The swordfish (genus: Xiphias) enters the LRT

Xiphias gladius
is the extant swordfish, a large, fast, open seas predator.

Figure 1. Extant swordfish (Xiphias) to scale with Eocene swordfish (Blochius).

Figure 1. Extant swordfish (Xiphias) to scale with Eocene swordfish (Blochius).

The swordfish lacks facial bones
the barracuda retains as vestiges rimming the orbit. That’s why it does not nest with the barracuda, but between the barracuda and the clade of halibut + electric eel, which also lacks those post-orbital bones… given the present paltry number of fish presently tested by the large reptile tree (LRT, 1490 taxa, subset Fig. 6).

Figure 2. Swordfish skull compared to barracuda skull. Some bones are reidentified here based on homology with the barracuda.

Figure 2. Swordfish skull compared to barracuda skull. Some bones are reidentified here based on homology with the barracuda.

Even tiny post-hatchling swordfish
(Fig. 3) have a large sword-like rostrum… and a matching mandible that shortens relative to the rostrum with maturity.

FIgure 3. Hatchling swordfish with 'sword'.

FIgure 3. Hatchling swordfish with ‘sword’.

The swordfish vertebral column
(Figs. 1, 4) has far fewer vertebrae than in deep ancestor taxa, like Amia.  Eocene swordfish ancestors, like Blochius (Fig. 1), already have fewer, but more elongate vertebrae and also lack pelvic fins.

Figure 4. Xiphias growth series. Note hatchlings more closely resemble tiny barracudas.

Figure 4. Xiphias growth series. Note hatchlings more closely resemble tiny barracudas. Juveniles more closely resemble sailfish.

Xiphias gladius (Linneaus 1758; Gregory and Conrad 1937; up to 4.5m in length) is the extant swordfish, derived from a sister to the barracuda, Sphyraena. 1cm long hatchlings more closely resemble little barracudas,,, then they look like little sailfish… then they reduce the long dorsal fin, keeping the portion just posterior to the skull.

The sword-like rostrum
is not used to spear, but to slice and maim smaller fish traveling in dense schools. Larval swordfish feed on zooplankton including other fish larvae. Juvenile swordfish eat squid, fishes, and pelagic crustaceans.

Egg production in a phylogenetic and size context:

Small Amia females produce 2000–5000 eggs.
Larger Sphyaena females produce 5000–30,000 eggs.
Even larger Xiphia females produce up to 29 million eggs.

Figure 5. Another Blochius specimen from Eocene strata.

Figure 5. Another Blochius specimen from Eocene strata.

An oil producing gland adds to speed.
Videler et al. 2016 report, “the discovery of a complex organ consisting of an oil-producing gland connected to capillaries that communicate with oil-excreting pores in the skin of the head. The capillary vessels transport oil to abundant tiny circular pores that are surrounded by denticles. The oil is distributed from the pores over the front part of the head. The oil inside the gland is identical to that found on the skin and is a mixture of methyl esters. We hypothesize that the oil layer, in combination with the denticles, creates a super-hydrophobic layer that reduces streamwise friction drag and increases swimming efficiency.” (Fig. 7).

Figure 6. Subset of the LRT with Xiphias added.

Figure 6. Subset of the LRT with Xiphias added.

Figure 6. Swordfish oil glands from Videleer et al. 2016 (color added).

Figure 7. Swordfish oil glands from Videleer et al. 2016 (color added).

Figure 1. Eurhinosaurus, a derived ichthyosaur, in several views.

Figure 8. Eurhinosaurus, a derived ichthyosaur, in several views.

And finally,
it’s worthwhile to compare the Jurassic ichthyosaur, Eurhinosaurus (Fig. 8) with the recent swordfish, Xiphias. The ichthyosaur does not have the oil gland, does not lose teeth from the ‘sword’, does not reduce the number of vertebrae, does not lose the pelvic fins, and does not have deep caudal region. Don’t expect Eurhinosaurus to compete for speed with the swordfish.


References
Gregory WK and Conrad GM 1937.
The comparative anatomy of the swordfish (Xiphias) and the sailfish (Istiophorus). The American Museum Novitates, 952:1-25.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Linneaus C von 1766. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio duodecima, reformata. pp. 1–532. Holmiæ. (Salvius)
Videler JJHaydar DSnoek RHoving H-JT and Szabo BG 2016. Lubricating the swordfish head. 

wiki/Amia
wiki/Xiphias
wiki/Sphyraena

The barracuda (genus: Sphyraena) enters the LRT

Sphyraena barracuda
is one of the terrors of the sea (Fig. 1), but it’s skull is a work of art and an engineering marvel (Fig. 2).

