An ‘amphibian’ with an antorbital fenestra

Surprised to find this: 
Acheloma (Cope 1882; Dilkes and Reisz 1987; Early Permian, 275 mya; aka Trematops), a trematopsid amphibamid lepospondyl basal tetrapod had a confluent antorbital fenestra and naris. Bolt 1974 considered this a “very elongate external naris” and then considered two hypotheses for its origin and use:

  1. as a nasal salt gland (rather improbable, but still possible, according to Bolt)
  2. to transfer of forces away from the antorbital bar (Bolt’s preferred hypothesis)

Bolt also noted
that earlier papers referred this morphology to a confluent antorbital vacuity, but dismissed the notion by saying, “There is no evidence that any labyrinthodont, including the ancestors of trematopsids, possessed such an [completely separate] antorbital vacuity.” IMHO, this convergent trait need not have been completely separate to qualify as an antorbital vacuity/fenestra. As Bolt noted, in nearly every case, there is a slight constriction in this vacuity marking the end of the naris and the beginning of the antorbital vacuity (Fig. 1).  A nasal flange descends inside the vacuity.

Earlier
we looked at the antorbital fenestra in other tetrapods here.

Figure 1. Acheloma dunni skull with a confluent antorbital fenestra and naris.

Figure 1. Acheloma dunni skull with a confluent antorbital fenestra and naris. Scale bar = 5 cm.

Perhaps of interest to this discussion
is the relatively large diameter palatal teeth on the vomers, palatines and ectopterygoids (Fig. 1). Bolt also found evidence for a nasal flange in related Doleserpeton and Tersomius, but not in unrelated Seymouria and Eryops.

Olson 1941 had this odd explanation:
The anterior part was for smelling, the longer posterior part was for respiration and the reason for this was the internal naris lies beneath only the posterior part. Bolt noted the shortest route was not always the only route in tetrapods. Air passages can be quite complicated.

The odd otic notch
that likely housed an eardrum in related taxa, is long and narrow in Acheloma. Dilkes and Reisz (1987) noted, “The shape of the otic notch, however, argues against an impedence-matching hearing system because the vibrational properties of the postulated tympanum would be profoundly different from one with the same surface area but circular in outline.”

Acheloma cumminsi was originally considered a temnospondyl, but here nests between Dendrerpeton and Cacops within the lepospondyls with many traits convergent with temnospondyls, like that large wide skull and large overall size. The related Acheloma dunni (Fig. 1) had giant palatal teeth.

As promised earlier:
lepospondyl traits of Acheloma and Cacops not present in temnospondyls from the character list of the LRT. Let me know if you see errors here:

  1. Ventral naris chiefly maxilla in lateral view
  2. Prefrontal separate from postfrontal
  3. Preorbital length of skull sub-equal to postorbital length of skull
  4. Naris shape in lateral view < 2x longer than tall
  5. Palatine exposure on the external skull below orbit.
  6. Squamosal posterior rim is a ‘big curve’
  7. Squamosal descends to ventral skull
  8. Mandible tip straight, does not rise
  9. Cervical centrum longer than tall
  10. Cervical neural spines not taller than centra
  11. Pleurocentra larger than intercentra
  12. Two sacral vertebrae
  13. Sacral spines not > acetabulum depth
  14. Anterior chevron shapes, not wider proximally
  15. Anterior caudal neural spines not higher than centra
  16. Clavicle shorter than scapula
  17. Humerus not ‘L’-shaped
  18. Manual metacarpals 1-3 align
  19. Longest metacarpals: 2, 3 and sometimes 4
  20. Longest manual digit: three and four
  21. Manual unguals sharp pointed
  22. Metacarpal 5 absent – except in Cacops. Acheloma has 5 carpals.
  23. Posterior ilium not longer than anterior ilium
  24. Pubic apron wide
  25. Longest metatarsals: 3 and 4
  26. Pedal 3.1 not > p2.1
  27. Overall size not > 60 cm in length

Shifting
Acheloma, Broilleus and Cacops to Eryops adds 24 steps at present. Shifting those three + Dendrepeton and Tersomius adds 17 steps at present. Shfting those five + the three members of the Amphibamus clade adds 35 steps at present.

