Academic and publicity headlines:
“The most primitive osteichthyan braincase?”
“Neurocranial anatomy of an enigmatic Early Devonian fish shed light on early osteichthyan evolution.”
No. Far from it. Living moray eels (genus: Gymnothorax) have a much more primitive osteichthyan braincase than Ligulalepis (Fig. 1). A proper phylogenetic context (Fig. 2) is essential, but was not present in prior studies of this tiny fish due to taxon exclusion.
More publicity headlines:
“The ancestor of all bony fish has a surprising link to humans.”
“400-Million Year-Old Fish Discovered Is Ancestor of Dinosaurs, Humans”
In the LRT Ligulalepis was a fish with no descendants (Fig. 2). A long list of omitted fossil taxa, along with extant moray eels, lungfish and coelocanths were ancestral to dinosaurs and humans (Fig. 2), but that story was overlooked. Once again, taxon exclusion creates a phylogenetic misstep and a journalistic headline grab.
From the Wikipedia page:
“Ligulalepis was first described from isolated scales found in the Taemas-Wee jasper limestones of New South Wales (Early Devonian age) by Dr Hans-Peter Schultze (1968) and further material described by Burrow (1994). A nearly complete skull found in the same general location was described in Nature by Basden et al. (2000) claiming the genus was closely related to basal ray-finned fishes (Actinopterygii). In 2015 Flinders University student Benedict King found a more complete new skull of this genus which was formally described by Clement et al. (2018), showing the fish to be on the stem of all osteichthyans.”
(Schultze 1968; Basden et al. 2000; Clement et al. 2018; Late Silurian; Fig. 1) is a tiny partial skull considered basal to ray-fin fish. In the large reptile tree (LRT, 2006 taxa; subset Fig. 2) tiny Ligulalepis nests with slightly larger Early Devonian Kenichthys (FIg. 3), one of several tiny taxa related to larger Youngolepis and these are related to bottom-dwelling porolepiformes like the rounder Porolepis and the flatter Tungesnia, Guiyu and Stensioella (Fig. 4).
in the LRT (subset Fig. 2) lobe-finned porolepiformes (e.g. Porolepis) and the coelocanths (e.g. Latimeria) are NOT closely related to tetrapods or tetrapodomorphs. So don’t “Pull a Larry Martin” and focus on those lobed fins alone. Test all the traits. In the LRT lobed fins convergently appear several times in the ‘lobe fin’ clade — and lobes disappear to become ray fins, also by convergence with ray fin taxa. Traditionally omitted herring (Engraulis) and bichirs (Polypterus) are closer to tetrapods in the LRT.
Despite its antiquity
Ligulalepis is not close to the origin of bony fish in the LRT (FIg. 2). Rather it is one more highly derived Late Silurian taxon that documents a great radiation in the Silurian that has not been discovered in fossil strata yet.
Earlier studies with fewer taxa
found a basal split between lobe-fins and ray-fins. It’s more complicated than that. In the LRT porolepiformes converge with unrelated lungfish sharing similar external and internal nares. These taxa also converge with lobe-finned coelocanths in their lobe fins. Many of these taxa are low, slow, flat and small fish. Others are low, slow, fat and large fish. Many are known from fused skull bones that often don’t preserve the wide cheek bones (e.g. jugal, postorbital, squamosal, quadratojugal). Stensioella (Fig. 4) is a rare exception that preserves virtually the whole specimen, at least on one side.
What does the small size of Ligulalepis tell us?
Phylogenetic miniaturization usually retains juvenile traits (e.g. short rostrum, large eyes, small size) into adulthood. These changes can lead to novel clades. Presently no new clades arise from Ligulalepis. That can change. Presently Ligulalepis is tiny without descendants, just the opposite of what the headlines said back in 2018.
Basden AM et al. 2000. The most primitive osteichthyan braincase? Nature 403(6766): 185–188.
Burrow CJ 1994. “Form and function in scales of Ligulalepis toombsi Schultze, a palaeoniscoid from the Early Devonian of Australia”. Records of the South Australian Museum. 27 (2):175–185.
Chang M-M and Yu X-B 1981. A new crossopterygian Youngolepis praecursor gen. et sp. nove.form Lower Devonianof E. Yunna, China. Scientia Sinica 24:89–97.
Chang M and Zhu M 1993. A new Middle Devonian osteolepidid from Qujing, Yunnan. Mem. Assoc. Australas. Palaeontol. 15 183-198.
Clement AM et al. 2018. Neurocranial anatomy of an enigmatic Early Devonian fish shed light on early osteichthyan evolution. eLife 7. doi:10.7554/eLife.34349
Schultze H-P 1968. Palæoniscoidea-Schuppen aus dem Unterdevon Australiens und Kanadas und aus dem Mitteldevon Spitzbergens. Bulletin of the British Museum. 16: 343–368.
Zhu M and Ahlberg P 2004. The origin of the internal nostril of tetrapods. Nature 432:94-97.