FIgure 1. Sphyraena barracuda in vivo. Note the anterior placement of the pelvic fins relative to the very long tail.

FIgure 1. Sphyraena barracuda in vivo. Note the anterior placement of the pelvic fins relative to the very long tail.

Today
the barracuda enters the large reptile tree (LRT, 1489 taxa (and see below)) alongside the swordfish (Xiphias gladius). Both are open water, fast, predatory swimmers derived from the bottom dwelling, slow-moving, sometimes air-breathing bowfin, Amia (Fig. 3). Note (Fig. 1) the reduction (but not absence!) of the postorbital and jugal bones in the barracuda along with the lack of teeth on the maxilla relative to the bowfin (Fig. 3).

Figure 1. The skull of the barracuda (genus: Sphyraena) with bones identified with colors.

Figure 1. The skull of the barracuda (genus: Sphyraena) with bones identified with colors.

Sphyraena barracuda (originally Esox sphyraena Linneaus 1758; up to 165cm in length) is the extant barracuda. Note the tiny remnants of the postorbital and jugal rimming the sclerotic ring. The maxilla terminates anterior to the orbit, but the jaw joint does not. The caudal region makes up most of the body based on the anterior migration of the pelvic fins. Barracudas are fast and wide-ranging open water swimmers. They have lost the ability or need to breathe air at the surface. Females can release 5000 to 30,000 eggs and hatchlings resemble little adults.

FIgure 3. The bowfin, Amia calva, is basal to both the electric eel and halibut in the LRT.

FIgure 3. The bowfin, Amia calva, is basal to both the electric eel and halibut in the LRT.

The point of these figures
is to simplify and illustrate the evolutionary paths derived taxa take, gaining and losing traits to more effectively adapt to their niche or to exploit a new niche, in this case, open water predation.

To those of you who thought
a small set of generalized character traits could not possibly lump and separate 360 tetrapod taxa, I hope you tone down your attacks now that the taxon list is 4x that number and includes everything from fish to birds.

To those of you who thought
digitally painting bones with transparent colors was a bad idea, I hope you have been won over by a technique that helps readers understand graphic images better than with line drawings and arrows attached to labels. I was not the first to employ this graphic method, which is gaining wider acceptance and use.

Figure 6. Subset of the LRT with Xiphias added.

Figure 4. Subset of the LRT with Xiphias and Sphyraena added.

Figure 3. Subset of the LRT focusing on bony ray fin fish and kin. Here Devonian Cheirolepis nests with extant deep sea Malacosteus.

Figure 4b. Subset of the LRT focusing on bony ray fin fish and kin when the LRT included 1524 taxa.

The LRT continues to document
a gradual accumulation of derived traits at every node, more accurately echoing evolutionary events than prior attempts employing fewer taxa and those excluding key taxa based on tradition and bias. Please use it as a guide when selecting taxa for your more focused studies.


References
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.

wiki/Amia
wiki/Xiphias
wiki/Sphyraena

Reconstructing the wicked skull of Onychodus

Today a GIF animation
provides a great way to put together the skull and palate of the lobe-fin fish with a dental whorl, Onychodus (Fig. 1) that just cannot be matched by the printed page. And there’s no pay wall.  :  )

FIgure 1. Several views of the Onychodus skull.

FIgure 1. Several views of the Onychodus skull. Contra xx the vomer fangs are present.

Onychodus sigmoides (Newberry 1857; Andrews et al. 2006; Late Devonian; 10cm skull, 47 cm length with larger specimens up to 2m in length) is a sister to the much smaller Strunius (Fig. 2). Both are members of the Onychodontida. The large, rotaring tooth whorl at the dentary and long premaxilla extending below the orbit are key traits. The jugal and postorbital are fused. The nasal contacts the orbit. Like Strunius, Onychodus links lobe fin fish to derived ray fin fish.

Figure 1. Strunius enlarged to show detail. Inset shows the second origin of the dual external naris as the original apparently splits by the addition of a skin bridge creating two openings.

Figure 1. Strunius enlarged to show detail. Inset shows the second origin of the dual external naris as the original apparently splits by the addition of a skin bridge creating two openings. A reminder, this is a late-survivor of an earlier radiation.

