The origin of jaws in the placoderm clade

The jawless taxon Rhinopteraspis (above) demonstrates the jawless condition in pre-gnathostomes like Poraspis (below). The mobile oral elements are animated.

The genesis of jaws in Qilinyu (middle) demostrates the transition from parasagittal elements in jawless pre-gnathostomes to lateral elements in basal gnathostomes by the disappearance of all but the anterior tips of the mobile oral elements, now tentatively joined to one another, but still moving in a limited arc.

The jaw evolves a greater arc and the margin of the oral cavity (= mouth) moves anteriorly by shrinkage of the nasal elements in phylogenetically miniaturized Bianchengichthys (bottom). Whether the mandible elements were jointed strongly or not cannot be determined from the tiny fossil and its gracile mandible. Either way, it was still fragile.

Note: The origin of stem-tetrapods did not go through these steps nor these taxa. See Birkenia for that evolutionary lineage.

Poraspis brevis (Kiaer 1930; Klaer and Heintz 1935; Late Silurian to Early Devonian, 410mya; 10.5cm) is traditionally considered a member of the Heterostraci. Here Poraspis nests basal to Qilinyu, a basal member of the Placodermi. Both share spike-like fins at the genesis of fins for this clade.

Qilinyu rostrata (Zhu et al. 2016; Late Silurian, 419mya) nested close to Arandaspis and Manta in the LRT. This small basal placoderm had a pointed rostrum and a small face with cranial armor split from the thorax armor, as in placoderms. The large 'forehead' posterior to the eyes housed the gill basket atrium with excurrent water exiting at the new cranial-thoracic split. The ventral nostrils were likely blind pockets. The oral cavity included transverse 'jaws' without an anterior arc. The torso armor protected the small, nascent pectoral fins and tiny pelvic fins. Labial cartilages (= external quadrates, red) were preserved lateral to the early hyomandibulars (dark green) likely due to taphonomy. Those labial cartilages are replaced here to their in vivo positions to match those in Manta and Rhinoptera.

Note: The ventral view was taken prior to the further excavation of the matrix seen at the top of this page.

Brindabellaspis stensioi (Young 1980; King B, Young GC and Long JA 2018; Early Devonian) was originally considered a strange sort of long-rostrum placoderm. Here it nests with a very primitive, short-rostrum placoderm, Qilinyu. Skull proportions have changed, plus the thorax shield is reduced. Enlargeing prefrontals separate the frontals.