Molnar et al. 2021: Forelimb function across the fish–tetrapod transition

Molnar et al. 2021
bring us a deep and complex look into the hypothetical muscles (based on muscle scars) of Eusthenopteron (Fig. 1), Acanthostega (Fig. 1) and Pederpes. The authors compare these distinctly different taxa to “show that early tetrapods share a suite of characters including restricted mobility in hurmerus long-axis rotation, increased muscular leverage of humeral retraction, but not depression/adduction, and increased mobility in elbow flexion-extension.” The authors infer the earliest ‘steps’ in tetrapod forelimb evolution were related to limb-substrate interactions. Weight support appeared later.

Figure x. The fin to finger transition in the LRT with the addition of Elpistostege.

Figure 1. The fin to finger transition in the LRT with the addition of Elpistostege.

Unfortunately, 
without a valid phylogenetic context, what these authors deliver is not quite germane to the topic of their headline. The actual fin-to-finger transition occurred between Panderichthys (Figs. 1, 2) and the extremely similar Trypanognathus (Figs. 1, 2). The former had fins. The latter had fingers and toes. Otherwise they were very much alike.

Molnar et al. looked at the wrong taxa. Neither Panderichthys nor Trypanognathus are mentioned in the Molnar et al. text.

What can we conclude given
the similarities and differences of Panderichthys and Trypanognathus?

  1. Small fins and limbs at the transition were incapable of weight bearing
  2. Elbows and knees were incapable of bending, pushing, pulling
  3. Torso much longer than tail, lots of flexible ribs
  4. Low, wide, flexible torso at the transition provided serpentine locomotion
  5. Little risk of tipping over due to low center of gravity
Figure 6. Dorsal and ventral views of Panderichthys and several basal tetrapods demonstrating the low, flat skulls and bodies with small limbs and relatively straight ribs.

Figure 2. Dorsal and ventral views of Panderichthys and several basal tetrapods demonstrating the low, flat skulls and bodies with small limbs and relatively straight ribs.

Molnar et al. conclude:
“Together, these results suggest that competing selective pressures for aquatic and terrestrial environments produced a unique, ancestral “early tetrapod” forelimb locomotor mode unlike that of any extant animal.”

Not really. Consider the moray eel chasing crabs on land without fins or fingers. Click the pic to view video on YouTube. David Attenborough is the narrator.

Now put predator and prey in a Devonian swamp setting,
with lots of growing and rotting vegetation and no rocky place to find safety. Note (in figure 2), the rather slow phylogenetic growth of the limbs relative to the torso in this sequence. Other lineages did their own thing in their own time. Ossinodus, for instance (Fig. 1), had a shorter torso and longer limbs, and was a phylogenetic ancestor to Ichthyostega and Acanthostega.


References
Molnar JL, Hutchinson JR, Diogo R, Clack JA and Pierce SE. 2021. Evolution of forelimb musculoskeletal function across the fish-to-tetrapod transition. Science Advances 2021; 7: eabd7457 22 January 2021

Latest fins-to-fingers paper stumbles due to taxon exclusion

Dickson et al. 2020 bring us their views
on the transition from fins to feet at the base of the Tetrapoda.

Unfortunately,
taxon exclusion (Fig. 1), once again, mars this study published in Nature.

Figure 1. Cladogram from Dickson et al. 2020 with an overlay indicating taxa found in the LRT and key LRT taxa with limbs lacking in Dickson et al. 2020.

Figure 1. Cladogram from Dickson et al. 2020 with an overlay indicating taxa found in the LRT and key LRT taxa with limbs lacking in Dickson et al. 2020.

Cherry-picking taxa in Dickson et al. 2020
nested Ichthyostega and Acanthostega as the first taxa to have fingers and toes. These two were highly promoted in earlier works that included co-author, Jennifer Clack, but that should not excuse the exclusion of pertinent taxa. When more taxa are added to a cladogram (Fig. 2), these two famous basal tetrapods, with their large, well-formed limbs, nest not as transitional taxa, but as derived taxa leaving no descendants (subset Fig. 2). Their polydactyl extremities were evolutionary dead-end experiments perhaps reflecting one of the first returns to a more aquatic existence.

Employing a wider gamut of taxa,
in the large reptile tree (LRT, 1766 taxa; subset Fig. 2), pre-tetrapods with flat morphologies and small fins, like Panderichthys (Fig. 3) had only four finger buds. Strongly similar taxa, like the late-surviving basalmost tetrapod, Trypanognathus, likewise had a flat morphology, small limbs and only four fingers. This finger number is typical of basal tetrapods until the advent of finger 5 in five unrelated clades (Fig. 2) leaving no living descendants, except for reptilomorphs, beginning with Tulerpeton, Utegenia and kin, the clade that ultimately evolved reptiles, mammals, primates and humans.

Figure 4. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Figure 2. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Figure 6. Dorsal and ventral views of Panderichthys and several basal tetrapods demonstrating the low, flat skulls and bodies with small limbs and relatively straight ribs.

Figure 3. Dorsal and ventral views of Panderichthys and several basal tetrapods demonstrating the low, flat skulls and bodies with small limbs and relatively straight ribs.

Adding taxa 
resolves this problem and many others. Excluding taxa only perpetuates traditional myths and hobbles present research.


References
Dickson BV, Clack JA, Smithson TR et al. 2020. Functional adaptive landscapes predict terrestrial capacity at the origin of limbs. Nature (2020). https://doi.org/10.1038/s41586-020-2974-5

SVP abstracts 17: Pederpes is a junior synonym for Whatcheeria

Otoo et al. 2020 bring us
a new reconstruction of Whatcheeria (Figs. 1, 2), evidently updated from a 2018 abstract by the same authors (less one).

Figure 1. Whatcheeria fossil.

Figure 1. Whatcheeria fossil.

From the Otoo et al. abstract:
“The early tetrapod Whatcheeria is represented by hundreds of specimens from the Mississippian Delta locality (Iowa, U.S.A.). Research on the postcranial anatomy allows a full-body reconstruction to be produced for the first time. The ribcage is strongly regionalized, with long anterior trunk ribs bearing large uncinate processes, and short posterior trunk ribs. The girdles and limbs are massive; in particular, the processes of the humerus are very large, and imply bulky forelimb and shoulder musculature, especially relating to the retraction of the forelimb. The cervical region is elongated and the tail is reduced in length relative to contemporary tetrapods such as embolomeres and colosteids.”

