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The evolution of temnospondyl (amphibian) larvae

Friday, April 12, 2024 15:01

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In the LRT
(Fig 3) temnospondyls precede lissamphibians, including their extant representatives, frogs and salamanders. On another branch, temnospondyls also precede reptilomorphs (including Amphibamus, Fig 1) which lead to reptiles = amniotes.

Schoch and Witzmann 2024 report,
“Most taxa in which small specimens are preserved had aquatic larvae with external gills that superficially resemble larval salamanders.”

Figure 4. Late Carboniferous Amphibamus is a potential trackmaker for the Grand Canyon latest Early Carboniferous tracks. with medial digits the longest, like the trackmaker. ” data-image-caption=”

Figure 4. Late Carboniferous Amphibamus is a potential trackmaker for the Grand Canyon latest Early Carboniferous tracks. with medial digits the longest, like the trackmaker.

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2020/08/amphibamus588-1.jpg?w=259″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2020/08/amphibamus588-1.jpg?w=584″ class=”size-full wp-image-48017″ src=”https://pterosaurheresies.files.wordpress.com/2020/08/amphibamus588-1.jpg?w=584&h=677″ alt=”Figure 4. Late Carboniferous Amphibamus is a potential trackmaker for the Grand Canyon latest Early Carboniferous tracks. with medial digits the longest, like the trackmaker. ” width=”584″ height=”677″ srcset=”https://pterosaurheresies.files.wordpress.com/2020/08/amphibamus588-1.jpg?w=584&h=677 584w, https://pterosaurheresies.files.wordpress.com/2020/08/amphibamus588-1.jpg?w=129&h=150 129w, https://pterosaurheresies.files.wordpress.com/2020/08/amphibamus588-1.jpg?w=259&h=300 259w, https://pterosaurheresies.files.wordpress.com/2020/08/amphibamus588-1.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 1 Late Carboniferous Amphibamus is a potential trackmaker for the Grand Canyon latest Early Carboniferous tracks. with medial digits the longest, like the trackmaker.

The authors identified several ontogenetic paths for temnospondyls:

Basal taxa (edopoids, dvinosaurus, eryopiforms):
Skull developed faster than axis and and appendicular elements plus no drastic transformation.

Dissorophoids:
Skull developed slower than limbs.

Micromelerpeton:
long and steady metamorphosis.

Amphibamus:
(Fig 1) rapid metamorphosis.

The authors wrote,
“We distinguish three different types of metamorphosis (morphological, ecological and drastic) that evolved cumulatively in early tetrapods and within temnospondyls.”

The authors do not dive into frog and salamander metamorphosis.
Rather this paper is focused on temnospondyl larvae.

Schoch and Witzmann bring up an interesting set of phylogenetic questions

that were not delved into in their paper.

In the LRT
(Fig 3) basal members of Lissamphibia, Microsauria, Amphibamus and Seymouriamorpha are all amniote precursors. Silvanerpeton (Fig 2) is the last common ancestor of reptiles = amniotes. Gephyrostegus and a long list of traditional anamniotes are also on the amniote side of this divide in the LRT.

Is there an unrecognized connection or disconnection
between the two modes of reproduction and ontogeny? Embryo metamorphosis, whether inside or outside of the confines of the egg, appears to be a bifurcation, rather than a gradual evolution. So what happened ecologically to encourage this split? What environment made Silvanerpeton depart from and distinct from its ancestor, Amphibamus?

Embryos in reptile = amniote eggs swim in their own little ponds
within the waterproof amnion until more fully developed than amphibian larvae. Based on the presence of Latest Devonian Tulerpeton just outside the Reptilia = Amniota, all prior phylogenetic splits must be Devonian, not Carboniferous, where all the fossils are.

Figure 2. Eusauropleura to scale with ancestral and descendant taxa including Eucritta, Utegenia, Silvanerpeton and Gephyrostegus, the last common ancestor of all reptiles. ” data-image-caption=”

Figure 2. Eusauropleura to scale with ancestral and descendant taxa including Eucritta, Utegenia, Silvanerpeton and Gephyrostegus, the last common ancestor of all reptiles.

