New basal vertebrates added to the lineage of humans

The long line of taxa closest to the lineage of humans
(Fig. 1; genus: Homo) are particularly fascinating because they show how we (both writers and readers) came to acquire all our big and little bits and pieces.

Figure 4. Subset of the LRT updated with new basal vertebrates.

Figure 1. Subset of the LRT updated with new basal vertebrates. Now you can see why the origin of tetrapods has been controversial as lungfish and sarcopterygians are both offshoots of the main line.

Figure x. Subset of the LRT focusing on fish and other basal vertebrates.

Figure x. Updated subset of the LRT focusing on fish and other basal vertebrates with more taxa

With the expansion
of the current taxon list deep into the Silurian, several non-traditional human ancestor taxa have made it to ‘The List’ (Figs. 1–3) in the large reptile tree (LRT, 1489 taxa).  At the same time, many former favorites have dropped off ‘The Traditional List’ because taxon inclusion has shown them to be off-shoot taxa basal to other non-human lineages. It’s a bushy family tree, but only one lineage leads to humans, and it no longer includes Osteolepis and Eusthenopteron. Note the gradual accumulation of traits from one sister to another.

Figure 1. Basal vertebrates in the LRT closest to the lineage that gave rise to tetrapods and ultimately humans. This is a slow-moving, low-slung, flat-headed bunch. Swifter, rounder taxa, better suited to open waters are not on this list.

Figure 2. Basal vertebrates in the LRT closest to the lineage that gave rise to tetrapods and ultimately humans. This is a slow-moving, low-slung, flat-headed bunch. Swifter, rounder taxa, better suited to open waters are not on this list.

Polypterus, the extant bichir,
(Fig. 2) is a bit out of synch with the rest of the gradual flow, but then again, we’re comparing an extant taxon with Silurian and Devonian sisters. Evolution never stops. And no other taxon nests better at that node. Same for Clarias, the walking catfish, which looks more like its sisters when you take the skin off and just compare the skulls and skeletons.

Figure 2. Basal tetrapods in the LRT closest to the lineage of humans. Note the continuation of slow, low-slung, flat-head taxa crawling onto land after crawling along the sea and lake bottoms.

Figure 3. Basal tetrapods in the LRT closest to the lineage of humans. Note the continuation of slow, low-slung, flat-head taxa crawling onto land after crawling along the sea and lake bottoms. Only with the appearance of long legs and long digits do tetrapods assume a rounder, less flat morphology.

Sometimes it’s better to step back
and get a wider view to see the overall patterns emerging in evolution… and then dive into the details.

Traditional tetrapod family trees,
like Clack et al. 2019 (Fig. 4) still include taxa like Eusthenopteron, Ichthyostega and Acanthostega. These three taxa are no longer on the list of taxa closest to the lineage of humans in the LRT.

Figure 6. Cladogram of basal vertebrates by Clack et al. 2019. Compare to figure 1.

Figure 4. Traditional, not confirmed cladogram of basal vertebrates by Clack et al. 2019. Compare to figure 1.

Worse yet,
when you take a look at the details in the Clack team cladogram, there are many fatal mismatches, like big-limbed and flat Ossinodus with tiny-limbed and eel-like Crassigyrinus.  And look at all that loss of resolution at several nodes. The Clack team seems to not know that Silvanerpeton is the last common ancestor of all reptiles. Perhaps this is so because they do not include enough reptiles in their cladogram to make this apparent. Eusthenopteron made their list (by tradition, not testing), but Stensioella, Cabonnichthys and Tinirau did not. These are key taxa in the lineage of higher vertebrates that need to be included.


References
Clack JA, Ruta M, Milner AR, Marshall JEA, Smithson TR and Smithson KZ 2019. Acherontiscus caledoniae: the earliest heterodont and durophagous tetrapod. Royal Society Open Science 6: 182087. http://dx.doi.org/10.1098/rsos.182087

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