Whatcheeria nests in the large reptile tree (LRT, subset Fig. 3) alongside Pederpes (Fig. 4). The two share all traits scored in the LRT and are coeval in the Early Carboniferous.

Figure 2. Whatcheeria skull.

Figure 2. Early Carboniferous Whatcheeria skull.

More from the Otoo et al. abstract:
“The resulting proportions are more similar to terrestrial taxa such as Seymouria and Eryops. These taxa also share with Whatcheeria robust humeri with large processes, large olecranon processes, large scapular blades, and regionalized ribcages. Such similarities suggest convergent life habits, with an anteriorly stiffened trunk to increase the effectiveness of the powerful forelimbs and reduce lateral motion of the body.”

Figure 4. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Figure 3. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

More from the Otoo et al. abstract:
“We hypothesize that Whatcheeria represents an independent experiment in appendicular-dominated locomotion, with improved ability to explore terrestrial environments The large (>2 m maximum) body size of Whatcheeria is larger than most Mississippian tetrapods, particularly those for which there is the most compelling evidence of terrestriality (e.g., Balanerpeton, Westlothiana). Aquatic locomotion may have been accomplished by bottom-walking, or rowing with the forelimbs.”

“Our new data include additional synapomorphies between Whatcheeria and Pederpes, and suggest that the latter is a juvenile.

Whatcheeria and Pederpes nest together in the LRT (Fig. 4). Of 235 traits, none differ between the two. Based on scale bars the two are identical in size, with 10cm measuring the snout to the posterior orbit on both. Pederpes (Clack 2002) is thus a junior synonym for Whatcheeria (Lombard and Bolt 1995). Hmmm. Wonder how this one got away from the experts over the last 18 years. Whatcheeria entered the LRT in 2017, so I had three years to see this, too.

Figure 3. Pederpes is a basal taxon in the Whatcheeria + Crassigyrinus clade.

Figure 4. Early Carboniferous Pederpes is a basal taxon in the Stegocephalia.

More from the Otoo et al. abstract:
These data contribute to a new diagnosis for the Whatcheeriidae and a reassessment of material and taxa referred or compared to the family; significantly, Ossinodus is not a whatcheeriid and represents a distinct morphotype.

The LRT (subset Fig. 3, Fig. 5) agrees with this.

Figure 2. Ossinodus, Pederpes were more primitive than the more aquatic Icthyostega.

Figure 5. Ossinodus,is more primitive than the more aquatic Icthyostega. Pederpes is more derived, but close. The black areas of Ossinodus are known. The rest is restoration.

More from the Otoo et al. abstract:
“However, these data do move Whatcheeria crownward in phylogenetic analyses. Rather, our findings highlight the disparity of stem tetrapods, and emphasizes Whatcheeria’s status as an early-diverging experiment in a morphology later revisited by crown tetrapods.”

The LRT (subset Fig. 3) does not agree with this conclusion. Ossinodus (Fig. 5) nests basal to both stegocephalians (including Whatcheeria) and crown tetrapods. It is the most basal tetrapod with substantially larger limbs than those of basalmost tetrapods like Trypanognathus. Ichthyostega and Pederpes are taxa leaving no Permian and Mesozoic descendants.


References
Ahlberg PE and Milner AR 1994. The origin and early diversification of tetrapods. Nature 368, 507-514.
Clack JA 2002. Gaining Ground: The origin and evolution of tetrapods. Indiana University Press.
Clack JA 2002. An early tetrapod from ‘Romer’s Gap’. Nature. 418 (6893): 72–76. doi:10.1038/nature00824
Lydekker R 1890. On two new species of labyrinthodonts. Quarterly Journal of the Geological Society, London 46, 289-294.
Lombard RE and Bolt, J.R 1995. A new primitive tetrapod, Whatcheeria deltae, from the Lower Carboniferous of Iowa. Palaeontology 38(3):471–495.
Otoo B, Bolt J, Lombard E and Coates M 2020. A new reconstruction of Whatcheeria and the ecomorpholigcal disparity of early tetrapods. SVP abstracts 2020.
Otoo BK, Bolt JR, Lombard E 2018. A leg up: Whatcheeria and its new contributions to tetrapod anatomy. SVP abstracts.
Panchen AL 1991. The early tetrapods: classification and the shapes of cladograms in: Origins of the Higher Groups of Tetrapods: Controversy and Consensus. Eds. Schultze HP and Trueb L. Comstock Publishing Associates, Cornell University Press, Ithaca and London.

wiki/Pederpes
wiki/Whatcheeria

https://pterosaurheresies.wordpress.com/2018/10/30/svp-2018-new-whatcheeria-data-from-nearly-100-specimens/

SVP abstracts 13: Tiny Tiktaalik-like tetrapod

Lembert et al. 2020 bring us
a much smaller Tiktaalik-like tetrapod.

From the Lembert et al. abstract:
“The elpistostegalian stem-tetrapod Tiktaalik roseae (Fig. 1) is known from a single locality (NV2K17) within the Fram Formation of Ellesmere Island, Nunavut Territory, Canada. Specimens from this locality represent subadult to adult specimens, including specimens up to 61% larger than the holotype specimen (NUFV 108) and reaching an estimated 3 meters in length.”

Figure 3. Tiktaalik specimens compared to Ossinodus.

Figure 1. Tiktaalik specimens compared to Ossinodus.

“Here we present fossil material of a much smaller elpistostegalian specimen (NUFV 137) from a second, slightly older locality within the Fram Formation on Ellesmere Island (NV0401), possibly representing a juvenile T. roseae specimen or a new taxon.”

Not mentioned in this abstract, tiny Koilops (Figs. 2, 3) nests basal to Tiktaalik in the large reptile tree (LRT, 1251 taxa).

Figure 2. Koilops is a flat-headed sister to Spathicephalus, but with teeth, larger orbits and a shorter snout

Figure 2. Koilops is a flat-headed sister to Spathicephalus, but with teeth, larger orbits and a shorter snout

Figure x. The fin to finger transition in the LRT with the addition of Elpistostege.

Figure 3. The fin to finger transition in the LRT with the addition of Elpistostege.