That tooth whorl
reduced the need for vomer fangs (though still present in Onychodus) and required space in the palate to sheath them.

Interesting note, previously overlooked due to taxon exclusion:
Taxa following onychodontids have a steeply arched palate that continues to the present in extant and extinct teleosts (most bony fish sans those listed below).

Additionally,
several derived taxa, like Strunius, and derived clades among the Teleosti, have an upturned premaxilla. This is where it starts.

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

Figure 2. Subset of the LRT updated with new basal vertebrates.

Many prior studies
nest onychodontids basal to Osteoleopis (Fig. 3) and similar lobefins with an exit naris (= choana) inside the palate.

By contrast, the LRT tests taxa without bias or tradition. 
Here (Fig. 2) Onychodus and Strunius are derived sarcopterygians that lose the lobe in their fins. The twin narial opening on the side of their rostrum is also a derived trait, going the opposite way that tradition imagined, and giving rise to most of the ray-fin fish.

Figure 2. Ostelepis has a large bone basal to the pelvic fin. IMHO it is too far back to be a possible ischium, contra Panchen.

Figure 2. Ostelepis has a large bone basal to the pelvic fin. IMHO it is too far back to be a possible ischium, contra Panchen.

Earlier we looked at the smaller, more derived sister of Onychodus,
Strunius, and its phylogenetically transitional traits linking lobefins to ray fins in the LRT here.  Phylogenetic miniaturization is once again at play at the base of new major clades.


References
Andrews M, Long J, Ahlberg P, Barwick R, Campbell K 2006. The structure of the sarcopterygian Onychodus jandemarrai n. sp. from Gogo, Western Australia: with a functional interpretation of the skeleton. Transactions of the Royal Society of Edinburgh. 96: 197–307.
Newberry JS 1857. Fossil fishes from the Devonian rocks of Ohio. Geological
Survey of Ohio: Bulletin National Institute: 1–120.

wiki/Onychodontida
wiki/Strunius
wiki/Onychodus

A small Cheirolepis gets a new face

The skull of the basal ray-fin fish,
Cherolepis (Agassiz 1835a, b; Figs. 1–4), was restored/reconstructed to perfection by Pearson and Westoll 1979 (Fig. 1). I pulled scores from that data with high confidence. Everyone who is into fish uses those images. It’s a landmark study on a key taxon.

Giles et al. 2015 wrote,
“As the sister lineage of all other actinopterygians, the Middle to Late Devonian (Eifelian–Frasnian) Cheirolepis occupies a pivotal position in vertebrate phylogeny.”

Figure 1. Old and new versions of Cheirolepis based on DGS methods.

Figure 1. Old and new versions of Cheirolepis based on DGS methods.

Cheirolepis nested
in the large reptile tree (LRT, 1487 taxa; Fig. 5) with high Bootstrap scores, but several autapomorphic traits (= unique traits that do not blend well with sister taxa) showed  up prompting the need to go back to the fossil itself (Fig. 2).

Figure 2. Skull tracings of two specimens of Cheirolepis.

Figure 2. Photos from Giles et al. 2015.  Skull tracings of two specimens of Cheirolepis using DGS methods. See figure 3 for reconstruction.

Using DGS methods
I traced the NMS.1956.19 specimen of Cheirolepis (Figs. 2) and graphically slid the bones back to their invivo positions (Fig. 3). This technique avoids the pitfalls that attend freehand drawing, as in Pearson and Westoll 1979. Try as I might, I was not able to confirm the boxy profile that Pearson and Westoll provided to Cheirolepis (Figs. 1, 4).

Figure 3. Cheirolepis DGS tracing and reconstruction, distinct from original but in one with sister taxa in the LRT.

Figure 3. Cheirolepis DGS tracing and reconstruction, distinct from original but in one with sister taxa in the LRT. Using transparent colors enables one to place lateral views of palatal elements inside the lateral view of the specimen itself.

Moreover
the new scores entered into the LRT proved to be much more gradual, blending in with sister taxa. The traditional lobe-fins, Onychodus and Strunius, are more primitive. The  ray-fin fish, Amia (Fig. 6), and spiny sharks, like Brachyacanthus are more derived (Fig. 5).

The NMS specimen
is much smaller than the Pearson and Westoll reconstruction (Fig. 4) and could represent variation within the genus. Certainly the spiny sharks, with an extremely large eye and short rostrum, have a facial profile more like the larger specimen than the smaller specimen, which more closely resembles the bowfin, Amia.