On a side note:

Having a fifth finger on basal tetrapods (no matter how you count them, 1-4 or 2-5) is rare after Acanthostega partly because a complete manus is rare in basal tetrapods and partly because many taxa have only four fingers. Proterogyrinus, Seymouria, Cacops and basal reptiles all have five fingers preserved. Presently that’s a discontinuous list, but those five fingers could be homologous. If you know of any other related examples, let me know. I need that data.

References
Bolt JR 1974. Osteology, function, and evolution of the trematopsid (Amphibia: Labyrinthodontia) nasal region. Fieldiana: Geology 33(2): 11-30.
Cope ED 1882. Third contribution to the history of the vertebrata ofthe Permian Formation of Texas. Proc. Phil. Soc., 20: 447-461.
Dilkes DW and Reisz R 1987. Trematops milleri identified as a junior synonym of Acheloma cumminsi with a revision of the genus. American Museum Novitates 2902.
Olson EC 1941. The family Trematopidae. Journal of Geology 49:149-176.

wiki/Acheloma

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Teyujagua: Not “transitional between archosauriforms and more primitive reptiles”

A new paper by Pinheiro et al. 2016
reports that the small skull (UNIPAMPA 653) of a new genus, Teyujagua paradoxa (Figs. 1, 2), is “transitional in morphology between archosauriforms and more primitive reptiles. This skull reveals for the first time the mosaic assembly of key features of the archosauriform skull, including the antorbital and mandibular fenestrae, serrated teeth, and closed lower temporal bar. Phylogenetic analysis recovers Teyujagua as the sister taxon to Archosauriformes…”

Well, that might be true if
you restrict the taxon list to the few (44) taxa employed by Pinheiro et al.

But when you expand the taxon list
to the size of the large reptile tree (660+ taxa) where we already have a long list of Youngina, Youngoides (Fig. 1) and Youngopsis sisters to Archosauriformes, then Teyujagua nests as a phylogenetically miniaturized sister to the NMQR 1484/C specimen attributed to Chasmatosaurus alexandri (Fig. 1). Like another phylogenetically miniaturized descendant of chasmatosaurs, Elachistosuchus huenei MB.R. 4520 and BPI 2871 (Figs. 3, 4), Teyujagua also turned its once large antorbital fenestra into a vestige (Figs. 1, 2).

Figure 1. Teyujagua compared to sister taxa, including Youngoides, Proterosuchus and Chasmatosaurus. Teyujagua is a phylogenetic miniature in which the antorbital fenestra became a vestige.

Figure 1. Teyujagua compared to sister taxa, including Youngoides, Proterosuchus and Chasmatosaurus. Teyujagua is a phylogenetic miniature in which the antorbital fenestra became a vestige. Note the posterior jugal, which may or may not have supported a now missing quadratojugal anterior process.

I can see why the authors got so excited about their discovery.
The Teyjjagua skull looks like a little Chasmatosaurus skull without the antorbital fenestra. That’s because it IS one. In their own words, “This skull represents a previously unknown species that is the sister taxon to Archosauriformes and which fills a major morphological gap in understanding of early archosauriform evolution.”

Unfortunately, the authors were dealing with an antiquated cladogram
in which Youngina is basal to lizards and archosaurs… among many, many other atrocities.  They report, “Our novel cladistic analysis recovered two most parsimonious trees with 872 steps. The strict consensus of these topologies positions Teyujagua as the sister taxon of Archosauriformes, a position previously occupied by the Lower Triassic Prolacerta.”