Continuing from the Lembert et al. abstract:
“Preserved remains of NUFV 137 include fragmentary lower and upper jaws, gular plates, fragments of the rostrum, articulated body scales, articulated pectoral fin elements, and several other currently unidentified endoskeletal pieces. Linear proportions between homologous landmarks of lower jaws of NUFV 137 and NUFV 108 suggest an animal approximately 61% smaller than the holotype of T. roseae, and, with a reconstructed total jaw length of approximately 12.4 cm, NUFV 137 is similar in size to one of the smallest known elpistostegalian taxa (Rubrognathus kuleshovi).”

Taxon exclusion has evidently excluded the even smaller Koilops (Fig. 2) from the Lembert et al. studies.

“If NUFV 137 represents a juvenile T. roseae individual, it would expand the known size range of T. roseae specimens, with implications for understanding allometric growth in a tetrapodomorph taxon.

Figure 4. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Figure 4. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Continuing from the Lembert et al. abstract:
“While lower jaw characters appear to be similar to those in T. roseae, it is uncertain if some features, such as a posteriorly displaced postsplenial pit line, reduced adsymphyseal dentition, and varying postcranial proportions, are the result of differences in ontogeny or warrant a separate taxonomic grouping. These differences, and the presence of a potential operculum, indicate NUFV 137 might represent a distinct but similar, Tiktaalik-like taxon.”


References
Lemberg JB, Stewart TA, Daeschler E and Shubin NH 2020. Tomography of a tantalizingly tiny Tiktaalik-like taxon. SVP abstracts 2020.

Woltering et al. 2020 study genes to elucidate finger origins

Woltering et al. 2020
attempted to elucidate the transition from fins to fingers by studying the genes of extant lungfish, which don’t have fingers and their ancestors never had fingers.

From the abstract
“How the hand and digits originated from fish fins during the Devonian fin-to-limb transition remains unsolved.

No. The large reptile tree (LRT; subset Fig. 1) solved that problem in 2019 following the work of Boisvert, Mark-Kurik and Ahlberg 2008. These authors found four finger buds on Panderichthys. Thereafter four fingers appear on all basalmost tetrapods in the LRT, like Trypanognathus (Fig. 2), a taxon found in Carboniferous strata with Middle Devonian origins. Taxon exclusion is once again the problem here.

Late Devonian taxa with supernumerary digits, like Acanthostega and Ichthyostega, are the traditional ‘go-to’ taxa for the fin-to-finger transition. That was supplanted in 2019 by phylogenetic analysis in the LRT (subset Fig. 1). Simply adding taxa recovers Acanthostega and Ichythyostega as terminal taxa. They have more derived skulls and bodies sporting larger limbs and more digits.

Figure 4. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Figure 4. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Woltering et al. 2020 report,
“Controversy in this conundrum stems from the scarcity of ontogenetic data from extant lobe-finned fishes. We report the patterning of an autopod-like domain by hoxa13 during fin development of the Australian lungfish, the most closely related extant fish relative of tetrapods.”

In other words, Woltering et al. looked at genes in lungfish that never had digits.

Figure 6. Dorsal and ventral views of Panderichthys and several basal tetrapods demonstrating the low, flat skulls and bodies with small limbs and relatively straight ribs.

Figure 2. Dorsal and ventral views of Panderichthys and several basal tetrapods demonstrating the low, flat skulls and bodies with small limbs and relatively straight ribs at the fin-to-finger transition. Acanthostega and Ichythyostega have more derived bodies with larger limbs and more digits.

Why study lungfish
when we have fossil taxa (Fig. 3) in the lineage of tetrapods? Why study genes when genomic studies produce false positives in deep time? Taken together the Woltering et al. study seems like a waste of effort on both fronts, but they didn’t realize this at the time. Paleontologists love genomics like Isaac Newton loved alchemy.

Figure 3. Forelimb of several basal tetrapods rearranged to more closely fit the LRT. Four fingers turns out to be the primitive number. Five is a recent mutation. Six was a short-lived experiment in Tulerpeton.

Figure 3. Forelimb of several basal tetrapods rearranged to more closely fit the LRT. Four fingers turns out to be the primitive number. Five is a recent mutation. Six was a short-lived experiment in Tulerpeton.

Woltering et al. 2020 report,”
“Differences from tetrapod limbs include the absence of digit-specific expansion of hoxd13 and hand2 and distal limitation of alx4 and pax9, which potentially evolved through an enhanced response to shh signaling in limbs. These developmental patterns indicate that the digit program originated in postaxial fin radials and later expanded anteriorly inside of a preexisting autopod-like domain during the evolution of limbs. Our findings provide a genetic framework for the transition of fins into limbs that supports the significance of classical models proposing a bending of the tetrapod metapterygial axis.”

Be wary of genetic studies over deep time. They have been shown to deliver false positives way too often to be trusted, or even attempted. Fossils and phenomic studies are better in all respects because they recover cladograms in which all taxa demonstrate a gradual accumulation of derived traits.


References
Boisvert CA, Mark-Kurik E and Ahlberg PE 2008.
 The pectoral fin of Panderichthys and the origin of digits. Nature 456:636–638.
Woltering JM et al. (5-co-authors) 2020. Sarcopterygian fin ontogeny elucidates the origin of hands with digits. Science Advances 6(34): eabc3510 DOI: 10.1126/sciadv.abc3510
https://advances.sciencemag.org/content/6/34/eabc3510

Grand Canyon tetrapod tracks: odd, or misinterpreted?

Short summary for those in a hurry.
There are several reasons to think only the original interpretation (Fig. 1) is odd.

Rowland, Caputo and Jensen 2020 bring us their interpretation
of an odd 313mya trackway (Figs. 1–3) from the latest Early Carboniferous (Pennsylvanian) in Grand Canyon National Park (AZ, USA).

Figure 1. Animation of the interpretation of Rowland, Caputo and Jensen 2020 of the Grand Canyon Early Carboniferous trackmaker.

Figure 1. Animation of the interpretation of Rowland, Caputo and Jensen 2020 of the Grand Canyon Early Carboniferous trackmaker.

From the abstract
“We report the discovery of two very early, basal-amniote fossil trackways on the same bedding plane in eolian sandstone of the Pennsylvanian Manakacha Formation in Grand Canyon, Arizona. 

Only trackway 1 (Figs. 1–3) is under review here. And that may be more than one trackway.

It displays a distinctive, sideways-drifting, footprint pattern not previously documented in a tetrapod trackway. We interpret this pattern to record the trackmaker employing a lateral-sequence gait while diagonally ascending a slope of about 20˚, thereby reducing the steepness of the ascent.”

