The lancet fish (Alepisaurus): a swordfish with fangs and no sword

According to Wikipedia
The lancet fish (Alepisaurus) is a traditional member of the Aulopiformes, the clade of lizardfish and their allies. Lizardfish, as you might remember, have eyeballs as far anteriorly as possible, the opposite of lancet fish.

Figure 1. Alepisaurus, the lancet fish nests with swordfish and eels in the LRT.

Figure 1. Alepisaurus, the lancet fish nests with swordfish and eels in the LRT.

By contrast,
in the large reptile tree (LRT, 1810+ taxa) the lancet fish nests close to European eels (Anguilla), but closer to swordfish (Xiphias). All three have similar skulls with a straight rostrum and very little (if any) post-orbital region anterior to the massive hyomandibular. Of course, neither eels nor swordfish have anything like those massive fangs found in the lancet fish. All three had a last common ancestor (LCA) distinct from other fish, and each evolved from that LCA their own special way leaving few clues, other than a similar skull, as evidence of their relationship to one another. That’s the way the LRT recovered it.

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

Figure 2. Extant swordfish (Xiphias) to scale with Eocene swordfish (Blochius), which looks more like Alepisaurus overall.

That’s why we run all taxa through phylogenetic analysis. 
To do otherwise, to cherry-pick one trait or a dozen, is to “Pull a Larry Martin” which we are cautioned (for good reason) never to do. Professor Martin taught us well.

Figure 5. Skull of Anguilla, the European eel, compares well with that of Bavarichthys. Note the loss and reduction of preorbital bones.

Figure 3. Skull of Anguilla, the European eel, compares well with that of Bavarichthys. Note the loss and reduction of preorbital bones.

Alepisaurus ferox
(Lowe 1833, 1835a b, 215cm) is the extant lancetfish. Traditionally considered a aulopiform (= lizard fish. likeTrachinocephalus) here Alepisaurus nests as a swordfish sister without a rostrum sword, but with giant fangs and vestigial pelvic fins. During prey capture the fangs pierce swimming muscles, stopping the struggle. This hermaphrodite taxon can descend more than a mile into deep unlit seas.

Figure 4. Swordfish ontogeny (growth series). Hatchings have teeth, a short bill and an eel-like body still lacing pelvic fins.

Figure 4. Swordfish ontogeny (growth series). Hatchings have teeth, a short bill and an eel-like body still lacing pelvic fins.

It’s worth noting
that swordfish hatchlings have teeth and a long dorsal fin, like lancet fish. So the resemblance is closer in younger swordfish. It’s also worth noting that fossil swordfish precursors, like Blochius (Fig. 4, Eocene; 60-150cm) also had proportions closer to lancet fish.

Fierstine 2006, in his history of billfishes,
mistakenly included the convergent sailfish, Istophorus, and excluded the European eel and the lancet fish. This is what I mean when we talk about “Pulliing a Larry Martin.” It looks right. It feels right. It gets published. However, whenever you cherry-pick taxa, you are less likely to get the big picture you are chasing: an accurate hypothesis of interrelationships that models real evolutionary events. Instead you should  employ lots and lots of taxa. Then the big picture (= the accurate model) will come to you. The cladogram will recover the correct tree topology.


References
Fierstine HL 2006. Fossil history of billfishes (Xiphoidei). Bulletin of Marine Science 79(3):433–453.
Lowe RT 1833. Description of a new genus of acanthopterygian fishes, Alepisaurus ferox. Proc. zool. Soc. Lond ,1:104.
Lowe RT 1835a. Description of a new genus of acanthopterygian fishes, Alepisaurus ferox. Trans. zool. Soc. Lond., 1: 123-128, pl. 19.
Lowe RT 1835b. Additional observations on Alepisaurus ferox. Trans. zool. Soc. Lond., 1: 395-400, pl. 59.

wiki/Aulopiformes
Alepisaurus_ferox

Mystery solved: Thylacoleo is a giant sugar glider…

no doubt, a little too big to glide…
and Thylacoleo (Fig. 2) is looking even less carnivorous in phylogenetic bracketing.

Sugar gliders
(Fig. 1) are phalangers (Fig. 6), a marsupial clade nesting between kangaroos and wombats (Fig. 5).

Figure 1. Petaurus breviceps skeleton in two views, plus a skull with mandible, lacking in the skeleton.

Figure 1. Sugar glider, Petaurus breviceps, skeleton in two views, plus a skull with mandible, lacking in the skeleton.