So this is where it really pays off
to use several specimens from the Youngina grade and several specimens from the Proterosuchus grade along with 660+ opportunity taxa to nest with.

Figure 2. The rostrum of Teyujagua with the vestigial antoribital fenestra circled here. You can see how the maxilla grew over the opening. Once again, this is data that should have been announced from firsthand observation by PhD level paleontologists, not from a casual observer of photographic data.

Figure 2. The rostrum of Teyujagua with the vestigial antoribital fenestra circled here. You can see how the maxilla grew over the opening. Once again, this is data that should have been announced from firsthand observation by PhD level paleontologists, not from a casual observer of photographic data.

Diagnosis (from the paper)
“Archosauromorph with the following unique character combination: confluent, dorsally positioned external nares; maxilla participating in orbital margin; antorbital fenestra absent; trapezoidal infra temporal fenestra with incomplete lower temporal bar; teeth serrated on distal margins; surangular bearing a lateral shelf; external mandibular fenestrae present and positioned beneath the orbits when the lower jaw is in occlusion (autapomorphic for Teyujagua).”

Comments on the diagnosis
The NMQR 1484/C specimen of Chasmatosaurus (Fig. 1) is pretty well preserved except for the premaxilla/narial region. Given the morphology of the Teyujagua rostrum, the NMQR specimen likely shares the trait of a dorsal naris, perhaps with a slender ascending process of the premaxila, which might be lost in both specimens. The maxilla actually does not appear to reach the orbit. The antorbital fenestra remains present as a closed over vestige. The lower temporal bar might be incomplete, but just as likely the anterior process of the quadratojugal might be taphonomically missing, as in the NMQR specimen (Fig. 1). Other proterosuchids have similar tooth serrations. The mandibular fenestra is further forward, but the posterior mandible is also deeper. The specimen is indeed distinct enough to merit a unique generic name, as is the case with several of the Chasmatosaurus/Proterosuchus specimens, which do NOT represent a growth series.

Phylogenetic miniaturization
has reduced the antorbital fenestra in BPI 2871 and Elachistosuchu, which nest at the base of the Choristodera. Both nest as descendants of larger Chasmatosaurus specimens in the large reptile tree.

Figure 1. Elachistosuchus (Janensch 1949, Sobral et al. 2015) is a sister to BPI 2871, a basal choristodere.

Figure 3. Elachistosuchus (Janensch 1949, Sobral et al. 2015) is a sister to BPI 2871, a basal choristodere.

Not mentioned by the authors
The miniaturized skull of Teyujagua has fewer teeth than in sister or ancestors, but matching the condition in Euparkeria (Fig. 1), a related taxon only one node away at the base of a sister clade.

Rostral area of BPI 2871, formerly considered a younginid and here nesting at the base of the Choristodera.

Figure 4. Rostral area of BPI 2871, formerly considered a younginid and here nesting at the base of the Choristodera as a descendant of Chasmatosaurus.

If you don’t remember
this earlier post (2011), Youngoides (UC1528, Fig. 5) had the genesis of an antorbital fenestra. It is the current proximal sister to the Archosauriformes in the large reptile tree.

Figure 1. GIF movie tracing the antorbital fenestra with fossa and surround bones of the FMNH UC 1528 specimen of Youngoides romeri. This is one of the earliest and most primitive appearances of the archosauriform antorbital fenestra, previously overlooked.

Figure 5. GIF movie tracing the antorbital fenestra with fossa and surround bones of the FMNH UC 1528 specimen of Youngoides romeri. This is one of the earliest and most primitive appearances of the archosauriform antorbital fenestra, previously overlooked.

In summary, the authors report
“Teyujagua presents an unexpected combination of basal archosauromorph and typical archosauriform features. For example, Teyujagua resembles basal archosauromorphs in lacking an antorbital fenestra and retaining open lower temporal bars1. However, Teyujagua possesses external mandibular fenestrae and serrated teeth, features previously considered unique to Archosauriformes.”