Only one interpretation was provided by the authors. Here you’ll see one more.

“These trackways are the first tetrapod tracks reported from the Manakacha Formation and the oldest in the Grand Canyon region. The narrow width of both trackways indicates that both trackmakers had relatively small femoral abduction angles and correspondingly relatively erect postures.”

Is this the correct interpretation? Another (Fig. 2) is presented.

“They represent the earliest known occurrence of dunefield-dwelling amniotes―either basal reptiles or basal synapsids―thereby extending the known utilization of the desert biome by amniotes, as well as the presence of the Chelichnus ichnofacies, by at least eight million years, into the Atokan/Moscovian Age of the Pennsylvanian Epoch.”

“The depositional setting was a coastal-plain, eolian dunefield in which tidal or wadi flooding episodically interrupted eolian processes and buried the dunes in mud.”

Could these tracks be interpreted more parsimoniously?
What if the trackmaker was just an ordinary lissamphibian, like Celtedens (Figs. 2, 6) or a reptilomorph, like Amphibamus (Fig. 4)? Both have more of a matching manus and pes than any coeval amniote (details below). What if the trackmaker had a more sinuous spine, similar to that of coeval tetrapods and Celtedens or Amphibamus? What if the trackmaker had sprawling limbs, similar to other coeval tetrapods and Celtedens or Amphibamus? The trackmaker did not leave a tail drag mark, so what if the trackmaker had a short tail, like Celtedens or Amphibamus? What if there were multiple trackmakers? Is it possible that one or two trackmakers closely followed another one in a mating ritual or pursuit?

Figure 2. The Chelichnus-like tracks together with Celtedens, an amphibian trackmaker with a short tail, sinuous spine, splayed limbs and fewer digits than coeval amniotes.

Figure 2. The Chelichnus-like tracks together with Celtedens, an amphibian trackmaker with a short tail, sinuous spine, splayed limbs and fewer digits than coeval amniotes. The unused tracks would have been created by a pursuing Celtedens-like trackmaker.

If so, 
here’s an animation based on an alternate taxon walking in a more typical fashion (Fig. 2) with less freehand invention. More splayed limbs and a sinuous spine are employed here matching coeval tetrapods in morphology and gait. In this scenario the unused tracks (Fig. 2) were created by a second and third Celtedens-like trackmaker pursuing in lock step with the first trackmaker.

Figure 3. Imagery from Rowland, Caputo and Jensen 2020, with color overlays and PILs added.

Figure 3. Figure from Rowland, Caputo and Jensen 2020, with color overlays and PILs added at left.

From the first line of the Introduction
Amniotes evolved early in the Pennsylvanian or late in the Mississippian Epoch.”

The authors were so sure the tracks were made by amniotes
they plugged the word “Amniotes” into the first line of the Introduction. In the large reptile tree (LRT, 1725+ taxa) we have several amniotes (= reptiles) from the EARLY Mississippian (Viséan). These were overlooked by Rowland, Caputo and Jensen 2020. The authors and those they cited were not up to date with the most recent phylogenic hypotheses of interrelationships.

Still on the subject of amniotes, the authors note,
“Because this trackway records the presence of relatively long digits with acuminate claws, we infer that the trackmaker was an amniote.” The Early Cretaceous lissamphibian, Celtedens (Figs. 2, 5) and the Late Carboniferous reptilomorph, Amphibamus (Fig. 4), also have long, slender digits with claws that taper to a point. The longest digits are medial on each manus and pes. Amniotes had more asymmetrical extremities with digit 4 typically the longest. The authors followed their initial bias and did not consider morphologically similar, but phylogenetically dissimilar trackmakers.

The keyword “Lissamphibian”
is not found in the Rowland, Caputo and Jensen text. Celtedens is a lissamphibian known only from two Early Cretaceous specimens. However, given the presence of related Gerobatrachus, Apteon and Doleserpeton specimens in the Early Permian, the radiation of Celetedens-like taxa was likely in the Carboniferous. The Late Carboniferous basal reptilomorph, Amphibamus, is likewise not mentioned and was not considered a potential trackmaker despite its appropriate match both morphologically and temporally.

The manus and pes of the Grand Canyon trackmaker 
were nearly equal in size. The pes in Carboniferous amniotes is typically larger than then manus. The authors agree, noting, “the manus prints of the Manakacha tracks are not conspicuously smaller than the pes prints, contrary to the typical pattern in Chelichnus.” Celtedens also has a pes that is larger than the manus, but the lissamphibians Apteon, Doleserpeton and Triassurus have subequal extremities. So does the reptilomorph, Amphibamus (Fig. 4).

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. 

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.

The digits of the trackmaker
were relatively symmetrical with digits 2, 3 and 4 making impressions. The digits in Carboniferous amniotes are typically asymmetrical with 4 the longest and largest. The authors note, “Chelichnus tracks typically consist of only three or four digits of a pentadactyl trackmaker.” 

Taxonomic affinity of the trackmaker
The authors report, “Impressions of three digits are present in each track (Figs 4 and 6), however no plausible Pennsylvanian candidate trackmaker taxon was tridactyl. Thus, we interpret the prints to be shallow undertracks made by a pentadactyl animal whose lateral digits were not impressed deeply enough into the sediment to translate into the preserved bedding plane. Without impressions of all five digits on each foot we are unable to measure foot slenderness and other characters that are useful for distinguishing among the tracks of various basal amniote taxa.”

Figure 4. Microbrachis slightly revised with a new indented supratemporal here rotated to the lateral side of the skull above the squamosal and quadratojugal. Otherwise this image is from Carroll, who did not indent the supratemporal.

Figure 5. Microbrachis slightly revised with a new indented supratemporal here rotated to the lateral side of the skull above the squamosal and quadratojugal. Otherwise this image is from Carroll, who did not indent the supratemporal.

Yes, three digits is a little unsettling for a tetrapod trackmaker. 
The Middle Pennsylvanian microsaur, Microbrachis (Fig. 5), had a three digit manus and a five digit pes with #1 and #5 smaller than the medial digits, but these were mere vestiges, unable to support the animal on a terrestrial substrate.

Figure 3. Celetendens is the closest relative to Karaurus in the LRT.

Figure 6. Celetendens is the closest relative to Karaurus in the LRT.