Adding the marsupial sugar glider,
Petaurus (Figs. 1, 3), and the cuscus, Phalanger (Fig. 6), to the large reptile tree (LRT, 1231 taxa) resolves a decades-old phylogenetic problem because Petaurus, the sugar glider, nests as a sister to Thylacoleo, the marsupial lion (Figs. 2, 4). Phalanger, the cuscus, nests as their last common ancestor, which has been suggested earlier.

According to the AustraliaMuseum website
“Most palaeontologists think that the ancestors of thylacoleonids were herbivores, an unusual occurrence since most carnivores evolved from other carnivorous lineages. One proposal suggests that thylacoleonids evolved from a possum ancestor (Phalangeroidea) based on dental formula, the skull of the cuscus Phalanger, and on a phalangerid-like musculature. Alternatively, evidence from certain skull features may show that thylacoleonids branched off the vombatiform line, the lineage that includes wombats and koalas.”

In the LRT,
wombats and koalas are now sister taxa to the cuscus clade. Without the sugar glider and the cuscus, the marsupial lion earlier nested with the wombat, Vombatus.

Just to be clear,
Phalanger is not an ancestor to Didelphis, the Virginia opossum, in the LRT, even though the Australian Museum called it a ‘possum ancestor.’

Figure 2. Thylacoleo skeleton compared to Petaurus skeleton to scale.

Figure 2. Thylacoleo skeleton compared to Petaurus skeleton to scale.

Long thought to be a super predator, 
in the midst of a clade of gentle wombat-like herbivores, Thylacoleo had, for its size, the strongest bite of any mammal, living or extinct, despite having tiny upper canines. This linking with sugar gliders further erodes the carnivorous hypothesis. 

Figure 3. Skulls of the genus Petaurus with many more teeth than in Thylacoleo, but in the same general pattern. Note the lower third premolar and its similarity to the same tooth in Thylacoleo.

Figure 3. Skulls of the genus Petaurus with many more teeth than in Thylacoleo, but in the same general pattern. Note the lower third premolar and its similarity to the same tooth in Thylacoleo. The big organe tooth at the tip of the dentary is the canine. The lower incisors are absent.

Arboreal or not?
Wikipedia reports, “The claws [of Thylacoleo] were well-suited to securing prey and for climbing trees.” And now we know how that came to be. Petaurus, despite its arboreal abilities, does not have a divergent thumb, like the one found in Thylacoleo.

Dentary canines
traditionally considered large, rodent-like incisors due to their placement, the anterior-most (medial-most) dentary teeth are actually canines. The incisors and their alveoli have disappeared. This can only be traced via phylogeny (see Arctocyon and Didelphis). The ancestrally small lower incisors are gone, replaced with ancestrally large large lower canines that meet medially like typical incisors. Notably, the lower canines maintain their traditional placement relationship to the upper canines (Fig. 6).

Even more interesting,
some marsupial taxa that experience a phylogenetic miniaturization, like Eurygenium (basal to Toxodon) the incisors reappear and the canines are not much larger than the incisors. That’s called a reversal or an atavism.

Figure 4. Thylacoleo skull. Many times larger than Petaurus, with fewer larger teeth, this is a giant sugar glider.

Figure 4. Thylacoleo skull. Many times larger than Petaurus, with fewer larger teeth, this is a giant sugar glider. The large orange tooth is the lower canine. The upper canine is a vestige. 

Size
Thylacoleo was 71 cm tall at the shoulder, about 114-150cm long from head to tail tip, about the size of a jaguar.

Petaurus is 40cm long to the tail tip, about the size of a ‘flying’ squirrel. Loose folds of skin spanning the fore and hind limbs to the wrists and ankles are used to extend glides from tree to tree, or up to 140m. The diet includes sweet fruits and vegetables.

The sugar glider in vivo.

Figure 5. The sugar glider, Petaurus, in vivo. Note the wrinkled fur between the fore and hind limb. That’s the gliding membrane.

Petaurus species
According to Wikipedia, “There are six species, sugar glidersquirrel glidermahogany glidernorthern glideryellow-bellied glider and Biak glider, and are native to Australia or New Guinea.” Whichever one is closest to Thylacoleo has not been tested or determined.

Figure 2. Thylacoleo skeleton compared to Petaurus skeleton to scale.

Figure 5. Subset of the LRT focusing on Marsupialia, Metatheria and then nesting of Thylacoleo.

Petaurus breviceps (Waterhouse 1839; Early Miocene to present; up to 30cm) is the extant sugar glider, a nocturnal squirrel-like marsupial able to climb trees and glide with furry membranes between the fore and hind limbs. An opposable toe is present on each hind foot. Sharp claws tip every digit.

