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.

 

 

 

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.

Parmastega: not “a sister group to all other tetrapods”

Beznosov, Clack, Lusevics, Ruta and Ahlberg 2019
describe a new Russian basal ‘tetrapod’ Parmastega (Fig. 1), based on a nearly complete skull and pectoral girdle. Dated at 372 million years ago, or 12 million years before Acanthostega (Fig. 1), Ichthyostega and Tulerpeton, this taxon offers insights into the acquisition of basal tetrapod traits.

Parmastega was described as
“a sister group to all other tetrapods.” Of eight published analyses in this paper, most nest Paramastega close to Ventastega (Fig. 1).

Unfortunately,
the Beznosov et al. taxon list includes only the traditional tetrapods typically tested in studies like this and Eusthenopteron. Their taxon list omits many, if not most of the taxa listed in yesterday’s list of taxa transitional between jawless fish and reptiles.

Figure 1. Parmastega compared to scale with Acanthostega and Ventastega.

Figure 1. Parmastega compared to scale with Acanthostega and Ventastega. Both are similar to Parmastega in most regards. The placement of the naris in Ventastega might not have been ventrally, on the jawline, but higher on the snout as shown here.

By contrast,
the large reptile tree (LRT, 1587 taxa) nests Parmastega so close to Acanthostega. As reported yesterday and earlier, a larger taxon list indicates that Acanthostega is not a taxon transitional between fins and feet, but is a derived tetrapod apparently returning to a more aquatic niche. This runs counter to traditional hypotheses put forth by co-authors Ruta, Clack and Ahlberg, the top experts in this niche of paleontology.

A concave rostrum,
elevated orbits and large size mark it as a derived taxon, despite its antiquity. The intertemporal is fused to the supratemporal.

The post-cranial skeleton of Parmastega
was described as “weakly ossified”. That is in direct contrast to Acanthostega and virtually all other taxa in the LRT.

The chronology offered by Parmastega
supports the hypothesis of a radiation of tetrapods much earlier, with the few fossils found in the Late Devonian representing late-surviving radiations of that radiation.


References
Beznosov PA, Clack JA, Lufsevics E, Ruta M and Ahlberg PE 2019. Morphology of the earliest reconstructable tetrapod Parmastega aelidae. Nature 574:527–531.

 

Tetrapod evolution without Ichthyostega and Acanthostega

Two recent papers,
(Clack 2009, Long et al. 2018, Figs. 1, 2), included traditional cladograms of tetrapod evolution ranging from taxa with fins to taxa with legs. Both included Ichthyostega and Acanthostega, taxa traditionally considered essential to any discussion of taxa documenting the transition from fins to legs.

Figure 1. Modified from Clack 2009 showing the taxa in the transition from fins to feet.

Figure 1. Modified from Clack 2009 showing the taxa in the transition from fins to feet.

The two studies do not have the same taxon list.
In Clack 2009 (Fig. 1) Panderichthys is a penultimate most basal taxon. In Long et al. 2018  (Fig. 2) Panderichthys is nearly a penultimate most derived taxon.

From Long et al. 2018, their cladogram of taxa in the transition from fins to feet.

Figure 2. From Long et al. 2018, their cladogram of taxa in the transition from fins to feet.

By contrast
the large reptile tree (LRT, 1586 taxa; subset Fig. 3), which employs many more pertinent taxa, nests Ichthyostega and Acanthostega distinctly off the main line leading from jawless Silurian fish to amniotes (= reptiles) and relegates them to the sidelines where they give rise to no other taxa. Apparently these two terminal (= dead end) taxa were evolving secondarily to a more aquatic niche or role. They both have no known descendants in the LRT. The LRT represents a new hypothesis of interrelationships from 2017 requiring confirmation or refutation with a similar taxon list.

Today I’ll summarize the subset topology recovered by the LRT
by graphically listing the included taxa that were transitional between jawless fish in the Silurian and basalmost reptiles in the Early Carboniferous. The list includes many taxa that have been traditionally omitted from prior more focused studies, like Clack 2009 and Long et al. 2018. The LRT minimizes taxon exclusion by testing all 1586 included taxa against one another, minimizing traditional biases and omissions.