Celtedens and Amphibamus had four fingers and five toes. 
So, they are not a perfect match for the Grand Canyon trackmaker, but they are close, at least one finger closer than any coeval amniote. Early Cretaceous Celtedens (Fig. 5) is too small to be the trackmaker. However, two hundred million years separate the two. On the other hand, Amphibamus (Fig. 4) is a better size match to the trackmaker and much closer temporally/stratigraphically.

The authors note, 
“a lateral-sequence gait is the most parsimonious footfall-sequence interpretation that is compatible with the pattern of tracks in this trackway. Tetrapods, in fact, routinely use a lateral-sequence gait when walking slowly; while one foot is off the ground, this gait provides a larger stable triangle than other footfall sequences. Moreover, a lateral-sequence gait facilitates undulations of the spine, which lengthen the step.” 

Actually, the diagram provided by Rowland, Caputo and Jensen minimizes undulations of the spine and the steps are not lengthened, but shortened. What they inadvertently describe is the more parsimonious and typical movement of the lissamphibian Celtedens presented here (Fig. 2).

“As indicated by expulsion rims adjacent to many of the tracks (Figs 4,5B,5C and 6), interpreted to occur on the downhill side, the trackmaker’s body was oriented straight up the slope.”

In figure 2 the Celtedens-like tetrapod also ascends the hill, but diagonally, taking big steps, not tiny lateral steps.

“Fossil trackways that record diagonal movement on the slope of a sand dune are common in the ichnology literature.”

“Francischini et al. documented an occurrence within the Permian eolian Coconino Sandstone of Arizona in which the angle of progression of an Ichniotherium trackway―inferred to have been made by the diadectid reptiliomorph Orobates ―differs markedly from the angle that the feet were pointing, similar to the case documented here in the Manakacha Formation. However, none of such previously documented cases of a tetrapod moving diagonally across the face of a sand dune record such a regular pattern of impressions of all four feet, as does Trackway 1 described here, and none have been interpreted to record a lateral-sequence gait.”

The possibility of two or three trackmakers in quick succession creating trackway 1 did not occur to the authors of this paper.

The possibility of an Amphibamus-like or Celtedens-like trackmaker did not occur to the authors of this paper. Instead they went straight for an imagined, headline-generating anachronistic atypical trackmaker, a taxon not present in coeval strata walking unlike any known taxon past or present.

Best to go with Occam’s razor and maximum parsimony.

Add taxa, especially when matching tracks to trackmakers, to make sure you don’t overlook more obvious matches.

Add pursuing trackmakers if there are too many tracks for one ordinary trackmaker.


References
Rowland SM, Caputo MV and Jensen ZA 2020. Early adaptation to eolian sand dunes by basal amniotes is documented in two Pennsylvanian Grand Canyon trackways. PLoSONE 15(8): e0237636. https://doi.org/10.1371/journal.pone.0237636

The first four citations found in Rowland, Caputo and Jensen 2020:
Ahlberg PE and Milner AR 1994. The origin and early diversification of tetrapods. Nature 1994; 368: 507–514.
Clack JA 2002. Gaining Ground: the origin and evolution of tetrapods. Bloomington: Indiana University Press.
Benton MJ. 2005. Vertebrate Palaeontology. 3rd ed. Blackwell Science.
Ford DP and Benson RBJ 2020. The phylogeny of early amniotes and the affinities of Parareptilia and Varanopidae. Nature Ecology & Evolution 2020; 4: 57–65.

Genesis of air breathing in basal tetrapods

The genesis of limbs and toes 
from lobes and fins gets most of the publicity in transitional fish-tetrapods.

Today we look at the less popular transition
from water breathing with gills to air breathing with a nose and lungs.

Like most fish,
Onychodus (Fig. 1) drew in oxygenated water by opening its mouth. At this moment, the gill covers are closed to prevent backdraft. Closing the mouth and raising the basihyal (medial bone between the mandibles) until it presses against the solid palate reduces the mouth volume, pushing that mouthful of  water posteriorly past the gills where oxygen and carbon dioxide are transferred. At this time the gill covers are open to permit that water to exit, completing the cycle. The dual nares have nothing to do with respiration at this point, only olfaction, with water passively entering the anterior opening and passively exiting the posterior opening (Fig. 1). The air bladder arising from the gut tube anterior to the stomach is not involved in respiration at this stage.

Among lobefin fish,
coelacanths, like Latimeria, have this primitive system.

Figure 1. Onychodus is typical of most fish having dual external nares strictly for olfactory sensing. Gill covers are part of the respiratory apparatus.

Figure 1. Onychodus is typical of most fish having dual external nares strictly for olfactory sensing. Gill covers are part of the respiratory apparatus.

Among lobefin lungfish (Late Silurian to present),
like Kenichthys (Fig. 2), Youngolepis, Polypterus (the extant bichir) and Howidipterus, oxygen-poor water, supplemented by gulps of dry air, once again enters the mouth and is passed back over the gills and out the gill covers. Both the incurrent and excurrent nares migrate ventrally. (Not sure why.) Worthy of a Nature article, the excurrent opening is parked on the jaw margin between the premaxilla and maxilla in Kenichthys, so half the excurrent exited outside the mouth, while the other half exited inside the mouth (see ventral view in Fig. 2), all passively. (Not sure why this migration took place either, except that with the lips sealed inhalation and exhalation can still take place… slowly… in and out of both openings, perhaps to retain mouth moisture during aestivation (hibernation in dry mud.) Note the pinprick size of each opening.

Figure 1. Kenichthys Images from Zhu and Ahlberg 2004, colors added. The authors made a convincing argument that Kenichthys represented a transitional taxon between Youngolepis and Eusthenopteron. Note the lack of vomer fangs and a distinctly different set of skull sutures in Kenichthys, which does not nest with Eusthenopteron in the LRT.

Figure 2. Kenichthys Images from Zhu and Ahlberg 2004, colors added. The authors made a convincing argument that Kenichthys represented a transitional taxon between Youngolepis and Eusthenopteron. Note the lack of vomer fangs and a distinctly different set of skull sutures in Kenichthys, which does not nest with Eusthenopteron in the LRT.

Among basal lobefin crossopterygians (Early to Late Devonian),
like Gogonasus, Eusthenopteron, and elongate, flattened Cabonnichthys, Elpistostege, Tiktaalik and Panderichthys the tiny excurrent nasal opening just barely enters the rim of the mouth cavity and is thereafter considered a choana. The tiny external incurrent opening is thereafter considered a naris. Based on their tiny sizes, both remain useless for respiration. Large gill covers and a solid palate are retained for traditional water respiration supplemented by dry air gulping as needed.