Phalanger orientalis (Pallas 1766; 34 cm in length) is a nocturnal arboreal folivore marsupial known as thte Northern common cuscus. Commonly considered a ‘possum’ the cuscus nests between wombats and kangaroos, basal to sugar gliders and marsupial lions.

Figure 6. The cuscus (genus: Phalanger orientalis) nests with Petaurus and Thylacoleo in the LRT.

Figure 6. The cuscus (genus: Phalanger orientalis) nests with Petaurus and Thylacoleo in the LRT. Those anterior dentary teeth look like incisors, but phylogenetically are actually canines.

Thylacoleo carnifex (Owen 1859; Pliocene-Pleistocene; 1.14 m long) was a giant sugar glider like Petaurus. Thylacoleo had the strongest bite of any mammal with the largest, sharpest molars of any mammal. It had fewer but larger teeth than Petaurus. The manus included retractable claws. The pes had a very large heel bone (calcaneum). This supposedly carnivorous ‘marsupial lion’ nests with herbivores. Pedal digit 1 likely had a phalanx and claw, but it has not been shown.

References
Goldingay RL 1989. The behavioral ecology of the gliding marsupial, Petaurus australis. Research Online. University of Wollongong Thesis Collection. PDF
Owen R 1859. On the fossil mammals of Australia. Part II. Description of a mutilated skull of the large marsupial carnivore (Thylacoleo carnifex Owen), from a calcareous conglomerate stratum, eighty miles S. W. of Melbourne, Victoria. Philosophical Transactions of the Royal Society 149, 309-322. 
Waterhouse GR 1838. Observations on certain modifications observed in the dentition of the Flying Opossums (the genus Petaurus of authors). Proceedings of the Zoological Society of London. 4: 149–153.

wiki/Petaurus
wiki/Thylacoleo
https://australianmuseum.net.au/thylacoleo-carnifex

Stenocybus and Haptodus

In lateral view Stenocybus and Haptodus are close matches. Even so, the two are distinct taxa separated by others according to the large reptile tree now up to 315 taxa (not counting the therapsid or pterosaur trees). Stenocybus was twice as large, but this specimen of Haptodus was one of the smallest ones known.

Figure 1. Comparing the basal therapsid Stenocybus to the basal sphenacodont, Haptodus. The similarities are great here, but Stenocybus still nested closer to Ophiacodon.

Figure 1. Comparing the basal therapsid Stenocybus to the basal sphenacodont, Haptodus. The similarities are great here, but Stenocybus still nested closer to Ophiacodon.

Haptodus
The maxilla was shorter than the lacrimal in Haptodus. The teeth were smaller. The quadratojugal was not visible. The ascending process of the premaxilla was shorter.

Stenocybus
The skull was narrower in ventral view and the palate was smaller in Stenocybus. The palatal elements (other than the vomers) were largely behind all the teeth in Stenocybus. Haptodus represents the plesiomorphic condition. The pterygoid was smaller in Stenocybus as were the quadrates. The frontal and parietal was smaller producing a more visible lateral termporal fenestra in dorsal view. The canine tooth was more prominent and the maxilla housing its root was much deeper, overlapping the shrinking lacrimal. The septomaxilla shifted more toward the surface and the prefrontals formed larger ‘eyebrows.’

Of course Tetraceratops, the darling of traditional paleontologists, is not related and does not resemble these two enough to drag it away from a closer nesting with Tseajaia.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again. 

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Credner H 1888. Die Stegocephalen un d’Saurier aus dem Rothliegnden des Plauen’schen Grudes bei Dresden: Zeitschrift der Deutschen Geologischen Gesellschaft, v. 40, p. 490-558.
Currie PJ 1977. A new Haptodontine Sphenacodont (Reptilia: Pelycosauria) from the Late Pennsylvanian of North America: Journal of Paleontology, v. 51, n. 5, p. 927-942.
Huene F von 1925. Ein neuer Pelycosaurier aus der unteren Permformaiton Sachens: Geologische und Palaeontologische Abhandlungen, Jena., v. 18, NF 14, p. 215-264.
Kammerer CF 2011. Systematics of the Anteosauria (Therapsida: Dinocephalia), Journal of Systematic Palaeontology, 9: 2, 261 — 304, First published on: 13 December 2010 (iFirst)
Romer AS and Price LW 1940. Review of the Pelycosauria. Geological Society of America Special Papers 28: 1-538.

wiki/Haptodus
wiki/Stenocybus