More surprising is that placoderms had abdominal muscles running perpendicular to the body's midline, says coauthor Per Ahlberg of Sweden's Uppsala University.
To find out, he says, the researchers first need to confirm that the muscles were present in all types of placoderms, not unique to the one placoderm group studied.
The assemblage comprises the psammosteids Pycnosteus palaeformis Preobrazhensky, Tartuosteus giganteus (Gross) and Psammolepis proia Mark-Kurik (Glinskiy 2012); the placoderms Actinolepis tuberculata Agassiz, Heterostius ingens Asmuss, Homostius latus Asmuss, Coccosteidae, Asterolepis sp.
The ichthyoassemblage belongs to the Pycnosteus tuberculatus psammosteid Zone and the Asterolepis delli placoderm Zone, and includes the psammosteids Pycnosteus tuberculatus (Rohon), Ganosteus stellatus Rohon (Glinskiy 2012); the placoderms Actinolepis sp.
mainly acanthothor-acids have been reported, one of them identified as Romundina sp.
The representation of various skeletal elements of the placoderms
Bothriolepis ornata and B.
are very common in the Middle-Upper Devonian units, particularly in the Givetian and Frasnian.
But between the latest Devonian Period and the subsequent Carboniferous period, placoderms
disappeared and ray-finned fishes rapidly replaced lobe-finned fishes as the dominant group, a demographic shift that persists to today.
Other groups of vertebrates, such as psammosteids, placoderms
, acanthodians and porolepiforms, predominate in those assemblages while the chondrichthyans are occasional accessory elements.
He said that their work earlier this year suggests the reproductive structure in the dominant group of placoderms
, called arthrodires, was similar to present day sharks.
In May 2008, Dr John Long and colleagues announced in Nature they had discovered a 380 million-year-old fossil of a pregnant female placoderm
(Materpiscis attenboroughi), complete with an umbilical cord and embryo.
Scales and tesserae or fragments of plates of heterostracans, placoderms
, acanthodians, chondrichthyans, and osteichthyans (sarcopterygians and actinopterygians) were found (see Table 1).