobranchs
What structure in chondrichthyans is responsible for attaching developing teeth behind the functional teeth?
Dental lamina
How frequently do young sharks replace their upper-jaw teeth on average based on the text?
Every 7.8 days
Which group of organisms is now considered to be the sister group to chondrichthyans?
Acanthodians
Tooth replacement in extant sharks varies significantly due to all of the following EXCEPT:
Dietary preferences
What is a secondary modification from the original tooth condition in chimaeras according to the text?
Grinding tooth plates
Which extinct group mentioned in the text also exhibited tooth whorls similar to chondrichthyans?
Menaspoids
What is a distinguishing feature between stem chondrichthyians and modern sharks mentioned in the text?
Position of the mouth
Which era saw the appearance of the first elasmobranchs?
Early Devonian
What differentiated iniopterygians from crown chondrichthyians?
Primitive braincase
Which Paleozoic group had a feeding mechanism described as sharklike but lacked some characteristics of modern sharks?
Stem elasmobranchs
What characteristic shape were iniopterygians compared to in the text?
Chimaera-like
Which feature of Cladoselache distinguishes it as a representative example of a stem elasmobranch?
Terminal mouth
What was a unique characteristic of the Paleozoic group known as menaspoids?
Heavily armored heads
Which group of fishes survived until the end of the Triassic period, showing characteristics similar to stem elasmobranchs?
Xenacanths
When did the modern elasmobranch radiation, Neoselachii, first appear?
Late Triassic
What is the significance of Xenacanths in the context of elasmobranch evolution?
Represented stem elasmobranchs
What differentiated the Mesozoic elasmobranch radiations from Xenacanths?
Survival into Cenozoic
What was a key feature of the Late Cretaceous elasmobranch Hybodus?
Circular saw-like tooth whorl
What type of prey were the stout, blunt teeth of hybodonts primarily suited for?
Holding and crushing crabs
What morphological changes were observed in the posterior teeth of hybodonts compared to their anterior teeth?
They were broader and more blunt
What advancement in the structure of fins made hybodonts more mobile compared to earlier elasmobranchs?
Narrow, stalk-supported fins
What function is suggested by the ridges and barbs on the posterior surface of hybodont dorsal-fin spines?
Assisting in capturing prey
How did the narrow base of the hybodont fin contribute to their locomotion?
Allowed for rotation to control swimming angles
What was the significance of the disappearance of hybodonts at the end of the Mesozoic era?
They became increasingly rare and extinct
What is a distinguishing characteristic of Hybodonts based on their dentition?
Pointed teeth at the front and blunt teeth at the rear
In what period did Hybodonts first appear according to the text?
Devonian
What is the significance of the euselachian heterocercal tail mentioned in the text?
Aids in controlling the fish's shape and movement
What characteristic feature distinguishes Hybodonts' teeth from those of other fish?
Heterodont dentition
How does the hybodont tail's distribution of force aid in swimming?
Lifts the fish when swimming vertically
What distinguished Hybodus, a well-known genus of hybodonts, among other fish?
A flexible lower lobe in its tail
What geologic era saw the appearance of the first elasmobranchs?
Devonian
What is the significance of Neoselachii in the Late Triassic?
They represented a radiation of stem members of chondrichthyan lineages.
What distinguishes holocephalans from other chondrichthyans?
Their heavy armor on heads
During which era did paraselachians first appear?
Triassic
What was a characteristic feeding behavior of Cladoselache?
Carnivorous hunting
What is the defining characteristic of Elasmobranchii mentioned in the text?
Multiple gill openings on each side of the head
What feature distinguishes Holocephali from other groups mentioned in the text?
Undivided appearance of the head due to a single external gill opening
What is the distinguishing factor between sharks and rays (including skates) in Neoselachii?
Dorsoventrally flattened forms with five pairs of gill openings
What is the significance of the operculum in Holocephali anatomy?
Covers individual gill slits
Which characteristic is common among extant holocephalans mentioned in the text?
Fishlike body and long, flexible tail
What does Neoselachii's independently mobile upper jaw refer to?
Hyostyly, which is an independently mobile upper jaw
What was unique about Helicoprion among the paraselachians?
Exhibiting a bizarre, spiraling tooth whorl.
Why did paraselachians look like sharks according to the text?
Their upper jaw fused to the cranium, resembling modern sharks.
What did Helicoprion use its tooth whorl for?
Accessing hard-to-reach prey.
How did Helicoprion's tooth whorl differ from typical shark teeth?
Shark teeth were serrated while Helicoprion's were smooth.
What feature of Helicoprion's tooth whorl allowed its lifelong production of teeth?
Teeth that regenerated continuously.
How did the tooth whorl of Helicoprion differ from typical shark teeth structures?
Sharks have teeth that do not spiral, unlike Helicoprion's tooth whorl.
What Paleozoic group of fishes had heavily armored plates of dermal bone on their heads and paired spines projecting from the side of their mouths?
Menaspoids
Which group of fishes, known for a circular saw-like tooth whorl, probably fed on soft-bodied prey by trapping, piercing, and cutting them?