Figure 2. Updated subset of the LRT focusing on basal vertebrates (fish). Arrow points to Hybodus. This tree does not agree with previous fish tree topologies.

Figure 3. Updated subset of the LRT focusing on basal vertebrates (fish). Arrow points to Hybodus for no reason during this post.  This tree does not agree with previous fish tree topologies. See figures 1 and 2.

And here they are:
(Figs. 4–6) from Silurian jawless fish like Thelodus to Early Carboniferous Silvanerpeton.

Figure 3. Basal vertebrates in the lineage of reptiles, part 1.

Figure 4. Basal vertebrates in the lineage of reptiles, part 1.

Figure 2. Basal vertebrates in the lineage of reptiles, part 2.

Figure 5. Basal vertebrates in the lineage of reptiles, part 2.

Towards the end,
of figure two fingers and toes first appear in a phylogenetic sense, not a chronological sense. Greererpeton is Early Carboniferous (320 mya) while Ichthyostega and Acanthostega are Latest Devonian (360 mya). To most paleontologists those 40 million years make all the difference permitting omission of Greererpeton and similar taxa To the LRT, Greererpeton is a late survivor from an earlier, perhaps Middle Devonian, radiation.

Figure 3. Basal vertebrates in the lineage of reptiles, part 3.

Figure 6. Basal vertebrates in the lineage of reptiles, part 3.

In this final group,
(Fig. 6) we find Tulerpeton, another taxon from the Latest Devonian (360 mya). It is very nearly a reptile, just two nodes apart from Silvanerpeton, the last common ancestor of all living reptiles. So Silvanerpeton laid amniotic eggs despite its otherwise amphibian-like appearance, and this increases the probability that the more primitive Greererpeton was a late survivor of an earlier Mid Devonian radiation.

Figure 1. From the Beginning - The Story of Human Evolution was published by Little Brown in 1991 and is now available as a FREE online PDF from DavidPetersStudio.com

Figure 7. From the Beginning – The Story of Human Evolution was published by Little Brown in 1991 and is now available as a FREE online PDF from DavidPetersStudio.com

I wish I knew back then
what I know now when I designed, wrote and illustrated “From the Beginning—The Story of Human Evolution” (Wm. Morrow 1991; Fig. 7). But then, it would have been a much bigger book.


References
Clack JA 2009. The fish-tetrapod transition: new fossils and interpretations. Evolution: Education and Outreach 2(2):213–223.
Long JA, Clement AM and Choo B 2018. Early Vertebrate Evolution. New insights into the origin and radiation of the mid-Palaezoic Gondwann stem tetrapods. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 1–17.

Ventastega: not the transitional form they think it is

Ahlberg et al. 2008
introduced readers to Ventastega curonica, a Late Devonian tetrapod from Latvia known from “exceptionally preserved” 3D remains. The authors report Ventastega is, a transitional intermediate form between the ‘elpistostegids’ Panderichthys and Tiktaalik and the Devonian tetrapods (limbed vertebrates) Acanthostega and Ichthyostega. Ventastega is the most primitive Devonian tetrapod represented by extensive remains and casts light on a part of the phylogeny otherwise only represented by fragmentary taxa: it illuminates the origin of principal tetrapod structures and the extent of morphological diversity among the transitional forms.” 

Figure 2. Ventastega nests with Crassigyrinus in the LRT.

Figure 1. Ventastega nests with Crassigyrinus in the LRT.

By contrast
the large reptile tree (LRT, 1579 taxa, subset Fig. 2) nests Ventastega with the small-limbed aquatic predator, Crassigyrinus. The authors note, “A possibly homologous small interpremaxillary fontanelle is present in several Carboniferous forms such as Crassigyrinus and colosteids.” They also report, the shape of the otic capsule and ilium, also occur in much later and more derived tetrapods such as anthracosaurs and Crassigyrinus. We interpret these as persistent primitive traits rather than homoplastic reversals in the latter taxa.”

Figure 4. Subset of the LRT focusing on basal tetrapods. The light green line shows the most direct route to Tulerpeton.