Unfortunately, the authors appear to forget
that the antorbital fenestra can phylogenetically disappear and Chasmatosaurus demonstrates that the quadratojugal can wither phylogenetically or taphonomically disappear. It is a fragile bone.

References
Pinheiro FL, França MAG, Lacerda MB, Butler RJ and Schultz CL 2016. An exceptional fossil skull from South America and the origins of the archosauriform radiation. Nature Scientific Reports 6:22817 DOI: 10.1038/srep22817.

A new view of the first archosauriform antorbital fenestra

Earlier we looked at the FMNH UC 1528 specimen of Youngoides romeri. Today we’ll see another view of the same specimen in a GIF movie (Fig. 1). The antorbital fenestra identified here has been overlooked for several decades.

Figure 1. GIF movie tracing the antorbital fenestra with fossa and surround bones of the FMNH UC 1528 specimen of Youngoides romeri. This is one of the earliest and most primitive appearances of the archosauriform antorbital fenestra, previously overlooked.

Figure 1. GIF movie tracing the antorbital fenestra with fossa and surround bones of the FMNH UC 1528 specimen of Youngoides romeri. This is one of the earliest and most primitive appearances of the archosauriform antorbital fenestra, previously overlooked.

Youngoides romeri FMNH UC1528 (Olson and Broom 1937) Late Permian, Wuchiapingian, ~255 mya was derived from a sister to Youngina AMNH 5561 and preceded Proterosuchus at the base of the Eurchosauriformes.

This nesting of Youngina and Youngoides at the base of the Protorosauria and Archosauriformes is recovered in the large reptile tree. Prior studies tended to include squamates close to this node, but the large reptile tree found squamates nesting on a completely separate branch with a last common ancestor at the origin of the Amniota/Reptilia.

References
Olson EC and Broom R 1937. New genera and species of tetrapods from the Karroo Beds of South Africa. Journal of Paleontology 11(7):613-619

wiki/Youngina

Evolution basics – starring Jon Stewart and Babe Ruth

Evolution does not work in mysterious ways.
The basics (small variations leading over dozens of generations to larger changes) are simple:

GIF movie 1. Skull width as a variable demonstrated by Babe Ruth and John Stewart in this animated GIF file.

GIF movie 1. Skull width as a variable demonstrated by Babe Ruth and John Stewart in this animated GIF file.

  1. wider / narrower (skull, body, feet, etc.)
  2. taller-larger / smaller-shorter
  3. longer (more ribs) / shorter (fewer ribs)
  4. longer limbs / shorter limbs
  5. larger skull / smaller skull
  6. longer preorbital region / longer postorbital region
  7. longer neck / shorter neck
  8. sharp claws / rounded claws
  9. etc. / etc.

At left 
are extinct baseball star, Babe Ruth, and extant comedian/commentator, Jon Stewart, graphically demonstrating #1 on the above list, wider / narrower in the skull shape. Both are male members of the species Homo sapiens.

Other traits
one can add to this list include various perforations or fenestrae (which have several and often convergent origins and disappearances:

  1. fenestra between the naris and orbit (antorbital fenestra)
  2. fossa surrounding antorbital fenestra
  3. one or more fenestrae between the orbit and occiput
  4. fenestra in the mandible
  5. occipital fenestrae expand over braincase
  6. acetabulum perforated or not

And once fenestrae are formed:

  1. Loss of lower temporal arch
  2. Loss of upper temporal arch
  3. Loss of both

Then add
the size and shape of various bones and their processes compared to other bones and you have yourself a long character list. Enough of these (150+) provide a good matrix of characters and character states that can produce the menagerie of reptiles found in the large reptile tree, now numbering 566 taxa for 228 characters.