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2018/12/gephyrostegus_eusauropleura588.jpg?w=202″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2018/12/gephyrostegus_eusauropleura588.jpg?w=584″ class=”size-full wp-image-34503″ src=”https://pterosaurheresies.files.wordpress.com/2018/12/gephyrostegus_eusauropleura588.jpg?w=584&h=869″ alt=”Figure 2. Eusauropleura to scale with ancestral and descendant taxa including Eucritta, Utegenia, Silvanerpeton and Gephyrostegus, the last common ancestor of all reptiles.” width=”584″ height=”869″ srcset=”https://pterosaurheresies.files.wordpress.com/2018/12/gephyrostegus_eusauropleura588.jpg?w=584&h=869 584w, https://pterosaurheresies.files.wordpress.com/2018/12/gephyrostegus_eusauropleura588.jpg?w=101&h=150 101w, https://pterosaurheresies.files.wordpress.com/2018/12/gephyrostegus_eusauropleura588.jpg?w=202&h=300 202w, https://pterosaurheresies.files.wordpress.com/2018/12/gephyrostegus_eusauropleura588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 2. Eusauropleura to scale with ancestral and descendant taxa including Eucritta, Utegenia, Silvanerpeton and Gephyrostegus, the last common ancestor of all reptiles. Note the loss of lumbar ribs and the addition of gastralia, perhaps to hold larger eggs in gravid females.

The aforementioned reptile = amniote taxa
lose dorsal ribs in the lumbar region (Fig 2). This seems to be (= a guess) useful for storing larger eggs inside the mother’s body, eventually creating a noticeable bulge in certain living gravid lepidosaurs, offering yet another line of protection for the otherwise helpless eggs/embryos. Larger amniote eggs, held longer within the mother, provide their own ‘pond’ for the completion of development = metamorphosis prior to and after maternal egg-laying.

Perhaps the mother became the pond in the first amniotes = reptiles.
Perhaps Late Devonian to Early Carboniferous reptiles = amniotes were viviparous. If true, then only later did certain reptiles make nests and lay developing eggs. If true, then vivparity in some clades is a reversal of a reversal.

Figure 2. Subset of the LRT focusing on basal tetrapods (aka amphibians). ” data-image-caption=”

Figure 2. Subset of the LRT focusing on basal tetrapods (aka amphibians).

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/04/lrt_basal_tetrapod_cladogram588.jpg?w=143″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/04/lrt_basal_tetrapod_cladogram588.jpg?w=490″ class=”size-full wp-image-85303″ src=”https://pterosaurheresies.files.wordpress.com/2024/04/lrt_basal_tetrapod_cladogram588.jpg?w=584&h=1222″ alt=”Figure 2. Subset of the LRT focusing on basal tetrapods (aka amphibians). ” width=”584″ height=”1222″ srcset=”https://pterosaurheresies.files.wordpress.com/2024/04/lrt_basal_tetrapod_cladogram588.jpg?w=584&h=1222 584w, https://pterosaurheresies.files.wordpress.com/2024/04/lrt_basal_tetrapod_cladogram588.jpg?w=72&h=150 72w, https://pterosaurheresies.files.wordpress.com/2024/04/lrt_basal_tetrapod_cladogram588.jpg?w=143&h=300 143w, https://pterosaurheresies.files.wordpress.com/2024/04/lrt_basal_tetrapod_cladogram588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 3. Subset of the LRT focusing on basal tetrapods (aka amphibians).

The authors cite Schoch and Milner 2014 when they write,
“The origin of the temnospondyls remains rather obscure, as all other tetrapod groups are separated from them by wide morphological gaps.”

By contrast in the LRT (Fig 3) the origin of temnospondyls is clear and precise. Even so, the LRT is a hypothesis that requires confirmation, refutation or modification by independent workers with a similar taxon list, repeating the experiment.

References
Schoch RR and Witzmann F 2024. The evolution of larvae in temnospondyls and the stepwise origin of amphibian metamorphosis. Biological Reviews doi: 10.1111/brv.13084

Source: https://pterosaurheresies.wordpress.com/2024/04/13/the-evolution-of-temnospondyl-amphibian-larvae/

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