Figure 2. Subset of the LRT focusing on basal tetrapods. The light green line shows the most direct route to Tulerpeton.

It turns out
that Ventastega was reverting to becoming more aquatic, not more terrestrial.

Taxon exclusion may be the reason
why Ventastega nested apart from Crassigyrinus in Ahlberg et al., despite the obvious similarities (Fig. 1). Note the authors used only 21 taxa starting with Eusthenopteron and Panderichthys and ending with Silvanerpeton, Proterogyrinus and Eoherpeton. They did not include basal tetrapods recovered by the LRT, such as Koilops, Acroplous, Gerrothorax, Deltaherpeton and the collosteids. Ahlberg et al. nest Ventastega between Densignathus and Metaxygnathus, two taxa currently not tested in the LRT.

Densignathus is represented by a single upturned mandible with a convex ventral margin. Metaxygnathus is represented by a single mandible with a convex ventral margin. The Ahlberg et al. tree is a ladder, rather than the LRT bush (Fig. 2).


References
Ahlberg PE, Clack JA, Luksevics E, Blom H and Zupins I 2008. Ventastega curonica and the origin of tetrapod morphology. Nature 453(16):1199-1204.

The fin to finger transition: 1996, 2004 and 2019.

It is now early Fall, 2019
and everyone knows and agrees that certain lobe fin fish developed traits like choanae (internal nares), fingers and toes in their transition to the clade Tetrapoda (Fig. 3). Let’s see how far we’ve come.

Way back in 1996
Cloutier and Ahlberg presented cladograms that illustrated the fish families that contributed to tetrapod characters (Fig. 1). In short: ray-fin fish give rise to 1) coelacanths 2) onychodontids, and 3) porolepiformes. These gave rise to 4) lungfish, 5) rhizodontids, 6) osteolepiformes and 7) pre-tetrapods.

Figure 1. From Cloutier and Ahlberg 1996, colors added. Here ray fin fish give rise to onychodontids, the porolepiformes, lungfish, and pre-tetrapods.

Figure 1. From Cloutier and Ahlberg 1996, colors added. Here ray fin fish give rise to onychodontids, the porolepiformes, lungfish, and pre-tetrapods.

In 2004 Long and Gordon
presented two diagrams (here combined into one) that graphically show basically the same transition from Eusthenopteron at the bottom to Pederpes at the top (Fig. 2). Unfortunately Long and Gordon substituted a Tulerepton pes when otherwise they showed a series of hands (manus) for other taxa. Such mistakes rarely happen after referees and editors examine submissions, but should be pointed out whenever they appear.

Figure 2. Combined figures from Long and Gordon 2004 showing the traditional evolution of traits at the fin-to-finger transition.

Figure 2. Combined figures from Long and Gordon 2004 showing the traditional evolution of traits at the fin-to-finger transition.

 

In 2019
in the large reptile tree (LRT, 1578 taxa; subset Figs. 3, 4) many more taxa are listed and many more taxa nest between the primitive and derived members listed above.

FIgure 1. Subset of the LRT focusing on lobefin fish. Overlays indicate key traits in the origin of tetrapods.

Figure 3. Subset of the LRT focusing on lobefin fish. Overlays indicate key traits in the origin of tetrapods. Figure 4 follows.

Also shown are the derived traits
in the order of their appearance in this lineage.

Figure 4. Subset of the LRT focusing on basal tetrapods. The light green line shows the most direct route to Tulerpeton.

Figure 4. Subset of the LRT focusing on basal tetrapods immediately following figure 3. The light green line shows the most direct route to Tulerpeton, found in much the same stratum as pre-tetrapods and ichthyostegids.

What you might glean
from the above subset of the LRT (Fig. 4) is the rapid radiation of amphibian/basal tetrapods during the last of the Devonian, 365 mya. The stratigraphic dates (in green) remind us of the paucity of fossils known from this phylogenetic sequence in which some primitive taxa post-date several derived taxa. In other words, we should not be surprised to find representatives from nearly all of these clades in late Devonian strata someday.