The wider / narrower and smaller / larger dichotomies 
can also be seen in the variety of specimens attributed to Proterosuchus and Chasmatosaurus (Fig. 2, Broom 1903). Some paleontologists (Welman 1998, Ezcurra  and Butler 2015) consider these taxa congeneric. They think this variety constitutes an ontogenetic series. On the other hand, the large reptile tree recovered these taxa in distinct nodes and clades. Narrower-skulled forms nest together. So do wider-skulled forms and they lead to other even more distinct taxa, including some once again tiny forms. The tall-skulled proterosuchids do not lead to more derived taxa.

Figure 3. The many faces of Proterosuchus to scale and in phylogenetic order, among with their closest known relatives. Note the phylogenetic miniaturization, reduction of the drooping premaxilla and loss of the antorbital fenestra after the TM 201 specimen of Chasmatosaurus. Click to enlarge.

Figure 2. The many faces of Proterosuchus to scale and in phylogenetic order, among with their closest known relatives. Note the phylogenetic miniaturization, reduction of the drooping premaxilla and loss of the antorbital fenestra after the TM 201 specimen of Chasmatosaurus. Click to enlarge.

The smallest taxon
shown here (Fig. 2), Youngoides romeri, leads to euparkeriids and then to a long list of archosauriforms including dinosaurs, crocs and birds. This last common ancestor of proterosuchids and euparkeriids (all archosauriforms) also had a small antorbital fenestra.

Have a great weekend!
Keep those cards and letters coming.

References
Broom R. 1903. On a new reptile (Proterosuchus fergusi) from the Karroo beds of Tarkastad, South Africa. Annals of the South African Museum 4: 159–164.
Ezcurra MD and Butler RJ 2015. Post-hatchling cranial ontogeny in the Early Triassic diapsid reptile Proterosuchus fergusi. Journal of Anatomy. Article first published online: 24 APR 2015. DOI: 10.1111/joa.12300
Welman J 1998. The taxonomy of the South African proterosuchids (Reptilia, Archosauromorpha). Journal of Vertebrate Paleontology 18 (2): 340–347.

Did Champsosaurus and Tchoiria have an antorbital dimple?

The large reptile tree nests choristoderes, both large and small, with archosauriformes, derived from a long series of proterosuchids ending with the small, former younginid, BPI 2871 (Fig. 1). This tiny transitional taxon at the base of the Choristodera documents yet another case of phylogenetic miniaturization and independent loss of the antorbital fenestra.

All proterosuchids had an antorbital fenestra,
but tiny BPI 2871 had a dimple, sealed in back (Figs. 1, 7), not a fenestra.

BPI2871-3views588

Figure 1. Dorsal and palatal views of BPI 2871 with bones colorized above. Below, reconstructed images of BPI 2871 tracings. It is more complete than illustrated by Gow 1975. A vestige antorbital fenestra may be present here. Compare to figure 2.

I ran across this image
of the skull of Champsosaurus (Fig. 2), a large, long-snouted choristodere. It also appears to retain a small dimple (pink arrow). The dimple is otherwise undocumented (Fig. 4) and not always duplicated, as shown here (Fig. 4).

Figure 2. This specimen of Champsosaurus appears to retain a tiny antorbital fenestra. Is this replicated in other specimens?

Figure 2. This specimen of Champsosaurus appears to retain a tiny antorbital fenestra. Is this replicated in other specimens? Or is it a shadow? If it is a shadow, this is exactly where the AOF would be if present. The nares (nostrils) are above the jaw tips.

Getting back to ‘did Champsosaurus have an antorbital dimple’?
Apparently only sometimes. In this clade the dimple is a vestige at best, and not always present. Most other choristoderes do not have an antorbital dimple. But some do.

Figure 3. Cast of Champsosaurus from Triebold Palentology. Here again we see that dimple in front of the eye.

Figure 3. Cast of Champsosaurus from Triebold Palentology. Here again we see that dimple in front of the eye, below the lacrimal prefrontal and above the maxilla. Click to enlarge.