Figure 5. The genus Cheirolepis by Pearson and Westoll 1979 (line drawing) compared to DGS skull tracing. Evidently these were two different specimens based on scale alone.

Figure 4. The genus Cheirolepis by Pearson and Westoll 1979 (line drawing) compared to DGS skull tracing. Evidently these were two different specimens based on scale alone. Note the pectoral lobe-fin and the pelvic ray-fin.

Shortly,
I will be assembling images of a scaled evolutionary series of taxa that document a gradual accumulation of traits at every node, now that a sufficient number of gaps are being filled by taxon inclusion. As usual, it will tradition to a large extent, but certain untested relationships are going to show.

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

Figure 5. Subset of the LRT updated with new basal vertebrates.

FIgure 3. The bowfin, Amia calva, is basal to both the electric eel and halibut in the LRT.

FIgure 6. The bowfin, Amia calva, nests close to Cheirolepis in the LRT.


References
Agassiz JLR 1835a. Recherches sur les Poissons fossiles, 5 volumes. Imprimerie de Petitpierre et Prince, Neuchaatel, 1420 pp.
Agassiz JLR 1835b. On the fossil fishes of Scotland. Report of the British Association for the Advancement of Science, British Association for the Advancement of Science, Edinburgh.
Giles S, et al. (6 co-authors) 2015. Endoskeletal structure in Cheirolepis (Osteichthys, Actinopterygii), an early ray-finned fish. Palaeontology 58(5):849–870.
Pearson DM and Westoll TS 1979. The Devonian actinopterygian Cheirolepis Agassiz. Earth and Environmental Science Transactions of The Royal Society of Edinburgh. 70(13-14):337–399.

wiki/Cheirolepis

The nomenclature of skull bones in primitive lungfish

Campbell, Barwick and Pridmore 1995
labeled the many small bones of the lungfish skull in response to perceived nomenclature issues.

Unfortunately,
the authors continued the traditional practice of labeling the skull bones with letters and numbers (Fig. 1), thereby ignoring homologies with other vertebrates in which the skull bones have traditional names, like maxilla, frontal, etc.

Their abstract:
“An attempt is made to examine the problem of the nomenclature of the roofing bones in the most primitive dipnoans, which is still a matter of contention. The letter and number system that has been in use for the last half century was based on Dipterus, a Middle Devonian genus that has a reduced number of bones and a greatly shortened cheek in comparison with the Early Devonian Dipnorhynchus. Several attempts have been made to expand the Dipterus nomenclature to accommodate the more primitive condition in Dipnorhynchus and Uranolophus, but none has yet been generally accepted. The most recent attempt by Westoll (1989), which involves errors of reconstruction and interpretation, is discussed in this paper. The matter is of importance because assessment of the relationships of primitive dipnoans to other groups depends to a substantial extent upon these homologies.”

Figure 1. The Middle Devonian lungfish, Howidipterus, with subdivided skull bones colorized here to match those in the placoderm Entelognathus (Fig. 2).

Figure 1. The Middle Devonian lungfish, Howidipterus, with subdivided skull bones colorized here to match those in the placoderm Entelognathus (Fig. 2).

Schultze 2008 responded by
homologizing the skull bones of actinoperygians with those of sarcopterygians (as I just found out… learning as I go.) Not sure what the latest practice is.

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

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

We’ve already seen the breakup of skull bones in lungfish
into many little bones. These need to be homologized with those of other vertebrates, as shown at ReptileEvolution.com and at this prior blogpost. The way to do this is by way of phylogenetic bracketing.


References
Campbell KSW, Barwick RE and Pridmore PA 1995. On the nomenclature of the roofing and cheekbones in primitive dipnoans. Journal of Vertebrate Paleontology 15(1):28–36.
Schultze H-P 2008 (2007). Nomenclature and homologization of cranial bones in actinopterygians. Nomenclature and homologization of cranial bones in actinopterygians. In Mesozoic Fishes 4 – Homology and Phylogeny. Editors: Arratia G, Schultze H-P and MVH Wilson, Verlag Dr. F. Pfeil.

Kenichthys is supposed to be a basal tetrapodomorph

Zhu and Ahlberg 2004 presented a hypothesis of relationships that Kenichthys campbelli (Chang and Zhu 1993, Fig. 1) represented a transitional taxon between two purported ‘sarcopterygians’, Youngolepis and Eusthenopteron.

