Ahlberg 2018 discusses Middle Devonian tetrapod footprints, Acanthostega and Ichthyostega

Ahlberg 2018 wrote:
“The hypothesis that tetrapods evolved from elpistostegids during the Frasnian, in a
predominantly aquatic context, has been challenged by the discovery of Middle Devonian tetrapod trackways predating the earliest body fossils of both elpistostegids and tetrapods.”

Figure 1. The Middle Devonian tetrapod tracks from 395 mya are 35 million years older than Ichthyostega, which could not walk like this on land.
Figure 1. The Middle Devonian tetrapod tracks from 395 mya are 35 million years older than Ichthyostega, which could not walk like this on land.

“Here I present a new hypothesis based on an overview of the trace fossil and body fossil evidence. The trace fossils demonstrate that tetrapods were capable of performing subaerial lateral sequence walks before the end of the Middle Devonian. The derived morphological characters of elpistostegids and Devonian tetrapods are related to substrate locomotion, weight support and aerial vision, and thus to terrestrial competence, but the retention of lateral-line canals, gills and fin rays shows that they remained closely tied to the water.”

“Elpistostegids and tetrapods both evolved no later than the beginning of the Middle Devonian. The earliest tetrapod records come from inland river basins, sabkha plains and ephemeral coastal lakes that preserve few, if any, body fossils; contemporary elpistostegids occur in deltas and the lower reaches of permanent rivers where body fossils are preserved. During the Frasnian, elpistostegids disappear and these riverine-deltaic environments are colonised by tetrapods. This replacement has, in the past, been misinterpreted as the origin of tetrapods”.

Despite their important discoveries,
Per Ahlberg and Jennifer Clack promoted the wrong taxa as basalmost tetrapods due to taxon exclusion.

Figure 2. Basal tetrapods in the LRT. All of these fish-like taxa are more primitive than Acanthostega and Ichthyostega. Note the small fins on Pandericthys matching the small limbs on the other equally flat taxa.

By adding taxa,
in the large reptile tree (LRT, 1840+ taxa; subset Fig. 1) Trypanognathus and the Dvinosauria plus Greererpeton as basalmost taxa. These flat, small-limbed taxa most closely resemble tetrapod outgroup taxa like Panderichthys and Elpistostege.

In the LRT
Acanthostega and Ichthyostega left no polydactyl descendants. Contra Ahlberg, polydactyly is not primitive, but highly derived in taxa heading back to a more aquatic existence.

Four fingers and five toes
is the primitive number of digits. Small limbs sprouting from a long low torso with a short tail is primitive for tetrapods, matching outgroup panderichthyids and eplistostegids.

Figure 1. Brindabellaspis skull from King et al. 2020. Colors added here. 
Figure 1. Brindabellaspis skull from King et al. 2020. Colors added here. 

Originally misidentified as a placoderm,
Brindabellaspis was an Early Devonian elpistostegid in the LRT. Unfortunately only the partial skull is known, so let’s keep looking in coeval strata for more basalmost tetrapods.

References
Ahlberg PE 2018. Early Vertebrate evolution. Follow the footprints and mind the gaps: a new look at the origin of tetrapods. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 2018: 1–23.

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