| Incisoscutum | |
|---|---|
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| Incisoscutum specimen WAM 03.3.28 | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | †Placodermi |
| Order: | †Arthrodira |
| Suborder: | †Brachythoraci |
| Clade: | †Eubrachythoraci |
| Clade: | †Coccosteomorphi |
| Superfamily: | †Incisoscutoidea |
| Family: | †Incisoscutidae |
| Genus: | †Incisoscutum Dennis & Miles, 1981 |
| Species | |
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| Synonyms | |
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Incisoscutum is anextinctgenus ofarthrodireplacoderm from the EarlyFrasnianGogo Reef, from LateDevonianAustralia. The genus contains two speciesI. ritchiei,[1] named after Alex Ritchie, apalaeoichthyologist and senior fellow of theAustralian Museum, andI. 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[2] and ossified pelvicclaspers found in males.[3] This shows thatviviparity andinternal fertilization was common amongst these primitive jawed vertebrates, which are outside the crown groupGnathostomata.
In a study of fossil remains, comparison of theontogeny of fourteen dermal plates fromCompagopiscis croucheri and the more derived speciesIncisoscutum 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 thearthrodires, which include placoderms.[4]
Three-dimensional uncrushedIncisioscutum fossils with remarkable soft tissue preservation were discovered within the Western AustralianGogo Formation, a warm, shallow-sea reeffacies ofFrasnian age.[5]I. ritchiei was a small placoderm with an estimated body length of 30.3 cm (11.9 in).[6]
Incisoscutum's body size andmorphology are similar toTorosteus andCompagopiscis, suggesting a possiblepelagic lifestyle, although they were on differenttrophic levels of theirecosystem.[7] Jawmorphology suggests thatIncisoscutum wasdurophagous, and individual fossils have been found in numbers, suggesting possibleschooling behavior.[7]
WhenIncisoscutum 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 adultIncisoscutum.[1][2]
However, spurred by the discovery of preserved embryos inside theptyctodontsMaterpiscis attenboroughi[8] andAustroptyctodus gardineri, the ‘last meals’ of theIncisoscutum have now been reinterpreted as embryos, 5 cm in length, within pregnant adult females.[2]
The apparent gastric etching is now thought to be an early stage ofossification. The disarrangement is thought to be due to scattering as a result of the body cavity opening postmortem;worm casts throughout the body and surroundingmatrix 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 thearthrodires can now be excluded as stomach contents and interpreted as embryos.[2]
The discovery of embryos withinIncisoscutum is of evolutionary importance as these fossils reveal that arthrodires had advanced reproductive biology and were able to give birth to live young. TheIncisoscutum fossils show the oldest record ofviviparity in anyvertebrate, therefore internal fertilization and viviparity must have evolved in the vertebrates at least 380 million years ago and outside thegnathostomata.[2]
Viviparity inIncisoscutum and the ptyctodontids show that these placoderms were the firstK-strategists 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 theDevonianGogo Formation 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.[9]
Sexually dimorphic pelvicclaspers have been found in male and femaleIncisoscutum fossil specimens.[2][3] In males (WAM 03.3.28)[3] the completely ossified clasper is a slender rod attached to a square basal plate that articulates directly with thepelvic girdle. This contrasts tomodern sharks where the clasperarticulates with a basipterygialcartilage element. The tip of thedistal end has a small cap ofdermal bone with small pits anddenticles for clinging on to the female. Theproximal part of the clasper expands into a plate with fourforamina, two larger above two smaller. It is thought that this anatomy corresponds to the core of an erectile element as in extant sharks.[3]
FemaleIncisoscutum 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.[2]
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[8] suggestinghomology. It is therefore suggested that pelvic claspers may characterize all of the pytodontids and arthrodires.[3] As males and females have never been found in the same locality, it is possible thatIncisoscutum males and females inhabited different areas throughout most their life cycles, only coming together to mate, a behavior comparable to some extant sharks.[9]
Incisoscutum is a member of thesuperfamilyIncisoscutoidea, which belongs to thecladeCoccosteomorphi, one of the two major clades withinEubrachythoraci.[10][11]Incisoscutum was initially placed in thefamilyIncisoscutidae. However,Incisoscutidae is currently amonotypic family, with the genusIncisoscutum as the only member, and thus the family name Incisoscutidae is not widely used.[10] Alternatively,Incisoscutum could possibly be considered a member of the closely related familyCamuropiscidae, as shown in thecladogram below:[11]
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