From their abstract:
“The choana, a unique ‘internal nostril’ opening from the nasal sac into the roof of the mouth, is a key part of the tetrapod (land vertebrate) respiratory system. There is no consensus on the origin of the choana despite decades of heated debate. Here we present new material of Kenichthys, a 395-million-year-old fossil fish from China, that provides direct evidence for the origin of the choana and establishes its homology: it is indeed a displaced posterior external nostril that, during a brief transitional stage illustrated by Kenichthys, separated the maxilla from the premaxilla.”

From their caption:
“Figure 1. Nostril positions on the heads of sarcopterygian fishes.”

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 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. Caption first line from Zhu and Ahlberg: “Nostril positions on the heads of sarcopterygian fishes.”

Unfortunately,
in the Zhu and Ahlberg cladogram (Fig. 2) Kenichthys nests many nodes apart from Eusthenopteron. Compare to figure 3. Taxa at the bases of clades are more like outgroup taxa than others are. So care must be taken that all candidate sisters are included to test relationships.

Figure 2. Cladogram from Zhu and Ahlberg 2004. Note the large number of taxa separating Kenichthys from Eusthenopteron.

Figure 2. Cladogram from Zhu and Ahlberg 2004. Note the large number of taxa separating Kenichthys from Eusthenopteron.

Unfortunately
in the large reptile tree (LRT, 1482 taxa, subset Fig. 3) Kenichthys does not nest with either Youngolepis or Eusthenopteron. But Kenichthys does nest at the base of all taxa with an internal choana, including lungfish.

Note most of the ray-fin fish
are derived from one branch of lobefins (Fig. 3), and they reverted to a morphology in which both the in and out nares were on the side of the face, beginning with the lobefins Onychodus and Strunius.

Even so,
there are several air-breathing ray-fin taxa in this cladogram (Fig. 3) without choana.

Now the LRT needs to be confirmed
or refuted with a similar taxon list.

Figure 3. Subset of the LRT focusing on basal vertebrates. Here Kenichthys nests at the base of lungfish and Tetrapodomorpha.

Figure 3. Subset of the LRT focusing on basal vertebrates. Here Kenichthys nests at the base of lungfish and Tetrapodomorpha.

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

Figure 3b. Subset of the LRT updated with new basal vertebrates.

Apparently taxon exclusion
stung the Zhu and Ahlberg report. They assumed Kenichthys was a sarcopterygian and they did not test a long list of other taxa to make sure they were not confirming their own biases. They excluded the basal placoderm, Coccosteus (Fig. 4), which has a similar lateral nasal bone that extends nearly to the jaw line. Was there a tentative external naris present? A personal examination of the fossil with this possibility in mind is needed. If not present, then Kenichthys ‘invented’ the choana and thereafter it was free to move inside or outside the mouth. Contra the original hypothesis, according to the LRT Kenichthys does not represent a taxon transitional between an inner and outer choana.

In the Zhu and Ahlberg illustration
(Fig. 1) 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, but at a much more basal node, closer to coelacanths. The outgroup basal placoderm, Coccosteus, shares the trait of a large orbit (within an orbit’s length of the rostrum tip), distinct from competing sisters and outgroups, plus a long list of other traits.

Figure 3. Coccosteus is a placoderm that shares more traits with Kenichthys than any tested sarcopterygian.

Figure 3. Coccosteus is a placoderm that shares more traits with Kenichthys than any tested sarcopterygian. Also see Entelognathus (Fig. 4).

Entelognathus is a more primitive placoderm
(Fig. 4) without the derived infolding of the maxilla and enlargement of the cranial shield seen in derived Coccosteus (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 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 Polypterus (Fig. 1) and Tinirau (Fig. 3).

Kenicthys campbelli (Chang and Zhu 1993; Zhu and Ahlberg 2004; Early Devonian, 395 mya) was originally considered a basal sarcopterygian demonstrating the migration of the exit nostril to the jaw rim on its way to the palate to become the choana. Here Kenichthys nests basal to the coelocanth clade. Yesterday we looked at a sister taxon, Stensioella.