Not immediately apparent
is the hypothesis that both Acanthostega and Ichthyostega returned to a more aquatic (more tadpole-like) existence. These are only two of several phylogenetic trends reversing the overall trend toward a terrestrial niche (e.g. Gephyrostegus and Tulerpeton) and amniote reproduction as recovered in Silvanerpeton, the last common ancestor of all amniotes (=reptiles) in the LRT.


References
Cloutier R and Ahlberg PE 1996. Morphology, characters, and the interrelationships of basal sarcopterygians. Ch17 in Interrelationships of Fishes. Ed. Staissny MLJ, Parenti LR and Johnson GD. Academic Press NY PDF
Long JA and Gordon MS 2004. The greatest step in vertebrate history: a paleobiological review of the fish-tetrapod transition. Physiological and Biochemical Zoology 77(5):700–719. 

 

Devonian Ymeria not added to the LRT

Ymeria denticulata 
MGUH VP 6088 (Clack et al. 2012; ee-mer-ee-ah; Late Devonian; Fig. 1) was described as close to Acanthostega and Ichthyostega, but missing the skull roof. Ymeira preserves many bones in typical 3D fashion. Others like the maxilla, palatine and ectoptyergoid are shown in cross section. The coronoids are robustly denticulated (toothed). Bits of the clavicle + scapulocoracoid are also preserved.

Figure 1. Ymeria denticulata in situ with colors applied to show the alignment of the internal toothed coronoid with the external mandibular bones.

Figure 1. Ymeria denticulata in situ with colors applied to show the alignment of the internal toothed coronoid with the external mandibular bones. No labels are relabeled here.

Unfortunately,
not enough is known from this partial skull to nest Ymeria in the large reptile tree (LRT, 1480 taxa). Even so, several comparisons to Acanthostega (Fig. 2) should draw your attention to the many similarities, as the authors noted.

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

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

Clack et al. conclude
“A cladistic analysis not only suggests that Ymeria lies adjacent to Ichthyostega on the tetrapod stem, but also reveals substantial topological instability. As the third genus and the fifth species of tetrapod identified from North-East Greenland, it demonstrates the high diversity of Devonian tetrapods in that region.”


References
Clack JA, Ahlberg PE, Blom H and Finney SM 2012. A new genus of Devonian tetrapod from North-East Greenland, with new information on the lower jaw of Ichthyostega. Palaentology 55(1):73–86.

wiki/Ymeria

 

Temnospondyl evolution (Fortuny and Steyer 2019)

Adding taxa and reviewing scores
are slightly modifying the cladogram of basal tetrapods (Fig. 1), distinct from traditional cladograms.

Fortuny and Steyer 2019 report:
“Phylogenetic analysis of a large dataset (72 taxa, 212 characters) focuses on the in-group relationships of temnospondyls, the largest lower tetrapod clade. The following groups were unequivocally found to be monophyletic: Edopoidea (node), Dvinosauria (stem, excl. Brachyopidae), Dissorophoidea (node), Eryopidae (stem), and Stereospondyli (node). In all variant analyses, edopoids form the basalmost temnospondyl clade, followed by a potential clade (or grade) of small terrestrial taxa containing Balanerpeton and Dendrerpeton (‘Dendrerpetontidae’). All taxa higher than Edopoidea are suggested to form the monophyletic stem taxon Eutemnospondyli, tax. nov. The remainder of Temnospondyli fall into four robust and undisputed clades: (1) Dvinosauria; (2) Zatracheidae plus Dissorophoidea; (3) Eryopidae; and (4) Stereospondyli.”

By contrast
the large reptile tree (LRT, 1447 taxa, subset FIg. 1) finds Balanerpeton closer to reptiles and dvinosaurs closer to basalmost tetrapods. So Fortuny and Steyer’s traditional in-group includes out-group taxa.

Figure1. Subset of the LRT focusing on basal tetrapods, modified from earlier versions by adding taxa and re-scoring after better data was found.

Figure1. Subset of the LRT focusing on basal tetrapods, modified from earlier versions by adding taxa and re-scoring after better data was found. Here amphibians are temnospondyls and mammals are amphibians, by definition. Nomenclature needs to be reviewed. 

The traditional clade Stereospondyli
is paraphyletic in the LRT.

