From the Jones et al. 2021 In brief:
“Jones et al. disprove the long-held idea that mammal backbone evolution involved a transition from reptile-like lateral bending to sagittal bending.”
Unfortunately, the study was conducted without a proper phylogenetic context. Outgroups included three salamanders (taxa unrelated to reptiles in the large reptile tree (LRT, 1811+ taxa).
Diadectes (Fig. 1) was cherry-picked as a ‘stem amniote‘ and ‘the ancestral condition for amniotes‘, but in the LRT Diadectes is a deeply nested lepidosauromorph amniote (= reptile) derived from Milleretta and not far from limnoscelids, pareiasaurs, procolophonids and turtles. This is a traditional mistake still taught at the university level due to taxon exclusion. At least two of the co-authors are from Harvard. So, kids, don’t go there. Harvard is not up-to-date!
In their results section
Jones et al. report, “there is less phylogenetic signal than expected under a Brownian motion model of evolution and that vertebral shape varies substantially within clades.”
That’s because they did not employ enough pertinent taxa. More taxa = more understanding of reptile phylogeny, just like a larger mirror collects more light and increases resolution in telescopes.
The whole concept of a transition from lateral undulation
to dorso-ventral undulation in the synapsid ancestors of mammals has been known for several decades. It was even included in Peters 1991, and not as an original hypothesis.
From the Jones et al. abstract:
“We show that the synapsid adaptive landscape is different from both extant reptiles and mammals, casting doubt on the reptilian model for early synapsid axial function, or indeed for the ancestral condition of amniotes more broadly. Further, the synapsid-mammal transition is characterized by not only increasing sagittal bending in the posterior column but also high stiffness and increasing axial twisting in the anterior column. Therefore, we refute the simplistic lateral-to-sagittal hypothesis and instead suggest the synapsid-mammal locomotor transition involved a more complex suite of functional changes linked to increasing regionalization of the backbone.”
This was well-known decades ago.
Given this hypothesis, where do Jones et al. draw the transition zone?
Jones et al. indicate that dinocephalian synapsids walked like lizards by matching tracks (Fig. 1) and citing Smith 1993. They should have looked at dinocephalians more closely to see if Smith 1993 was correct. Turns out Smith 1993 was either incorrect or inaccurate.
This purported dinocephalian trackway
(Fig. 2, Smith 1993, Jones et al. 2021) does not match the hypothetical trackmaker (Fig. 2). The Smith 1993 trackway is so narrow the thumb prints overlapping the midline, something sprawling dinocephalians were unable to replicate (Fig. 3). That should have been checked, not just used ‘as is’ by Jones et al. 2021.
In similar fashion, too often workers use prior cladograms without checking for veracity. Science should be all about testing, checking, verifying. Too often it is about borrowing, trusting, accepting.
Perhaps a better trackmaker
can be found for the Smith 1993 track in a larger relative to Hipposaurus (Fig. 4), a basal therapsid with the required 1) narrow pectoral girdle, 2) long slender limbs and 3) extremities that match the narrow-gauge tracks in size and configuration.
Jones et al. 2021 discuss cervical, dorsal and lumbar regionalization,
without reporting that regionalization begins with Gephyrostegus (Fig. 5), a basalmost amniote (= reptile) in the LRT. This amphibian-like reptile was not mentioned by Jones et al. 2021. Smaller Diplovertebron (Fig. 5), a basal archosauromorph reptile, inherited and emphasized this regionalization.
Regionalization of the vertebral column
diminishes in some lepidosauriformes (Fig. 6) reaching a minimum in snakes. Regionalizaton increases in some owenettids, macrocnemids (including fenestrasaurs and pterosaurs) and iguanids.
regionalization did not diminish as much, as shown by Ophiacodon (Fig. 7) a basal synapsid in the lineage of therapsids.
a basal archosauromorph diapsid, Archaeovenator (Fig. 8) reduces regionalization to a minimum. Lepidosauromorph turtles minimize lateral undulations when they evolve a carapace. So regionalization comes and goes.
One good reason for a lack of ribs in the lumbar region
was to make room for larger amniote eggs in the Earliest Carboniferous that even today greatly distends the abdomen of gravid lizards (Fig. 9).
Giving credit where credit is due
Jones et al. measured and graphed vertebral dimension across a wide swath of taxa, many closely related to one another. Expanding the taxon list to a wider gamut might have helped them see beyond the synapsids, a clade already well-studied for this factor.
Jones KE, Dickson BV, Angielczyk and Pierce SE 2021. Adaptive landscapes challenge the ‘‘lateral-to-sagittal’’ paradigm for mammalian vertebral evolution. Current Biology https://doi.org/10.1016/j.cub.2021.02.009
Peters D 1991. From the Beginning – The story of human evolution. Wm Morrow.
Smith RM 1993. Sedimentology and ichnology of floodplain paleosurfaces in the Beaufort Group (Late Permian), Karoo sequence, South Africa. Palaios 8, 339–357.