Two Early Devonian flat head fish, Uranolophus and Janusiscus, now nest with another much more complete flat head, Guiyu, from the Late Silurian

More housekeeping in the fish subset
of the large reptile tree (LRT, 2192 taxa) moves two Early Devonian flat heads (known from little else) alongside the more completely known Late Silurian Guiyu. Corrections to the identities of the many dozen skull bones of these two prompted this new nesting. It took several months of relabeling fish bones to finally get to this node.

Previously and traditionally
Uranolophus was considered a lungfish.

According to Giles et al 2015,
tiny Janusiscus was considered a stem gnathostome, a possible ancestor to Chondrichthyes (sharks, etc.) or Osteichthyes (bony fish, etc.) or both.

Barely enough traits made these new nestings possible
in the LRT. Uranolophus was omitted from the the Giles et al cladogram. Employing fewer taxa, Giles et al nested Janusiscus between Entelognathus and Ramirosuarezia, two basal chondrichthyans in the LRT.

Guiyu oneiros
(Zhu et al. 2009; late Silurian; V17914) is the oldest, but not the most primtive osteichthyan in the LRT. The revised restoration gives this taxon almost a sea-turtle like appearance, with large pectoral paddles directed posteriorly. Guiyu is basal to a clade of flattened, ray-like lobe-fins, includin Uranolophus and Janusicus, on one branch, Porolepis on another. all from the Devonian.

Uranolophus wyomingensis
(Denison 1968; FMNJ PF 3805, 3792; Early Devonian) is a large, flat-head porolepiform ancestor with large pterygoid micro-tooth plates. Several cranial bones are broken up, as in lungfish. The nasal was transverse. The lateral sides of the skull, premaxilla and maxilla have not been described, likely not preserved.

From the Giles et al 2015 abstract on Janusiscus:
“The phylogeny of Silurian and Devonian (443-358 million years ago [Ma]) fishes remains the foremost problem in the study of the origin of modern gnathostomes (jawed vertebrates).

That question was answered here earlier this year. Jaws are not preserved in Janusiscus.

“A central question concerns the morphology of the last common ancestor of living jawed vertebrates, with competing hypotheses advancing either a chondrichthyan-or osteichthyan-like model.

The LRT resolved that problem earlier using the last common ancestor method. Fossils with jaws go back in the fossil record at least to the Early Silurian, and that’s a derived taxon, Shenacanthus. That puts the phylogenetic genesis of toothless jaws back to the Ordovician. Late surviving, latest Silurian Bianchengichthys is the most primitive taxon in the LRT with jaws, and they are feeble. In the LRT there are many nodes between Bianchengichthys and Shenacanthus.

“Here we present Janusiscus schultzei gen. et sp. nov., an Early Devonian (ca. 415 Ma) gnathostome from Siberia previously interpreted as a ray-finned fish, which provides important new information about cranial anatomy near the last common ancestor of chondrichthyans and osteichthyans.

Phylogenetic bracketing indicates Janusiscus, preserved without fins, was a lobe-fin close to Porolepis and others listed in this blogpost.

“The skull roof of Janusiscus resembles that of early osteichthyans, with large plates bearing vermiform ridges and partially enclosed sensory canals. High-resolution computed tomography reveals a braincase bearing characters typically associated with either chondrichthyans (large hypophyseal opening accommodating the internal carotid arteries) or osteichthyans (facial nerve exiting through jugular canal, endolymphatic ducts exiting posterior to the skull roof) and lacking a ventral cranial fissure, the presence of which is considered a derived feature of crown gnathostomes. A conjunction of well-developed cranial processes in Janusiscus helps unify the comparative anatomy of early jawed vertebrate neurocrania, suggesting primary homologies in ‘placoderms’, osteichthyans and chondrichthyans.

The LRT tests other traits (none using µCT scans) and includes more taxa.

“Phylogenetic analysis further supports the chondrichthyan affinities of ‘acanthodians’, and places Janusiscus and the enigmatic Ramirosuarezia in a polytomy with crown gnathostomes.

Seven years later, guided and supported by many intervening discoveries, acanthodians are the sister clade to chondrichthyans and Ramirosuarezia is no longer an an enigma in the LRT.

“The close correspondence between the skull roof of Janusiscus and that of osteichthyans strongly suggests an extensive dermal skeleton was present in the last common ancestor of jawed vertebrates, but ambiguities arise from uncertainties in the anatomy of Ramirosuarezia. The unexpected contrast between endoskeletal structure in Janusiscus and its superficially osteichthyan-like dermal skeleton highlights the potential significance of other incompletely known Siluro-Devonian ‘bony fishes’ for reconstructing patterns of trait evolution near the origin of modern gnathostomes.

Adding taxa resolves all phylogenetic issues.

It is pure coincidence and happenstance
that Janusiscus, named for the Roman god, Janus, was featured today, the first day of the month honoring Janus: January. WordPress starts every new day at midnight on the International Date Line = noon the day before here in Illinois. In any and every time zone, Happy New Year everyone! Thank you for your readership, comments and questions.

References
Denison RH 1968. Early Devonian lungfishes from Wyoming, Utah and Idaho. Fieldiana, Geology 17:353–413.
Giles S, Friedman M and Brazeau MD 2015. Osteichthyan-like conditions in an Early Devonian stem gnathostome. Nature 520(7545):82–85.
Zhu M, Zhao W, Jia Lu J, Qiao T and Qu Q 2009. The oldest articulated osteichthyan reveals mosaic gnathostome characters. Nature 458:469-474.

wiki/Guiyu
wiki/Janusiscus
wiki/Uranolophus