The traditional clade Eutemnospondyli (Schoch 2013)
is paraphyletic in the LRT.

Amphibia, by definition,
now includes Reptilomorpha, Reptilia, Mammalia and Primates.

Tetrapodomorpha requires a contrast with
lungfish (clade: Dipnoi) and coelacanths (clade: Actinista), which need to be added to the LRT.


References
Fortuny J and Steyer J-S 2019. New insights into the evolution of temnospondyls. Journal of Iberian Geology. https://doi.org/10.1007/s41513-019-00104-0
Schoch RR 2013. The evolution of major temnospondyl clades: an inclusive phylogenetic analysis. Journal of Systematic Palaeontology 11(6):673–705.

Air-breathing Polypterus: it’s in our direct ancestry

The Nile bichir
(genus: Polypterus; Fig. 1) is a small, long-bodied, extant fish at home in hot, swampy, oxygen-starved waters. It has lungs, but no trachea. It breathes through a spiracle. Juveniles have large, pink external gills for breathing underwater. For decades, the big question has been: “What is it?”

According to Wikipedia,
“Polypterus was discovered, described, and named in 1802 by Étienne Geoffroy Saint-Hilaire. It is a genus of 10 green to yellow-brown species. Naturalists were unsure whether to regard it as a fish or an amphibian. If it were a fish, what type was it: bony, cartilaginous, or lungfish? Some regarded Polypterus as a living fossil, part of the missing link between fishes and amphibians, helping to show how fish fins had evolved to become paired limbs.”

Figure 1. The Nile bichir (Polypterus), skull, skeleton and bones colorized for ease of comparison. Compare to the placoderm, Entelognathus, (Fig. 2) and the stem tetrapod Tinirau (Fig. 3).

Figure 1. The Nile bichir (Polypterus), skull, skeleton and bones colorized for ease of comparison. Compare to the placoderm, Entelognathus, (Fig. 2) and the stem tetrapod Tinirau (Fig. 3).

Surprisingly,
when added to the large reptile tree (LRT, 1447 taxa) Polypterus did not nest with the distinctly different basal actinopterygian, Cheirolepis (Fig. 4), but between the Silurian placoderm, Entelognathus (Fig. 2) and the Middle Devonian stem tetrapod/crossopterygian, Tinirau (Fig. 3). Thus, Polypterus is a very ancient fish, with a genesis predating all tested Devonian crossopterygians and actinopterygians.

Figure 2. The placoderm, Entelognathus, is widely considered the outgroup to the crossopterygians, the stem tetrapods. Compare the skull bones to those of Polypterus (Fig. 1) and Tinirau (Fig. 3).

Figure 2. The placoderm, Entelognathus, is widely considered the outgroup to the crossopterygians, the stem tetrapods. Compare the skull bones to those of extant Polypterus (Fig. 1) and Middle Devonian Tinirau (Fig. 3).

Romer (1946) wrote (quoted from Wikipedia),
“The weight of Huxley’s opinion is a heavy one, and even today many a text continues to cite Polypterus as a crossopterygian and it is so described in many a classroom, although students of fish evolution have realized the falsity of this position for many years…. Polypterus…is not a crossopterygian, but an actinopterygian, and hence can tell us nothing about crossopterygian anatomy and embryology.” If this were true, Polypterus would have nested with the actinopterygian, Cherolepis, in the LRT.

Hall (2001) reported, 
“Phylogenetic analyses using both morphological and molecular data affirm Polypterus as a living stem actinopterygian.” Remember, a ‘stem’ actinopterygian, by definition, is not an actinopterygian. It’s something else preceding that clade. Currently Polypterus cannot have its genes tested against any other placoderms or crossopterygians, but we can include this taxon in phylogenetic analysis.

So often
paleontologists keep looking, too often in frustration, where they think a taxon should nest (e.g pterosaurs as archosaurs, caseids as synapsids, whales as artiodactyls, etc.), instead of just letting the taxon nest itself in a wide gamut analysis, like the LRT. It’s really that easy and you can be confident of the results because all other candidates are tested at the same time.

Figure 3. The stem tetrapod, Tinirau. Compare to Polypterus (Fig. 1) and Entelogenathus (Fig. 2).