References
Chang M and Zhu M 1993. A new Middle Devonian osteolepidid from Qujing, Yunnan. Mem. Assoc. Australas. Palaeontol. 15 183-198.
Zhu M and Ahlberg P 2004. The origin of the internal nostril of tetrapods. Nature 432:94-97.

wiki/Kenichthys

Stensioella is supposed to be a basal placoderm

Updated Feb. 27, 2020 and May 4, 2020
with a higher resolution image (Fig. 1) and 80 more taxa, Stensioella now nests closer to Guiyu the oldest articulated bony fish, another flattened sister to the coelacanths with large lobe fins.

Figure 1. Updated tracing of the Stensioella skull.

Figure 1. Updated tracing of the Stensioella skull.

Here,
in the large reptile tree (LRT, 1482 taxa, Fig. 2) Stensioella (Fig. 1) now nests with Guiyu another lobefin close to coelacanths.

According to Wikipedia
Stensioella heintzi (“Heintz’s Little Stensio“) is an enigmatic placoderm of arcane affinity. It is only known from the Lower Devonian Hunsrück slates of Germany, where the only specimens have been found. [It] had armor made up of a complex mosaic of small, scale-like tubercles.”

“Some paleontologists believe that there are very few concrete reasons for S. heintzi’s placement in Placodermi. The paleontologist, Philippe Janvier suggests that it was actually a holocephalid, and not a placoderm at all. However, if this is true, then the holocephalids (chimaerasiniopterygianspetalodonts, et al.) diverged from sharks before the Chondrichthyan Devonian radiation.

The LRT tested a wide gamut of vertebrates
including all prior candidate sister taxa (Fig. 2).

Figure 2. The AMNH specimen of Stensioella in dorsal view along with a diagram in ventral view.

Figure 2. The AMNH specimen of Stensioella in dorsal view along with a diagram in ventral view.

Stensioella heintzi (Broili 1933; Emsian, late Early Devonian) is widely considered the most basal placoderm, but here nests with Silurian lobefins. Enlarged pectoral fins on a flattened head and torso mark this taxon. Distinct from most sisters, the posterior portion is also preserved. Here specific bones with tetrapod names are identified for the first time. Stensioella would have had a lifestyle like Squatina, the extant angel shark.

Figure x. Newly revised fish subset of the LRT

Figure x. Newly revised fish subset of the LRT

As mentioned previously
basal taxa in all the above fish clades are/were bottom feeders. Open water feeders with more specialized heterocercal tails, develop by convergence in every clade, except the tetrapoda… that is, until reptiles returned to the sea and evolved into ichthyosaurs, whales, etc.


References
Broili F 1933. Weitere Fischreste aus den Hunsrickschiefern. Situngsbirechte der bayerischen Akademie der Wissenschaften, Mathematisch-Naturewissenschaftliche Klasse 2: 269–313.

wki/Stensioella

The halibut and electric eel enter the LRT together

Aside

The halibut and electric eel enter the LRT together
for some reason this headline did not print originally.

A pleasant and unexpected surprise today.
The Atlantic halibut (genus: Hippoglossus; Fig. 1) and the electric eel (genus: Electrophorus; Fig. 2) enter the large reptile tree (LRT, 1480 taxa) together with high Bootstrap values. Their skulls, other than the migration of the eyeball in the halibut, and being flat and rotated in vivo to lie upon the sea floor, are nearly identical.

Both of these specialized fish are derived
from a sister to the more primitive bowfin (genus: Amia calva).

Figure 1. Skull of the halibut (Hippoglossus) in eye-side and blind-side views.

Figure 1. Skull of the halibut (Hippoglossus) in eye-side and blind-side views. Compare to the electric eel in figure 2 and the bowfin in figure 3.

Electrophorus (Linneaus 1766) is the extant electric eel. Not wide and flat, the ovate in cross-section murky water predator has likewise lost its cheek bones, narrowed the skull roof and came to rely on its toothy premaxilla for biting.

Figure 5. Skull of the electric eel (Electrophorus) distinct from the moray eel (Fig. 4) and European eel (Fig. 2).

Figure 5. Skull of the electric eel (Electrophorus) distinct from the moray eel (Fig. 4) and European eel (Fig. 2).

In traditional cladograms
these three taxa do not nest together except as bony fish in the clade Actinopterygii.

FIgure 3. The bowfin, Amia calva, is basal to both the electric eel and halibut in the LRT.

FIgure 3. The bowfin, Amia calva, is basal to both the electric eel and halibut in the LRT. Labels and colors match figures 1 and 2. The broad, primitive frontals and parietals narrow in descendent taxa.

