''Incisoscutum'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

arthrodire Arthrodira (Greek for "jointed neck") is an Order (biology), order of extinct armored, jawed fishes of the class Placodermi that flourished in the Devonian period before their sudden extinction, surviving for about 50 million years and penetratin ...

placoderm Placoderms (from Ancient Greek πλάξ 'plax'', ''plakos'''Plate (animal anatomy), plate' and δέρμα 'derma'''skin') are vertebrate animals of the class (biology), class Placodermi, an extinct group of prehistoric fish known from Pal ...

from the Early

Frasnian The Frasnian is one of two faunal stages in the Late Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Givetian Stage and followed by the Famennian Stage. Major reef-building was under way during ...

Gogo Reef, from Late

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

. The genus contains two species ''I. ritchiei'', named after Alex Ritchie, a palaeoichthyologist and senior fellow of the

Australian Museum The Australian Museum, originally known as the Colonial Museum or Sydney Museum. is a heritage-listed museum at 1 William Street, Sydney, William Street, Sydney central business district, Sydney CBD, New South Wales. It is the oldest natural ...

, and ''I. sarahae'', named after Sarah Long, daughter of its discoverer and describer, John A. Long. The genus is important in the study of early vertebrates as well-preserved fossilized embryos have been found in female specimens and ossified pelvic

claspers In biology, a clasper is a male anatomical structure found in some groups of animals, used in mating. Male cartilaginous fish have claspers formed from the posterior portion of their pelvic fin which serve to channel semen into the female's c ...

found in males. This shows that

viviparity In animals, viviparity is development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juv ...

and

internal fertilization Internal fertilization is the union of an egg and sperm cell during sexual reproduction inside the female body. Internal fertilization, unlike its counterpart, external fertilization, brings more control to the female with reproduction. For inte ...

was common amongst these primitive jawed vertebrates, which are outside the crown group

Gnathostomata Gnathostomata (; from Ancient Greek: (') 'jaw' + (') 'mouth') are jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all extant vertebrates, including all living bony fishes (both ray-finned ...

. In a study of fossil remains, comparison of the

ontogeny Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the ovum, egg to adult. The term can also be used to refer to t ...

of fourteen dermal plates from '' Compagopiscis croucheri'' and the more derived species ''Incisoscutum ritchiei'' suggested that lengthwise growth occurs earlier in the ontogeny than growth in width, and that dissociated allometric heterochrony has been an important mechanism in the evolution of the

s, which include placoderms.

Fossils

Three-dimensional uncrushed ''Incisioscutum'' fossils with remarkable soft tissue preservation were discovered within the Western Australian

Gogo Formation The Gogo Formation in the Kimberley region of Western Australia is a Lagerstätte that exhibits exceptional preservation of a Devonian reef community. The formation is named after Gogo Station, a cattle station where outcrops appear and fossi ...

, a warm, shallow-sea reef

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with distinctive characteristics. The characteristics can be any observable attribute of rocks (such as their overall appearance, composition, or con ...

age. ''I. ritchiei'' was a small placoderm with an estimated body length of .

Palaeobiology

Feeding and social behavior

''Incisoscutums body size and

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...

are similar to '' Torosteus'' and ''

Compagopiscis ''Compagopiscis'' is an extinct genus of placoderm known from the Gogo Formation. It lived in the Upper Devonian (Frasnian stage) of Western Australia. The genus is monotypic, with its only species being ''Compagopiscis croucheri''. Descripti ...

'', suggesting a possible

pelagic The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the sur ...

lifestyle, although they were on different

trophic level The trophic level of an organism is the position it occupies in a food web. Within a food web, a food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the ...

s of their

ecosystem An ecosystem (or ecological system) is a system formed by Organism, organisms in interaction with their Biophysical environment, environment. The Biotic material, biotic and abiotic components are linked together through nutrient cycles and en ...

. Jaw

suggests that ''Incisoscutum'' was

durophagous Durophagy is the eating behavior of animals that consume hard-shelled or exoskeleton-bearing organisms, such as corals, shelled mollusks, or crabs. It is mostly used to describe fish, but is also used when describing reptiles, including fossil t ...

, and individual fossils have been found in numbers, suggesting possible

schooling A school is the educational institution (and, in the case of in-person learning, the building) designed to provide learning environments for the teaching of students, usually under the direction of teachers. Most countries have systems of fo ...

behavior.

Embryos

When ''Incisoscutum ritchiei'' was first described, bony plates of smaller arthrodires were discovered within the body cavity of two specimens. Due to their disorganized arrangement, posterior position behind the trunk shield, apparent gastric etching and the fact that one of the small arthodires was posterior facing relative to the adult, these bony plates were thought to represent the stomach contents of the adult ''Incisoscutum''. However, spurred by the discovery of preserved embryos inside the

ptyctodont The ptyctodontids ("folded-teeth") are placoderms of the order Ptyctodontida, containing the family Ptyctodontidae. With their big heads, big eyes, reduced armor and long bodies, the ptyctodontids bore a superficial resemblance to modern day chim ...

s '' Materpiscis attenboroughi'' and '' Austroptyctodus gardineri'', the ‘last meals’ of the ''Incisoscutum'' have now been reinterpreted as embryos, 5 cm in length, within pregnant adult females. The apparent gastric etching is now thought to be an early stage of

ossification Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

. The disarrangement is thought to be due to scattering as a result of the body cavity opening postmortem;

worm cast A worm cast is a structure created by worms, typically on soils such as those on beaches that gives the appearance of multiple worms. They are also used to trace the location of one or more worms. References External links Soil science ...