Figure 4. How Barnum Brown illustrated the preorbital region of Champsosaurus, without a hint of a dimple or antorbital fenestra.

Figure 4. How Barnum Brown illustrated the preorbital region of Champsosaurus, without a hint of a dimple or antorbital fenestra.

Remember
living crocs also lack an antorbital fenestra. By convergence with choristoderes, crocs lost what their ancestors had. So there is precedence for such an evolutionary change. Brown 1905 did not illustrate a preorbital dimple in his treatise on Champsosaurus (Fig. 4).

Tchoiria is a basal choristodere
and this specimen (Fig. 5) appears to have a preorbital dimple along the same lines as the Triebold Champsosaurus cast (Fig. 3). But another Tchoiria (T. kauseni; “choy-er-ee-ya”??, Fig. 6, Ksepka Gao and Norell 2005) does not have such a dimple.

Figure 5. The skull of Tchoiria appears to also have a preorbital dimple.

Figure 5. The skull of Tchoiria appears to also have a preorbital dimple. This one appears to have a longer rostrum than in figure 6 and indeed, the two species are known to have different tooth counts.

 

What Tchoiria tells us about champsosaurs
Unlike other tetrapods, champsosaurs created an upper temporal arch from the postfrontal contacting the squamosal. In Tchoiria you can see that transition taking place (overlooked in the original paper, Fig. 6). Tchoiria also shows the separation of the prefrontal and nasal in their traditional places. By doing so, the very long ascending process of the premaxilla (not a narrow fused set of nasals) is revealed.

Figure 6. Tchoiria klauseni as originally interpreted and as interpreted using DGS. Note this specimen shows the the transition from the postorbital to the postfrontal contacting the squamosal. This also shows the extent of the premaxillary ascending process (yellow), nasals separate from prefrontals.

Figure 6. Tchoiria klauseni as originally interpreted and as interpreted using DGS. Note this specimen shows the the transition from the postorbital to the postfrontal contacting the squamosal. This also shows the extent of the premaxillary ascending process (yellow) and the nasals (pink_ separate from prefrontals (brown) Those would fuse in Champsosaurus (Fig. 4).

A closeup of the dimple on BPI 2871 might be instructive. 
The dimple here looks like the origin of the antorbital fenestra in Youngoides romeri (FMNH UC 1528 seen here), but it arrives at a much more derived node on the large reptile tree cladogram.

Rostral area of BPI 2871, formerly considered a younginid and here nesting at the base of the Choristodera.

Figure 7. Rostral area of BPI 2871, formerly considered a younginid and here nesting at the base of the Choristodera. Colors reappear every 5 seconds. Note the infilling of the antorbital fenestra. This is only known after cladistic analysis. The two sides of the same skull each provide clues as to the in vivo morphology. Splinters are often difficult to identify. This interpretation may change as time goes by. In the lower image, the fragile back of the dimple has been shifted dorsally. 

References
Brown B 1905. The osteology of Champsosaurus Cope. Memoirs of the AMNH 9 (1):1-26. http://digitallibrary.amnh.org/dspace/handle/2246/63
Ksepka, DT, Gao K-Q and Norell MA 2005. A New Choristodere from the Cretaceous of Mongolia. American Museum Novitates 3468. 22pp.

Haplocheirus: a basal alvarezsaroid and more…

Earlier we looked at two Cretaceous alvarezsauroid theropod dinosaurs that likely clung to trees while resting. Today we’ll look at a basal alvarezsauroid theropod dinosaur from the Jurassic with bigger claws and more primitive traits…

Figure 1. Theropods in the large reptile tree. Haplocheirus is highlighted in yellow.

Figure 1. Theropods in the large reptile tree. Haplocheirus is highlighted in yellow.