A bothersome conclusion is arising.
The LRT if recovering relationships that have been overlooked by prior academic workers, as it did in pterosaurs, birds, basal reptiles, basal tetrapods, whales, bats, snakes, caeids, mesosaurs, turtles and a long list of other taxa.

Amia calva (Linneaus 1766; up to 70cm in length) is the extant bowfin, a basal fish related to gars, able to breathe both water and air. Rather than two dorsal fins, an single elongate undulating fin is present. Hatchlings look like tadpoles or miniature placoderms. The squamosal an quadratojugal are absent. The postfrontal is fused to the fused frontal + parietal. In contrast, the lacrimal and jugal break apart into several bones.

Electrophorus electricus (originally Gymnotus electricus, Linneaus 1766; Gill 1864; up to 2m in length) is the extant electric eel, an obligate air breather nesting between Amia and Hippoglossus. The shape of the skull is like Amia, but the narrow dorsal bones and lack of lateral facial bones is like Hippoglossus. Electric organs that deliver shocks to enemies and prey make up 80% of the body.

Hippoglossus hippoglossus (Linneaus 1758; up to 4.7m in length) is the extant Atlantic halibut, one of the flounders, a fish that lives its life on its pale blind side. The lower eye migrates to the dark and camouflaged upper side as maturity approaches.

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

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

Figure 3. Subset of the LRT focusing on bony ray fin fish and kin. Here Devonian Cheirolepis nests with extant deep sea Malacosteus.

Figure 3. Subset of the LRT focusing on bony ray fin fish and kin. The LRT includes 1524 taxa.


References
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Linneaus C von 1766. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio duodecima, reformata. pp. 1–532. Holmiæ. (Salvius)

wiki/Amia
wiki/Electrophorus
wiki/Hippoglossus

wiki/Bowfin
wiki/Atlantic_halibut
wiki/Electric_eel

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

 

Guiyu and Psarolepis enter the LRT together

Today’s study confirms
the phylogenetic analyses of prior workers, like Zhu et al. 2012 and others cited therein. They considered Guiyu (Silurian, 419 mya) the earliest articulated bony fish. Zhu and Zhau 2009 described it as a basal lobe-finned fish with some ray-finned traits.

Figure 1. Guinyu in situ, as originally restored and as restored here based on the in situ data. Psarolepis shares the median spike seen here.

Figure 1. Guinyu in situ, as originally restored and as restored here based on the in situ data. Psarolepis shares the median spike seen here.

Today’s study offers a new reconstruction
for Guiyu: less like a sarcopterygian (Fig. 1) and more like the placoderm, Stensioella, from which it arises in the large reptile tree (LRT, 1480 taxa). So Guiyu documents the transition from placoderms to most bony fish (except catfish and sturgeons). Zhu et al. 2012 did not report the placoderm connection, perhaps because their reconstruction did not look like a placoderm. They did not notice the armored pectoral and pelvic fins were separate from the torso and tail.

Figure 2. Guiyu in situ, DGS colors added here and used to create the flatter, wider reconstruction with paddles preserved.

Figure 2. Guiyu in situ, DGS colors added here and used to create the flatter, wider reconstruction with paddles preserved. This rather complete taxon provides phylogenetic bracketing clues to the lateral skull and post-crania missing in a sister taxon, Psarolepis (Fig. 3). This taxon documents the transition from placoderms to bony fish.

Zhu et al. 2012 reported,
“Guiyu and Psarolepis have been placed as stem sarcopterygians in earlier studies, even though they manifested combinations of features found in both sarcopterygians and actinopterygians (e.g. pectoral girdle structures, the cheek and operculo-gular
bone pattern, and scale articulation). When Guiyu was first described based on an exceptionally well-preserved holotype specimen, it also revealed a combination of osteichthyan and nonosteichthyan features, including spine-bearing pectoral girdles and
spine-bearing median dorsal plates found in non-osteichthyan gnathostomes as well as cranial morphology and derived macromeric squamation found in crown osteichthyans. In
addition, Guiyu provided strong corroboration for the attempted restoration of Psarolepis romeri based on disarticulated cranial, cheek plate, shoulder girdle and scale materials.”

FIgure 2. Psarolepis skull restored from published data.

FIgure 2. Psarolepis skull restored from published data. Hypothetical opercula are not present, based on Guiyu.

