The vertebrate ‘new head’ is an evolutionary novelty dominantly generated by the neural crest and the cranial sensory placodes – embryonic tissues originating within the ectodermal borders of the neural plate. The anterior most placode is the adenohypophyseal placode, which later in development forms the anterior lobe of the pituitary – the major hormonal control organ of the vertebrate body. In all extant jawed vertebrates, the pituitary is of dual developmental origin, composed of the anterior adenohypophyseal lobe and a posterior neurohypophysis. The neurohypophysis is derived from the ventral evagination of the hypothalamus. The adenohypophyseal placode is initially incorporated into the ectoderm of the roof of the oral cavity. Later in embryogenesis, it invaginates to form an epithelial fold – the Rathke’s pouch – and connects with the neurohypophysis. It is widely accepted that the adenohypophyseal placode is unpaired in both jawless (where it is fused with the nasal placode to form a nasohypophyseal placode) and jawed forms. The division of the originally single nasal placode is considered a synapomorphy of jawed vertebrates.
The current phylogenetic consensus divides the extant jawed vertebrates (gnathostomes) into two major clades – Chondrichthyes and Osteichthyes. The greatest uncertainty arises in placing the fossil early gnathostomes relative to the osteichthyan–chondrichthyan node. The present majority view is that all ‘placoderms’ are stem gnathostomes arranged in a paraphyletic series with ‘acanthothoracids’ near the bottom and ‘maxillate placoderms’ at the top. All ‘acanthothoracids’ share an extended precerebral trabecular region – so-called ‘upper lip’ – a feature in some respects intermediate between the jawless and jawed conditions, which anchors their basal position on the tree. However, our research on the dentition of ‘acanthothoracids’ demonstrates that they differ fundamentally from the ‘placoderm’ dentitions, which had been thought to characterise stem gnathostomes, and instead resemble the dentitions of crown gnathostomes. The character similarity suggests that osteichthyans could be rooted close to the ‘acanthothoracids’, but in the consensus phylogeny they are widely separated.
Recently, we made an important anatomical discovery concerning the hypophyseal area of the ‘acanthothoracid’ genus Radotina. Its palate displays paired adenohypophyseal cavities separated from the neurohypophysis and opening directly into the mouth cavity. Our subsequent comparative study of hypophyseal cavities from tomographic data of selected early vertebrate fossils belonging to both gnathostome stem and crown groups shows a completely unexpected pattern. The character distribution provides evidence for an ancestral superficial and paired adenohypophysis and, by extension, paired adenohypophyseal placodes. Interestingly, the hypophyseal cavities of the stem chondrichthyan Gydoselache are almost indistinguishable from the morphology observed in most studied ‘acanthothoracids’. On the contrary, the ‘acanthothoracid’ Kosoraspis (which has the most osteichthyan-like dentition and upper jaw morphology among ‘acanthothoracids’) resembles the condition in the stem osteichthyan Ligulalepis – implying a major convergence between crown osteichthyans and chondrichthyans. This observation may have radical implications on the construction of the gnathostome tree with the chondrichthyan lineage branching off very early after the emergence of jaws. This configuration is supported by the fossil record and is in accordance with the proposed evolution of macromery.