Figure 4. Panderichthys palates. Note the lateral line below the naris is not continuous, contra Lombard and Bolt.

Figure 3. Panderichthys palates. Note the lateral line below the naris is not continuous, contra Lombard and Bolt.

When the gill covers disappear in fossil taxa
that signals the genesis of air-breathing from mouth to paired air bladders (now called ‘lungs’) rather than past the disappearing gills. According to the LRT, this occurred twice (if we don’t count the ontogenetic transformation of juvenile tadpoles (with gills) to adult frogs (with lungs) and other similar basal tetrapods).

In clade one: primitive Koilops retained and operculum (gill cover). Derived, but lobe-finned Tiktaalik and Spathicephalus did not have an operculum.

In clade two: weak limbed, four-fingered Trypanognathus (Fig. 4), Deltaherpeton, Collosteus, PholidogasterGreererpeton and Ossinodus, all lacked an operculum.

Figure 2. Animation of air-breathing in basal tetrapods with weak lungs inflated by contraction and expansion of the throat sac, rather than gill irrigation powered by the reduced here buccal bones.

Figure 4. Animation of air-breathing (tidal ventilation) in basal tetrapods with weak lungs inflated by contraction and expansion of the throat sac, rather than gill irrigation powered by the reduced ceratobranchials, still present at right. Air-tight nose flaps had to be present in order for this system to work. 

Clade two exceptions: Robust-limbed, eight-fingered Acanthostega (Fig. 5) and Ichthyostega retained tiny gill covers (operculum) as adults. And they had primitive tiny nares and choana, still not suitable for air-breathing. These convergent exceptions are here considered reversals due to a suite of derived traits nesting these two famous taxa apart from more primitive tetrapods and apart from each other in the LRT.

Figure 2. the MGUH VP 8160 specimen attributed to Acanthostega. Note the many similarities to Ymeria.

Figure 5. the MGUH VP 8160 specimen attributed to Acanthostega. Note the many similarities to Ymeria. Note the spiracle openings surrounded by the supratemporals. This provides an accessory respiration opening, convergent with bottom-dwelling skates and rays from the shark clade.

The signal that air-breathing respiration through the nostrils had begun
(Fig. 4) is when the nares and choana of fossil taxa enlarge to handle the larger volume of tidal ventilation coming through them. The nares also migrate higher on the skull so that they are at least partly visible in dorsal view. The internal nares are fully inside the mouth, which must be able to seal shut to divert air through the nares, rather than leaking past the lips. Gill covers are absent. Air-tight nose flaps had to be present in order for this system to work. The pterygoids reduce and retreat posteriorly (Fig. 4), creating large, pliable openings in the formerly solid palate (Fig. 3), expanding the potential volume of the mouth.

According to the LRT,
(subset Fig. 5) the enlargement and migration of the nares and choana occurred several times because several clades of derived basal tetrapods retained tiny lateral nares and choana despite having fully developed limbs.

Figure 3. Subset of the LRT focusing on basal tetrapods and their narial openings.

Figure 5. Subset of the LRT focusing on basal tetrapods and their narial openings.

Dorsal ribs
Basal tetrapods depend on an expanding and contracting the gular sac for tidal ventilation of the lungs, mimicking their lobe-finned ancestors. These same basal tetrapods (Fig. 6) were all low and wide with relatively straight, laterally-oriented ribs incapable of expanding and contracting the torso and lungs. Not until dorsal ribs elongated and started curling around the inside of an increasingly round (in cross-section) torso where they able to expand and contract the volume of the torso and the lungs inside. In that way mobile ribs gradually replaced a mobile throat sac for tidal ventilation.

Figure 6. Dorsal and ventral views of Panderichthys and several basal tetrapods demonstrating the low, flat skulls and bodies with small limbs and relatively straight ribs.

Figure 6. Dorsal and ventral views of Panderichthys and several basal tetrapods demonstrating the low, flat skulls and bodies with small limbs and relatively straight ribs, all to scale. Note the brevity of the tail in thee taxa.

The irony is
we know of Ichthyostega-grade tetrapods walking on land in the Middle Devonian. By that I mean, we know of tetrapods with relatively large limbs and supernumerary digits capable of elevating the belly off the substrate. Phylogenetic analysis indicates the trackmaker was a mouth-breather with tiny lateral nares. This was a short-lived experiment (as far as we know at present) leaving only Late Devonian descendants, like Icthyostega, that disappeared by the Early Carboniferous.

The longer lasting clade,
the one that produced all the other tetrapods including reptilomorphs, living amphibians and microsaurs, all had a long, low, flat body and skull with smaller 4-fingered limbs not capable of elevating the belly off the substrate, like Greererpeton and Trimerorhachis (Fig. 6). Only later, and by convergence did descendants rise off their belly with stronger limbs, mimicking those pioneer Middle Devonian tetrapod trackmakers.


References
Schoch RR and Voigt S 2019. A dvinosaurian temnospondyl from the Carboniferous-Permian boundary of Germany sheds light on dvinosaurian phylogeny and distribution. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2019.1577874.xxx

This blogpost comes not in response to a new academic paper, but to revisiting some of the taxa in the the large reptile tree (LRT, Figs. 5, 6) at this transition. Thanks to reader Dave M for the impulse to reexamine these taxa.

 

 

 

Elpistostege enters the LRT between fins and fingers

Cloutier et al. 2020 bring us
a new complete specimen of Late Devonian Elpistostege (Fig. 1) along with CT scans of the manus-in-the-lobefin revealing finger buds inside (Fig. 2). Despite these buds, Elpistostege had no elbows (Fig. 2) and was too large (relative to its small posteriorly-oriented fins) to support itself above the substrate. That would come later. Meanwhile it could have wriggled ashore, insured from tipping over by its wide body and low center of gravity.

Figure 1. Elpistostege from Cloutier et al. 2020 in situ, traced and reconstructed.

Figure 1. Elpistostege from Cloutier et al. 2020 in situ, traced and reconstructed.

Cloutier et al. focused on the fingers,
providing an external view and CT scans, colored like a diagram (Fig. 2). I added pastel colors to the four fingers that extend beyond the more numerous nascent metacarpals. Even at this early stage, parallel interphalangeal lines (PILs) can be drawn where phalanges work in sets during extension and flexion.

