Comp.ana. ostracoderms,acanthodians,placoderms

A DIVERSE ASSEMBLY OF ANCIENT ARMOURED

CRANIATES

Ostracoderms

TAXONOMYDomain: Eukarya

Kingdom: AnimaliaPhylum: Chordata

Subphylum: VertebrataSuperclass: Agnatha Class:

Ostracodermi

Characteristics Oldest known craniatesDate back to the middle of Ordovician

until the end of DevonianEntire body was covered with bony

dermal armourBony shield head coveringLacked jawsMost lacked paired finsMostly 2-3 cm long; a few reached up to 2

m longHeterocercal tail

Probably were slow, bottom-dwelling animals

Have median fins located down the midline of their backs

Gills were located in pouchesCartilaginous internal skeletonsA paired of side flaps aid in steeringMouth served to obtain oxygen and

retain bits of food and was permanently open

Gills were used exclusively for respiration

Used muscular gill pouch to pull in small and slow moving prey

Examples of the extinct Ostracoderms

According to E.A. Stensiö – paleontologist at the University of Stockholm

Head skeleton of cephalaspis (one species of an ostracoderm) was more or less flattened denticle-covered body shield with four dorsal apertures Two dorsal apertures accommodated a pair of

upward-staring eyes One accommodated the median or pineal eye Another accommodated a small, anterior

opening that was a single naris from which a nasohypophyseal duct led to an olfactory sac and beyond

eye

Dorsal field

nostril

Pineal foramen

lateral field

The body shield turned along its lateral edges

Beneath the gills the body covering were tile like scales

A small mouth that is lined by gills opens to the oropharyngeal chamber is found between the anterior edge of the shield and the scales

Curved row external gill slits extends from the corners of the mouth to the caudal margin of the head shield

Head contained an endoskeleton of endochondral bone and considerable cartilage

Acanthodians

Acanthodians?

Class of Extinct speciesShare features with Bony and

Cartilaginous fishesIn form they resemble sharks, but their

epidermis was covered by tiny rhomboid platelets like the scales of holosteans.Oldest jawed fishes

Holosteans – Bony fishes (Gars, Bowfins)

Acanthodians

Like Ostracoderms, their head and body were protected by a dermal armor of bony plates and scales

Acanthodians

Acanthodians did have cartilaginous skeletons

But their fins are bony based with a dentine spine

Most Acanthodians have heterocercal caudal fins.

Silurian to Permian Period.Marine/Freshwater

-Devonian Period: Freshwater species became dominant

Diplacanthus longispinus

Orders

Climatiiformes  had shoulder armor and many small sharp spines

Ischnacanthiformes  Had teeth fused to the jaw

Acanthodiformes filter feeders, with no teeth in the jaw, but long gill

 rakers.

Spiny Sharks they were superficially shark-shaped, with a

streamlined body, paired fins, and a strongly upturned tail; stout bony spines supported all the fins except the tail - hence, "spiny sharks".

However, they were not sharks.

Skeleton consisted of bone and cartilage.They had a large operculum.

Brief History

Despite being called "spiny sharks," acanthodians predate sharks. They evolved in the sea at the beginning of the Silurian Period, some 50 million years before the first sharks appeared.

Later the acanthodians colonized fresh waters, and thrived in the rivers and lakes during the Devonian  Period.

But the first bony fishes were already showing their potential to dominate the waters of the world, and their competition proved too much for the spiny sharks, which died out in Permian times (approximately 250 MYA).

Many paleonthologists consider that the acanthodians were close to the ancestors of the bony fishes. Although their interior skeletons were made of cartilage, a bonelike material had developed in the skins of these fishes, in the form of closely fitting scales. Some scales were greatly enlarged and formed a bony covering on top of the head and over the lower shoulder girdle. Others developed a bony flap over the gill openings analogous to the operculum in later bony fishes.

PLACODERMS

TAXONOMYDomain: Eukarya

Kingdom: AnimaliaPhylum: Chordata

Subphylum: VertebrataClass: Placodermi

Order: Arthrodira

Order AntiarchiOrder RhenanidaOrder

PetalichthyidaOrder

AcanthothoraciOrder

Ptyctodontida

Placoderms

Greek word “tablet & skin” referring to their heavy armoured bony plates

an extinct class of armored fishesPlacoderms evolved from agnathan (jawless) fishes

Abundant in the fresh waters of the Devonian era

First to evolve true jawsFirst animals to exhibit internalize egg fertilization

Distinct Characteristics

layer of segmented armor across the head and thorax

2 paired fins Depending on species, the remainder of the body would be scaled or naked.

sharpened points of bone which protrude from the head to grind their food.

Order Arthrodira

Dunkleosteus  sp.

Order Arthrodira

Best known placodermsA heavy dermal shield covered the

head and gill region and another covered much of the trunk

2 shield meet in a movable jointRemainder of the body: covered

with smaller bony scales or,in late species, was naked.

2 pairs upper jaw tooth plates

shark-like bodiesSome very large – e.g. Dunkleosteus

and Gorgonichthys to 6 - 9 m

The gigantic superpreditor Dunkleosteus terrelli 

Coccosteus sp.

Gorgonichthys sp.

Reconstruction of placoderm Gorgonichthys, Cleveland Shale - copyright John Long, Museum Victoria

Gorgonichthys sp.

Order AntiarchiWere small

placoderms reach a length of 15-20 feet but majority were shorter

Exhibit atypical pectoral fins & dorsal eyes

Have flattened ventral surface

Suggested to be bottom feeders

Have eyes on top of their heads

Pectoral fins are enclosed in bony appendages

Bothriolepis Canadensis

Order Rhenanida

Characteristics of Rhenanids:reduction of the thoracic

armour Body is covered by small

tesserae (small square of stones)

dorso-ventrally flattened shape

enlarged pectoral fins known from marine

environments and probably pursued a ray-like benthic lifestyle

Rhenanida, Gemuendina sp.

Order PetalichthyidaClosely related to

arthrodiresRarely exceeds a meterThey are freshwater

bottom dwellersUnlike the Arthrodires,

they are bottom-feeding fish

Flattened fish with short trunk and long, spine-like plates.

The nostrils and the anterior part of the head shield around the orbits, is covered by a number of tiny scales, as is the long trunk.

Lunaspis sp.

Order Acanthothoraci

Generalized by a stout spine emanating from the median dorsal plate

Some acanthothoracids are similar to rays, expanded pectoral fins and flattened appearance

Long head shields, very shortened trunk

With eyes and nares pointing upwards, suggesting a bottom dweller

Brindabellaspis stensioi

Order Ptyctodontida have enlongate

bodies whip-like tails reduced head thoracic armorExhibits sexual

dimorphism males of one

ptyctodontid, Rhamphodopis, have clasper-like intromittent organs similar to those found sharks and their kins 

Rhamphodopis

SUMMARYOrder Arthrodira (Arthrodires)– jointed neck–movable joint between the head and the thoracic

bodyOrder Antiarchi (Antiarchs)– pectoral fins are enclosed in bony tubes (pectoral

appendages)Order Rhenanida(Rhenanids) – enlarged pectoral fins – body covered by unfused scalesOrder Petalichthyida (Petalichthyids)– typified by their splayed (spread out) finsOrder Acanthothoraci – spiny trunk Order Ptyctodontida– some species exhibit sexual dimorphism– has beak-like mouth

Differences

Ostracoderms Acanthodians Placoderms

-oldest known vertebrate-jawless fishes-lacked paired fins

-oldest jawed fishes-their skeleton consist of bone and cartilage

-First animals to exhibit internalize egg fertilization-armored fishes with jaws