Figure 3. The stem tetrapod, Tinirau. Compare to Polypterus (Fig. 1) and Entelogenathus (Fig. 2).

So, where does that leave the basal actinopterygian, Cheirolepis?
In the LRT, Cheirolepis nests with the small crossopterygian, Gogonasus (Fig. 4) and these more circular cross-section taxa form a clade outside of the main line of flat stem tetrapods. That also solves several long-standing problems.

Now the late appearance of the basal fish, Cheirolepis
and other actinopterygians makes sense. They are derived from crossopterygians that have losing their lobe fins while retaining their fin rays.

Figure 4. The former most primitive ray-fin fish, Cheirolepis (Middle Devonian) nests with the crossopterygian, Gogonasus, in the LRT, distinct from Polypterus. Note the fleshy pectoral fins, the anterior advancement  and transformation of the pelvic fin and the loss of the anterior dorsal fin in Cheirolepis.

Figure 4. The former most primitive ray-fin fish, Cheirolepis (Middle Devonian) nests with the crossopterygian, Gogonasus, in the LRT, distinct from Polypterus. Note the fleshy pectoral fins, the anterior advancement  and transformation of the pelvic fin and the loss of the anterior dorsal fin in Cheirolepis.

Placoderms had fleshy fins.
The pectoral fins of Cheirolepis remained lobe-like, while the pelvic fins lost their lobes. That’s the progression, not the other way around.

If you see a fish with great distance
between the pectoral and pelvic fins (e.g. Polypterus, sharks, sturgeons, paddlefish, etc.) it is primitive. In Cheirolepis the distance is shortening. In many derived fish, the pelvic girdle is just beneath the gills. Very strange, when you think about it.

Not only can Polypterus breathe air,
it can lift its chest off the substrate with its robust forelimbs (Fig. 5). It can survive on land for several months at a time. Now those Middle Devonian trackmakers no longer seem so outlandish.

The three flathead fish, Entelognathus, Polypterus and Tinirau,
lead directly to Panderichthys, Tiktaalik and other flattened basal tetrapods.

That makes Polypterus,
like Didelphis, Caluromys and Lemur,  a living representative close to the direct lineage of tetrapods, mammals, primates and humans.

If you’re interested in Polypterus
the above YouTube videos will prove enlightening.


References
Geoffry Saint-Hillaire E 1802. Description d’un nouveau genre de poisson, de l’ordre des abdominaux. Bull. Sci. Soc. Philom., Paris, 3(61):97-98.
Hall BK 2001. John Samuel Budgett (1872-1904): In Pursuit of Polypterus, BioScience 51(5): 399–407.
Romer AS 1946. The early evolution of fishes, Quarterly Review of Biology 21: 33-69.

wiki/Polypterus
wiki/Tinirau
wiki/Cheirolepis
wiki/Entelognathus

When elbows and knees start bending in basal tetrapods

You might remember,
earlier we looked at the non-traditional origin of fingers and toes in the Tetrapoda in the basal dvinosaur/plagiosaur, Trypanognathus (Figs. 1, 7). 

Today
we’ll look at the origin of elbows and knees able to bend (distinct from lobefins and kin).

And then a quick peek
at bendable limbs large enough to sustain/lift the weight of the skull and body off the substrate, an ability chronicled in Middle Devonian tracks.

Backstory
Lobefin fish, like Eusthenopteron through Panderichthys, have one proximal limb bone, two more distally and many tiny bones further out, ending in lepidotrichia (fin filaments). In these taxa the radius is much longer than the ulna. The tibia is much longer than the fibula.

Figure 1. Basal tetrapods to scale. Yellow taxa are temnospondyls. Red taxa are reptilomorphs.

Figure 1. Basal tetrapods to scale. Yellow taxa are temnospondyls. Red taxa are reptilomorphs.

Dvinosaurs (basalmost tetrapods, and by definition, reptiles and humans; Fig. 2) like Trypanognathus (Figs. 1, 2), have the same limb bone arrangements (1 bones, 2 bones, then several bones), but the ulna length catches up to the radius and the fibula resembles the tibia. Notably this occurs on limbs too small to support weight. Only tiny fingers and toes are present. These replace the lepidotrichia. There is little indication that a strongly bendable elbow or knee is present.