s throughout the body and surrounding

matrix Matrix (: matrices or matrixes) or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the m ...

reveal that the fish carcass remained in an open environment after death. Furthermore, the lack of any taxa associated with the completely preserved delicate plates means that the

arthrodires Arthrodira (Greek for "jointed neck") is an order of extinct armored, jawed fishes of the class Placodermi that flourished in the Devonian period before their sudden extinction, surviving for about 50 million years and penetrating most marine eco ...

can now be excluded as stomach contents and interpreted as embryos. The discovery of embryos within ''Incisoscutum'' is of evolutionary importance as these fossils reveal that arthrodires had advanced reproductive biology and were able to give birth to live young. The ''Incisoscutum'' fossils show the oldest record of

in any

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

, therefore internal fertilization and viviparity must have evolved in the vertebrates at least 380 million years ago and outside the

gnathostomata Gnathostomata (; from Ancient Greek: (') 'jaw' + (') 'mouth') are jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all extant vertebrates, including all living bony fishes (both ray-finned ...

. Viviparity in ''Incisoscutum'' and the ptyctodontids show that these placoderms were the first

K-strategists In ecology, selection theory relates to the selection of combinations of traits in an organism that trade off between quantity and quality of offspring. The focus on either an increased quantity of offspring at the expense of reduced individual ...

in relation to breeding. They invested in rearing a smaller amount of eggs, rather than a huge spawn as their ancestors must have done. It is therefore likely that the warm reef environment of the

was stable and predictable, having a degree of ecological balance (for example hiding places for pregnant placoderms). This would have allowed the placoderms to invest more time and energy in producing and nurturing fewer, but more developed, offspring.

Pelvic claspers

Sexually dimorphic Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

pelvic

clasper In biology, a clasper is a male anatomical structure found in some groups of animals, used in mating. Male cartilaginous fish have claspers formed from the posterior portion of their pelvic fin which serve to channel semen into the female's ...

s have been found in male and female ''Incisoscutum'' fossil specimens. In males (WAM 03.3.28) the completely ossified clasper is a slender rod attached to a square basal plate that articulates directly with the

pelvic girdle The hip bone (os coxae, innominate bone, pelvic bone or coxal bone) is a large flat bone, constricted in the center and expanded above and below. In some vertebrates (including humans before puberty) it is composed of three parts: the Ilium (bone) ...

. This contrasts to modern sharks where the clasper articulates with a basipterygial

cartilage Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...

element. The tip of the

distal Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...

end has a small cap of

dermal bone A dermal bone or investing bone or membrane bone is a bony structure derived from intramembranous ossification forming components of the vertebrate skeleton, including much of the skull, jaws, gill covers, shoulder girdle, fin rays ( lepidotrich ...

with small pits and denticles for clinging on to the female. The

proximal Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

part of the clasper expands into a plate with four

foramina In anatomy and osteology, a foramen (; : foramina, or foramens ; ) is an opening or enclosed gap within the dense connective tissue (bones and deep fasciae) of extant and extinct amniote animals, typically to allow passage of nerves, arter ...

, two larger above two smaller. It is thought that this anatomy corresponds to the core of an erectile element as in extant sharks. Female ''Incisoscutum'' specimens differ from these male claspers and instead fossils have been found with a broad based pelvic plate that articulates with a posteriorly directed basipterygium, similar to modern sharks. The distal end has an articulation for an additional cartilaginous segment or series. Therefore, the difference in pelvic claspers between genders suggests that sexual dimorphism was already present in the arthrodires in the Devonian. Pelvic claspers have also been discovered in pyctodontid fossils suggesting homology. It is therefore suggested that pelvic claspers may characterize all of the pytodontids and arthrodires. As males and females have never been found in the same locality, it is possible that ''Incisoscutum'' males and females inhabited different areas throughout most their life cycles, only coming together to mate, a behavior comparable to some extant sharks.

Phylogeny

''Incisoscutum'' is a member of the

superfamily SUPERFAMILY is a database and search platform of structural and functional annotation for all proteins and genomes. It classifies amino acid sequences into known structural domains, especially into SCOP superfamilies. Domains are functional, str ...

Incisoscutoidea, which belongs to the

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

Coccosteomorphi Coccosteomorphi is an extinct clade of arthrodire placoderms within the Eubrachythoraci (of the suborder Brachythoraci), armored fish most diverse during the Devonian. Most are considered to be pelagic (open ocean) long-distance swimmers, leadin ...

, one of the two major clades within

Eubrachythoraci Eubrachythoraci is an extinct clade of arthrodire placoderms within the suborder Brachythoraci, armored fish most diverse during the Devonian. Most are considered to be pelagic (open ocean) long-distance swimmers, leading to their widespread dis ...

. ''Incisoscutum'' was initially placed in the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Incisoscutidae. However, Incisoscutidae is currently a monotypic family, with the genus ''Incisoscutum'' as the only member, and thus the family name Incisoscutidae is not widely used. Alternatively, ''Incisoscutum'' could possibly be considered a member of the closely related family Camuropiscidae, as shown in the

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

below:

References

{{Taxonbar, from=Q150924, from2=Q18618816, from3=Q112326560, from4=Q112326533 Arthrodires Arthrodire genera Placoderms of Australia