Most of the cranial bones of Psarolepis are fused to one another.
Unfortunately that provides few clues to figure out bone outlines. Here (Fig. 2) Psarolepis is restored based on patterns found throughout the clade. The colors applied to each bone makes this restoration challenge a bit easier and certainly easier to convey to readers.

FIgure 3. Guiyu skull reconstruction in closer view. Mandibles in dorsal view.

FIgure 3. Guiyu skull reconstruction in closer view. Mandibles in dorsal view.

Zhu et al. 2014 followed tradition in their abstract:
“Living gnathostomes (jawed vertebrates) include chondrichthyans (sharks, rays and chimaeras) and osteichthyans or bony fishes. Living osteichthyans are divided into two lineages, namely actinopterygians (bichirs, sturgeons, gars, bowfins and teleosts) and sarcopterygians (coelacanths, lungfishes and tetrapods). [1] It remains unclear how the two osteichthyan lineages acquired their respective characters and how their common osteichthyan ancestor arose from non-osteichthyan gnathostome groups. [2] Here we present the first tentative reconstruction of a 400-million-year-old fossil fish (Psarolepis) from China; this fossil fish combines features of sarcopterygians and actinopterygians and
yet possesses large, paired fin spines previously found only in two extinct gnathostome groups (placoderms and acanthodians). [3] This early bony fish provides amorphological link between osteichthyans and non-osteichthyan groups. It changes the polarity of many characters used at present in reconstructing osteichthyan interrelationships and offers new insights into the origin and evolution of osteichthyans.” [4]

The following notes answer issues raised above:

  1. The LRT separates actinopterygians into several ray-fin clades. Sturgeons, bichirs, and catfish nest apart from bowfins, gars and teleosts.
  2. That lack of clarity is resolved in the LRT.
  3. They are forgetting bichirs, sturgeons, sticklebacks, and Guiyu.
  4. All the more so if they had only included Entelognathus and Guiyu.
  5. Adding Tinirau and LRT taxa helps separate Eusthenopteron and kin from their traditional, and now offshoot link to Tetrapoda.

Zhu et al. 2014 also reported, 
“Psarolepis was first placed within sarcopterygians, as a basal member of Dipnormorpha or among the basal members of Crossopterygii. The new features revealed by the shoulder girdle and cheek materials reported here indicate that Psarolepis may occupy a more basal position in osteichthyan phylogeny.” The LRT resolved this historical issue by including pertinent and key taxa.

Zhu et al. 2014 produced two cladograms
when they introduced Psarolepis, neither of which included Guiyu. In cladogram A Zhu et al. nested Psarolepis between the spiny sharks (acanthodians) and ray-fin fish beginning with the bichir, Polypterus + lungfish. In cladogram B Zhu et al. nested Psarolepis between Polypterus and lobe-fin fish beginning with coelacanths. The placoderms, Entelognathus (Zhu et al. 2013) and Stensioella (Broilli 1933) were not mentioned or included. Both are ougroups to Guiyu and Psarolepis in the LRT. Acanthodians are not primitive or basal to bony fish. They are derived bony fish in the LRT as we learned earlier here. Contra traditions, the most interesting taxa that transition to tetrapods are all slow-moving bottom feeders, not swift open water predators.

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

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

Dr. Zhu Min
is the lead author on several of these new discoveries and publications shedding light on key taxa at the origin of bony fish, tetrapods and ultimately humans.


References
Broili F 1933. Weitere Fischreste aus den Hunsrickschiefern. Situngsbirechte der bayerischen Akademie der Wissenschaften, Mathematisch-Naturewissenschaftliche Klasse 2: 269–313.
Zhu M and Zhau W-J 2009. The Xiaoxiang Fauna (Ludlow, Silurian) – a window to explore the early diversification of jawed vertebrates. Abstract from: Rendiconti della Società Paleontologica Italiana. 3 (3): 357–358.
Zhu M, Yu X, Choo B, Qu Q, Jia L, et al. 2012.  Fossil Fishes from China Provide First Evidence of Dermal Pelvic Girdles in Osteichthyans. PLoS ONE 7(4): e35103. doi:10.1371/journal.pone.0035103
Zhu M, Yu X-B, Ahlberg PE, Choo B and 8 others 2013. A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature. 502:188–193.
Zhu M, Yu X-B and Janvier P 2014. A primitive fossil fish sheds light on the origin of bony fishes. Nature 287:607–610.

wiki/Psarolepis
wki/Stensioella