Figure 2. Subset of the LRT focusing on the basal tetrapods including Trypanognathus.

Figure 2. Subset of the LRT focusing on the basal tetrapods. Trypanognathus forelimb and hind limb shown at right. Also see figure 7.

In colosteids,
like Colosteus (Fig. 3) and Pholidogaster, the limbs have modern proportions, with bendable elbows and knees, but they remain far too small to support the weight of the head and torso. Another traditionally considered colosteid, Greererpeton (Fig. 1) nests at the base of the next derived clade in the large reptile tree (LRT, 1444 taxa).

Figure 5. Colosteus is covered with dermal skull bones and osteoderms. Those vestigial forelimbs are transitional to the limbless condition in Phlegethontia.

Figure 3. Colosteus is covered with dermal skull bones and osteoderms. Those vestigial forelimbs are transitional to the limbless condition in Phlegethontia.

Thereafter
limbs get bigger, as documented in Ossinodus (Fig. 1), able to support weight lifted above the substrate. At this stage and with this innovation basal tetrapods split into three clades: Temnospondyli, Lepospondyli and Reptilomorpha in the LRT. Even so, lateral undulation of the backbone is the main driver for stride length.

Temnospondyli
Limbs are larger in temnospondyls (Fig. 1). Bodies are rounder. Tails are longer. The limbs would have been advanced more by lateral undulation than by extension and flexion.

Acanthostega represents a rare reversal,
among temnospondyls. Phylogenetically it is a smaller, apparently neotonous taxon with extra fingers and toes, a reversal to a longer radius than ulna, smaller limbs, but also a smaller, narrower body and a robust tail. Both girdles were quite large and both the humerus and femur had large processes not seen in more primitive dvinosaurs.

Figure 2. Dendrerpeton without raised orbits from Holmes et al. 1998.

Figure 4. Dendrerpeton without raised orbits from Holmes et al. 1998. This configuration is similar to that of basal lepospondyls and reptilomorphs including microsaurs.

Lepospondyli
Trimerorhachis (Fig. 5) is either a basal taxon retaining a wide torso and relatively small limbs or it is yet another reversal because its sister taxon in the LRT is Dendrerepton (Fig. 4) a small taxon with robust limbs and a small body. This clade gives rise to frogs, like Rana, which hyper-emphasizes the limbs and reduces the torso, along with Cacops, which shortens the torso and emphasizes the girdles.

Figure 1. Trimerorhachis was considered a dvinosaurian temnospondyl. Here both Trimerorhachis and Dvinosaurus nest low on the basal tetrapod tree, close to the fin/finger transition.

Figure 5. Trimerorhachis was considered a dvinosaurian temnospondyl. Here both Trimerorhachis and Dvinosaurus nest low on the basal tetrapod tree, close to the fin/finger transition. The limbs in dorsal view would have been advanced more by lateral undulation than by extension and flexion.

Reptilomorpha
This clade includes reptiles, like Silvanerpeton, their proximal ancestors and microsaurs, like Tuditanus, in the LRT. Basal taxa were increasingly terrestrial with robust limbs on smaller bodies. Much later several clades within both Reptilia and Microsauria (e.g. Diplocaulus) returned to a more aquatic niche, typically reducing the limbs. In some reptiles (e.g. Ichthyosaurus, Orcinus) limbs evolved back into fins/flippers and tails evolved fish-like flukes.

Figure 1. Subset of the LRT focusing on basal tetrapods and showing those taxa with lobefins (fins) and those with fingers and toes (feet). Inbetween we have no data.

Figure 6. Subset of the LRT from an earlier post focusing on basal tetrapods and showing those taxa with lobefins (fins) and those with fingers and toes (feet). Inbetween we have now have data in the form of Trypanognathus (Fig. 7) at the base of the Dvinosauria.

Figure 1. Trypanognathus in situ, colorized to bring out ribs and limbs.

Figure 7. Trypanognathus in situ, colorized to bring out ribs and limbs, too small to support the body. Also see figure 2.