Is the Septomaxilla the Anterior Lacrimal in Synapsids?

In basal reptiles, the lacrimal bone extends from the naris to the orbit. This bone includes the tear duct that empties into the nasal cavity. The tear duct is the former second choana (the exit) in osteolepid rhipidistian fish.

In derived reptiles the lacrimal retreats from the naris but remains connected to the orbit.

In basal synapsids, the lacrimal continues to rim the naris. Among sphenacodontids, Haptodus and Edaphosaurus have this trait, but Sphenacodon and Dimetrodon do not. The nasal grows down further, touching the maxilla, covering the lacrimal.

Among protodiapsids, like Heleosaurus and Mesenosaurus, the maxilla rises to block the lacrimal from the naris.

Ophiacodon and the Origin of the Therapsida

Figure 1. Click to enlarge. Ophiacodon and its phylogenetic successors, the pelycosaurs and the therapsids. Note the extent of the lacrimal in blue.

In Ophiacodon it is not clear from available data whether or not the maxilla rises to block or overlie the lacrimal, but in all therapsids, including Nikkasaurus at the base, the maxilla overlaps the lacrimal, but not completely. Near the naris a part remains visible. Traditionally it is called the septomaxilla. Near the orbit the larger portion of the lacrimal remains visible. The interesting thing is the so-called septomaxilla always lines up with the lacrimal with the maxilla laminated over a portion of it.

In mammals the tear duct continues to empty into the nasal cavity.

Addendum: In Stenocybus we can see the septomaxilla taking over for a retreating lacrimal. So they are not the same bone.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.