Figure 2. Elpistostege manus/fin from Cloutier et al. 2020, rotated. Diagram at right with colors and PILs added.

Figure 2. Elpistostege manus/fin from Cloutier et al. 2020, rotated. Diagram at right with colors and PILs added. Only four digits are present primordially.

The skull presents a bit of a tracing challenge
as the rostrum is largely missing here (Fig. 3).

Figure 3. Elpistostege skull from Cloutier et al. 2020 with colors added. Note much of the rostrum is lost.

Figure 3. Elpistostege skull from Cloutier et al. 2020 with colors added. Note much of the rostrum is lost.

The large reptile tree
(LRT, 1658+ taxa, subset Fig. 4) tests more taxa than the traditional cladogram published in Cloutier et al. 2020. They nest Panderichthys as the more basal taxon. In the LRT Panderichthys is more derived (also see Fig. 5). Missing from Cloutier et al. are Spathicephalus (a sister to Tiktaalik) and Koilops, a neotonous form basal to Elpistostege and the rest of the Tiktaalik clade. In the LRT that’s an offshoot clade from the main line of fin to fingers taxa. Trypanognathus has the phylogenetically first known fingers, but that LRT hypothesis of relationships has been overlooked in favor of the multi-digit taxa, Acanthostega and Ichthyostega.

Figure 4a. Subset of the LRT focusing on basal tetrapods. Note the displaced positions of Acanthostega and Ichthyostega.

Figure 4a. Subset of the LRT focusing on basal tetrapods. Note the displaced positions of Acanthostega and Ichthyostega.

Figure 4b. Traditional cladogram produced by Cloutier et al. 2020. Compare to Figure 4a.

Figure 4b. Traditional cladogram produced by Cloutier et al. 2020. Compare to Figure 4a.

Traditional transitional taxa,
like Tiktaalik, Acanthostega and Ichthyostega, are phylogenetically shifting off to the side at various nodes in the LRT. Other taxa that never get talked about (Fig. 4) are more in the main line of tetrapod evolution leading to reptiles. Cloutier et al. seem to be perpetuating the traditional myth that basal tetrapods had more than five fingers (corrected in Fig. 5). The evidence shows that four is the primitive number. We looked at basal tetrapod finger evolution earlier here. and here.

Figure 5. Manus/Fin diagram from Cloutier et al. The phylogenetic order is wrong and Tulerpeton is not related (in the LRT). Greererpeton is substituted. 4 fingers is the primitive number.

Figure 5. Manus/Fin diagram from Cloutier et al. Two frames change every five seconds. The phylogenetic order is wrong and Tulerpeton is not related (in the LRT). Greererpeton is substituted. 4 fingers is the primitive number. Note the shrinking ulnare  in the revised version. Tiktaalik is off the main line of tetrapod evolution.

Elpistostege watsoni (Westoll 1938, Schultze and Arsenault 1985, Cloutier et al. 2020 is another flathead fish transitional to tetrapods, but off toward the Tiktaalik + Spathicephalus clade, derived from Koilops. Some tiny fingerbones are present in the lobefin. This fish is over a meter in length. It is the last taxon in this lineage with an anal fin. The radius continues to be as long as the ulna + ulnare with the intermedium separating them distally.

Figure x. The fin to finger transition in the LRT with the addition of Elpistostege.

Figure x. The fin to finger transition in the LRT with the addition of Elpistostege.

References
Cloutier R, Clement AM, Lee MSY, Noël R, Béchard I, Roy V and Long JA 2020. Elpistostege and the origin of the vertebrate hand. Nature https://doi.org/10.1038/s41586-020-2100-8
Schultze H-P and Arsenault M 1985. The panderichthyid fish Elpistostege: a close relative of tetrapods? Palaeontology 28, 293–309 (1985).
Swartz B 2012. A marine stem-tetrapod from the Devonian of Western North America. PLoS ONE. 7 (3): e33683. doi:10.1371/journal.pone.0033683
Westoll TS 1938. Ancestry of the tetrapods. Nature 141, 127–128 (1938).

wiki/Elpistostege

Sciam.com article on Elpistostege

Recalibrating clade origins, part 5

Earlier
we looked at the first part, second part, third part and fourth part of Marjanovic’s 2019 chronological recalibration of vertebrate nodes. Today we conclude.

Batrachia (Caudata + Salientia) = Amphibians (= extant Frogs + Salamanders) Marjanovic discusses Triadobatrachus from the Early Triassic (Olenekian, 249mya) and concludes that 249 mya “is a perfectly adequate hard minimum age for this calibration point.” And “290mya may be a defensible soft maximum value.”

After adding Triadobatrachus (Fig. 1) to the large reptile tree (LRT, 1631+ taxa), Gerobatrachus (Early Permian, 300mya) mentioned once by Marjanovic, nests basal to the few tested extant frogs and salamanders. So, 300mya is pretty close to his estimate of 290mya.

Figure 1. Triadobatrachus skull, as originally colorized and redone with tetrapod colors here.

Figure 1. Triadobatrachus skull, as originally colorized and dorsal surface redone with tetrapod colors here with bone fusion identified and the maxilla + premaxilla restored. Compare to Rana in figure 2. Distinct from extant frogs, a squamosal is present here.

Figure 2. Skull of the frog, Rana with colors matching those of Triadobatrachus.

Figure 2. Skull of the frog, Rana with colors matching those of Triadobatrachus. Here the squamosal and jugal are missing. The quadratojugal is present.

Chondrichthyes (Holocephalii + Elasmobranchii)
= (ratfish + sharks and skates) Marjanovic reports, “By current understanding (Frey et al., 2019), the oldest known crown-chondrichthyan is the stem elasmobranch Phoebodus fastigatus from the middle Givetian. The Givetian, part of the Middle Devonian, …so I propose 385 Ma as the hard minimum age of the chondrichthyan crown-group.”

By contrast, the LRT recovers the whale shark + manta ancestor, Loganellia (Early Silurian, 440mya; Fig. 3) as the oldest known ancestor of sharks and other fish. Sturgeons are more primitive, and therefore must be older, but Ordovician sturgeon and osteostracan fossils have not been found. Taxon inclusion recovers these novel interrelationships.

Figure 2. Loganellia, a thelodont with whale shark shape including dorsal fin. Image from OldRedSandstone.com. This appears to be Loganellia, not Thelodus (Fig. 7).