Figure 1. Haplocheirus sollers traced from several photos. This specimen is 15 million years older than Archaeopteryx and tens of million years older than dromaeosaurs and alvarezsarids. Click to enlarge. Note the robust pedal digit 2 and manual digit 1.Haplocheirus sollers (Choiniere et al. 2010 Late Jurassic, 150 mya, 2m long) is a a theropod dinosaur from the Jurassic that nests at the base of the alvarezsaurids (including Mononykus and Shuvuuia) and also basal to the Cretaceous dromaeosaurids (including Velociraptor), ~and~ basal to Jurassic proto-birds (including Aurornis, Fig. 2).

Figure 1. Haplocheirus sollers traced from several photos. This specimen is 15 million years older than Archaeopteryx and tens of million years older than dromaeosaurs and alvarezsarids. Click to enlarge. Note the robust pedal digit 2 and manual digit 1.Haplocheirus sollers (Choiniere et al. 2010 Late Jurassic, 150 mya, 2m long) is a a theropod dinosaur from the Jurassic that nests at the base of the alvarezsaurids (including Mononykus and Shuvuuia) and also basal to the Cretaceous dromaeosaurids (including Velociraptor), ~and~ basal to Jurassic proto-birds (including Aurornis, Fig. 2).

Despite the obvious similarities
to Velociraptor, Haplocheirus is treated only as a basal alvarezsauroid in Wikipedia. Certainly manual digit 1 is more robust in Haplocheirus, as in alvarezsaurids. But just as certainly, pedal digit 2 is more robust with a ginglymoid (pulley-shaped) joint (Fig. 1), as in dromaeosaurids (deinonychosaurs).

Figure 5. Balaur (in vertical and horizontal configurations) compared to Haplocheirus and Velociraptor, Aurornis, Archaeopteryx and Gallus. Balaur nests with Velociraptor in the large reptile tree.

Figure 5. Balaur (in vertical and horizontal configurations) compared to Haplocheirus and Velociraptor, Aurornis, Archaeopteryx and Gallus. Balaur nests with Velociraptor in the large reptile tree.

Phylogenetic bracketing indicates that Haplocheirus was covered with primitive feathers. The present tree (Fig. 2) is similar to that of other prior dinosaur cladograms.

The large reptile tree now includes 550 taxa.

References
Choiniere JN, Xu X, Clark JM, Forster CA, Guo Y, Han F 2010. A basal alvarezsauroid theropod from the Early Late Jurassic of Xinjiang, China. Science 327 (5965): 571–574.

The antorbital and lateral temporal fenestrae of the frog , Rana

Earlier we looked at the evolution of the frog, Rana. And it continues to be the most popular blog post of the past year.

Today, after adding Rana to the matrix of the large reptile tree (still not updated), I think it’s time we looked at the antorbital fenestra of Rana, and the lateral temporal fenestra as well (Fig. 1).

Figure 1. Rana, the bull frog, with naris in red, orbit in purple, antorbital fenestra in dark blue and lateral temporal fenestra in orange. The reduction of the the skull bones in Rana created these fenestrae.

Figure 1. Rana, the bull frog, with naris in red, orbit in purple, antorbital fenestra in dark blue and lateral temporal fenestra in orange. The reduction of the the skull bones in Rana created these fenestrae.

One usually thinks of additional skull fenestrae in the province of reptiles. As we saw earlier, the antorbital fenestra comes and goes in several reptiles. So does the lateral temporal fenestra. Amphibians (non-amniote tetrapods) typically do not have skull fenestrae. Neither to most basal reptiles.

Relative to the body, the skull of Rana is enormous. So are the hind limbs. Frogs leap, as everyone knows, and if the skull is going to be large it also has to be lightweight to enable longer leaps. So the skull bones are reduced to their bare minimum creating fenestrae.

Proximal outgroup taxa, including long-legged Triadobatrachus, likewise have reduced skull bones.

More distant outgroup taxa, including short-legged Gerobatrachaus and Doleserpeton and Utegenia have relatively smaller skulls and shorter hind limbs — and no skull fenestrae.