Menaspoids
Which modern chondrichthyan radiation first appeared in the Late Triassic according to the text?
Neoselachii
Which freshwater fish, similar to stem elasmobranchs, survived until the end of the Triassic period and possibly into the Jurassic?
Xenacanths
Which Paleozoic group will be considered in Chapter 7 as part of modern radiations, despite none of its members surviving into the Cenozoic era?
Elasmobranchs
Which Late Cretaceous elasmobranch is depicted in Figure 6.6 and mentioned in the text?
Hybodonts
What is the primary metabolic substrate of skeletal and cardiac muscles in extant chondrichthyans?
Ketones
What feature renders extant chondrichthyans neutrally buoyant in water?
Large, lipid-filled liver
Which division of extant chondrichthyans reflects a classification dating back almost to the start of chondrichthyan history?
Elasmobranchii
Which group is evolutionary related to the extinct sharklike paraselachians but is distinct from holocephalans?
Neoselachii
In which era did chondrichthyans likely exhibit their greatest diversity?
Paleozoic
Which group besides Holocephali can be seen to be part of a larger group known as Euchondrocephali?
Paraselachians
Study Notes
Chondrichthyan Evolution
- Chondrichthyans are a group of fishes characterized by a cartilaginous endoskeleton and the presence of dermal denticles.
- The earliest known chondrichthyans date back to the Early Ordovician period.
Late Triassic Neoselachii
- The appearance of Neoselachii in the Late Triassic marks a significant milestone in the evolution of chondrichthyans.
- Neoselachii is characterized by a modern elasmobranch radiation and is distinct from earlier stem elasmobranchs.
Holocephali and Cladoselache
- Holocephali are a group of chondrichthyans characterized by a unique head shape and a distinctive feeding mechanism.
- Cladoselache is a representative example of a stem elasmobranch and exhibits a shark-like feeding mechanism.
- The feeding mechanism of Cladoselache involves a tooth whorl used for grasping and piercing prey.
Menaspoids and Iniopterygians
- Menaspoids are a Paleozoic group of fishes characterized by heavily armored plates of dermal bone on their heads and paired spines projecting from their mouths.
- Iniopterygians are a group of fishes that resemble sharks but lack some characteristics of modern sharks.
- Iniopterygians are characterized by a unique tooth shape, with their teeth being compared to razors.
Xenacanthans and Stem Elasmobranchs
- Xenacanthans are a group of stem elasmobranchs that differed from other stem elasmobranchs in terms of their habitat and anatomy.
- Xenacanthans are characterized by a unique upper jaw and a distinctive spine-brush complex.
- The probable function of the spine-brush complex in Xenacanthans was for grasping and piercing prey.
Elasmobranchii and Holocephali
- Elasmobranchii refers to a group of chondrichthyans that includes modern sharks and rays.
- Holocephali are a group of chondrichthyans that are distinct from Elasmobranchii and are characterized by a unique head shape and a distinctive feeding mechanism.
- The term 'Elasmobranchii' refers to the Greek words "elasma" meaning thin plate and "branch" meaning gill.
Chondrichthyan Anatomy
- Chondrichthyans are characterized by a cartilaginous endoskeleton and the presence of dermal denticles.
- The unique skeletal characteristic of chondrichthyans is the presence of small calcium-containing units called tesserae.
- The hyoid arch in chondrichthyans is separate and not fused into a hyomandibula.
Chondrichthyan Radiation
- The modern elasmobranch radiation, Neoselachii, first appeared in the Late Triassic.
- The term 'Neoselachii' refers to the Greek words "neos" meaning new and "selachii" meaning shark.
- Neoselachii is characterized by a modern elasmobranch radiation and is distinct from earlier stem elasmobranchs.
Extant Holocephalans
- Extant holocephalans are characterized by a unique head shape and a distinctive feeding mechanism.
- Common names of extant holocephalans include ratfish, spookfish, and elephant fish.
Fossil Record
- The fossil record of chondrichthyans is primarily composed of teeth, which are the main remnants of extinct chondrichthyans.
- The unique characteristic of chondrichthyan teeth is that they are constantly replaced throughout their lifetime.
- The tooth replacement process in extant sharks varies significantly due to factors such as diet, habitat, and growth rate.
Hybodonts
- Hybodonts are a group of extinct elasmobranchs that appeared in the Early Cretaceous period.
- Hybodonts are characterized by a unique dentition, with their teeth being stout and blunt.
- The function of the ridges and barbs on the posterior surface of hybodont dorsal-fin spines is suggested to be for grasping and piercing prey.
Paraselachians and Helicoprion
- Paraselachians are a group of extinct chondrichthyans that resembled sharks but lacked some characteristics of modern sharks.
- Helicoprion is a well-known genus of paraselachians characterized by a unique tooth whorl.
- The tooth whorl of Helicoprion was used for grasping and piercing prey and was constantly replaced throughout its lifetime.
Learn about Elasmobranchii, a group of cartilaginous fish that includes sharks and rays. They are known for their unique anatomy with multiple gill openings on each side of the head. Discover more about these fascinating creatures!
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