Figure 2. Loganellia, a thelodont with a whale shark shape including dorsal fin. Image from OldRedSandstone.com. This appears to be Loganellia, not Thelodus.


Marjanovic reports, “There is not as much interest in phylogeny among specialists of early elasmobranchs than among specialists of early mammals or early dinosaurs.” The LRT does not have that problem. Enigmas are answered with this powerful tool that works well by avoiding tooth-only taxa.

Marjanovic considers the clade Batoidea (skates + rays) to be monophyletic.

In the LRT, so far, three origins for ray-like basal tetrapods have been recovered based on taxon inclusion.

Marjanovic considers the clade Neopterygii (Holosteomorpha + Teleosteomorpha) = (bowfins and gars + other teleosts or bony fish) to be monophyletic.

In the LRT, this hypothesis of relationships has been invalidated.

Marjanovic reports, “I cite 228 references for calibration purposes.”

In the LRT, I’m not sure how many citations I cite for 1631+ taxa, but once again, adding taxa brings new insights to hypothetical interrelationships. Marjanovic was testing the results of a previous publication, but should have done so with a greater authority, with more taxa and with no reference whatsoever to genomics.

Tomorrow: something new. 


References
Marjanovic D 2019. Recalibrating the transcriptomic timetree of jawed vertebrates.
bioRxiv 2019.12.19.882829 (preprint)
doi: https://doi.org/10.1101/2019.12.19.882829
https://www.biorxiv.org/content/10.1101/2019.12.19.882829v1

Late Devonian origin of four-fingered hand revisited and tweaked

Earlier we looked at the origin of fingers
in basal tetrapods (Fig. 1). The primitive number then, as now, was 4 fingers, 5 toes.

Figure 1. Graphing the presence of fingers and toes in basal tetrapods, updated today with the addition of 4 digits in Panderichthys.

Figure 1. Graphing the presence of fingers and toes in basal tetrapods, updated today with the addition of 4 digits in Panderichthys. Sharp-eyed readers will note the switching of Panderichthys with the Tiktaalik clade here.

Unfortunately,
I overlooked a paper (Boisvert, et al., 2008) that found four proto-digits in the lobefin of Panderichthys (Fig. 2) and provided good data for the Tiktaalik manus that I did not have. With those corrections, a quick review is in order.

Figure 1. From Boisert et al. 2008, colors added. This is their ordering for the evolution of manual digits. Compare to figure 2.

Figure 2. From Boisert et al. 2008, colors added. This is their ordering for the evolution of manual digits. Compare to figure 3 where Panderichthys and Tiktaalik switch places and several taxa are inserted transitional to Acanthostega.

For some reason, fingers are rarely preserved in basal tetrapods,
but most continue to have four. Proterogyrinus (Fig. 2) is an early exception with five. So is Acanthostega (Fig. 2) with eight. See chart above  (Fig. 1) for all tested taxa in the LRT.

Tradition holds that eight is a primitive number,
later reduced to five or four. The large reptile tree (LRT, 1590 taxa, subset Fig. 1) flips that around. Eight is a derived number on a terminal taxon (Acanthostega) leaving no descendants. The primitive number is four (subset Fig. 1).

Boisvert, Mark-Kurik and Ahlberg 2008 used a CT-scanner
to find four proto-digits on the manus of Panderichthys (Fig. 2) and compared those to the traditional basal tetrapod taxa: Eusthenopteron, Tiktaalik and Acanthostega. Note the big phylogenetic leap they show between Tiktaalik and Acanthostega. And note the apparent reversal in Tiktaalik as the metacarpal seems to revert to a ray. These problems are corrected in figure 3.

Figure 3. Forelimb of several basal tetrapods rearranged to more closely fit the LRT. Four fingers turns out to be the primitive number. Five is a recent mutation. Six was a short-lived experiment in Tulerpeton.

Figure 3. Forelimb of several basal tetrapods rearranged to more closely fit the LRT. Compare to figure 2. As discovered here earlier, four fingers turns out to be the primitive number. Five is a recent mutation. Six was a short-lived experiment in Tulerpeton.

The large reptile tree
(LRT, 1590 taxa) recovers a different topology (subset Fig. 1). In the LRT Acanthostega is not a transitional taxon, but an aberrant one returning to a more aquatic lifestyle and leaving no descendants. On the other hand, the four digits in Panderichthys are retained by a wide variety of basal tetrapods. The number jumps to five with the addition of a lateral digit in only a few taxa (Fig. 1). Importantly, in Utegenia (Fig. 4) we see the most primitive appearance of five digits in our lineage despite its late appearance in the fossil record. More derived, but earlier, Late Devonian Tulerpeton had six fingers representing a failed experiment leaving no descendants in the lineage of reptiles, represented by Silvanerpeton in the Early Carboniferous, also pre-dating Utegenia.

So frogs did not lose a finger.
They retained the four that Panderichthys provided them. Four, not five or eight, is the primitive number of digits for basal tetrapods, as discovered earlier here in the LRT. Let me know if there was an earlier discovery for this hypothesis of interrelationships and I will promote that citation.

Occasionally
a salamander will have six fingers. We’ll look at that strange case soon.

Figure 5. Utegenia diagram showing five fingers on each hand. This is the most primitive taxon in our lineage to have all five.

Figure 4. Utegenia diagram showing five fingers on each hand. This is the most primitive taxon in our lineage to have all five.

In the course of this study
I learned that the Tiktaalik clade and Panderichthys needed to switch places on the LRT. This has been updated in most cases (Fig.1).

Figure 2. Tulerpeton manus and pes in situ, reconstructed by Lebdev and Coates 1995 and newly reconstructed here.

Figure 5. Tulerpeton manus and pes in situ, reconstructed by Lebdev and Coates 1995 and newly reconstructed here.

Evidently Boisvert et al. were using
an outdated tree topology and did not recognize the problem that arose between the digits presented by Tiktaalik and those presented by Panderichthys (Fig. 2). Of course that puts Tiktaalik cousins, Koilops and Spathicephalus in the lobefin grade (Fig. 1), lacking fingers. Both currently lack post-cranial data, but were originally thought to be tetrapods.


References
Boisvert CA, Mark-Kurik E and Ahlberg PE 2008. The pectoral fin of Panderichthys and the origin of digits. Nature 456:636–638.