The Cleveland Shale fauna represents a unique view of the time after a major Devonian extinction event
(Frasnian–Famenian) with the recovery of arthrodires (Placodermi) best represented by this most specious
North American fauna. This time was followed by an additional event (Hangenberg Biocrisis) leading to the
extinction of arthrodires (and all other placoderms). An understanding of the diversity and interrelationships of
North American arthrodires can aid our understanding of this critical time in vertebrate evolution. A new aspinothoracid
arthrodire Hlavinichthys jacksoni gen. et sp. nov. is described from the Late Devonian of northern
Ohio, U.S.A., which adds to our knowledge of this group. It provides a point of comparison to other members
of the fauna whose interrelationships are poorly known. A phylogenetic analysis supports an assignment of
Hlavinichthys jacksoni gen. et sp. nov. among the aspinothoracid arthrodires. This work has drawn attention to
the continued need for descriptive and phylogenetic analyses of this unique fauna. Decades old species descriptions
need revision along with preparation and description of new taxa. The work on Hlavinichthys jacksoni
gen. et sp. nov. here is one step in that process.
Six enigmatic fossils from the Famennian (Devonian) Cleveland Shale in Ohio, U.S.A., are interpreted here as
arthrodiran (Placodermi) egg cases. Recognition as egg cases is confirmed based on the observation of layered
collagen fibers. The presence of a tuberculated bone fragment preserved within one case confirms a vertebrate
source. The nature of the tubercles and the unique morphology of the egg cases supports the interpretation of
an arthrodiran source. Reports of Devonian egg cases are limited to either assumed chondrichthyan producers
or a putative ‘egg sac’ with a morphology atypical for any vertebrate. The Cleveland Shale egg cases thus
represent the first record for a non-chondrichthyan producer. Among placoderms, behaviors of a pelagic life
style with obligate nesting sites, reef fishes with live birth, and estuarine and fluvial nurseries, along with eggcase
oviparity testifies to the diversity of reproductive strategies. As with modern fishes these strategies may
be ecologically driven and the derived and variable reproductive biology of extant chondrichthyans is actually
a primitive condition among gnathostomes. One consequence of the diversity of reproductive strategies (dependent
on the topology of relationships) is the independent origin of internal fertilization within placoderms,
possibly suggesting external fertilization as the primitive gnathostome reproductive mode.