What happened to the postfrontal and postorbital in birds?

Fauth and Rauhut 2020 bring us
“A short overview of the evolution of the skull of birds.”

From the first paragraph (Google translated from German)
“There are a number of advantages to being able to fly, be it the possibility of rapid geographical expansion, the settlement of trees, the escape from predators or the development of new feed sources, including prey capture. However, it cannot be regarded as the sole factor for the success of birds.”

Thereafter
the authors discuss and show (Fig. 1) skull traits, but make a traditional mistake based on a lack of attention to detail. Foth and Rauhut provide only one figure (Fig. 1), in which the postorbital is identified (in orange) only in Allosaurus (B) Archaeopteryx (C) and the enanthiornine, Shenqiornis (D). The postorbital is deemed absent in the extant Crax (A) and the extinct Ichthyornis (E) despite its presence in their diagram.

Figure 1. Theropod and bird skulls from Foth and Rauhut 2020. Postorbital is highlighted in orange, but not the same vestigial postorbital is not highlighted in bird skulls.

Figure 1. Theropod and bird skulls from Foth and Rauhut 2020. Postorbital is highlighted in orange, but not the same vestigial postorbital is not highlighted in bird skulls. Note: ‘Archaeopteryx’ is a wastebasket taxon with variation among the 13 known specimens.

Unfortunately
Foth and Rauhut took the easy way out by using previously provided oversimplified diagrams that lack the data needed to create a valid figure. They also followed paleontological tradition, which, at times like this, fail to provide valid data in the details.

Here are the missing details
in an actual Crax skull (Fig. 2) colorized using DGS methods. It shows a descending postfrontal (orange) and a vestigial postorbital (yelllow splint, but see caption for one more option). The postfrontal is largely fused to the frontal, but that does not negate its presence. No unfused frontal descends beyond mid depth in any vertebrate skull. We should label and score with reason, not with invalid traditions.

Figure 1. Crax tuberosa skull in three views.

Figure 2. Crax tuberosa skull in three views. Note the splint-like post0rbital (yellow). Alternate hypothesis: the splint is the postorbital process of the jugal (cyan, separate ossification from the base below the quadratojugal (olive). That would make the lumpy orange postfrontal the postfrontal + fused postorbital. Time to look at some embryos to see what is happening here: another great PhD dissertation.

The Eichstätt specimen of Archaeopteryx (= Jurapteryx)
shows the separation of the postfrontal (orange) from the frontal and the postorbital (in yellow) disarticulated and shifted slightly posteriorly in situ. This is the specimen basal to extant birds.

Figure 3. The Eichstätt specimen, Jurapteryx recurva, nests with the living ostrich, Struthio, presently in the LRT.

Figure 3. The Eichstätt specimen, Jurapteryx recurva, nests with the living ostrich, Struthio, presently in the LRT.

The tiny Early Cretaceous theropod, Scipionyx
(Fig. 4), demonstrates the separation of the frontal (blue), postfrontal (yellow-green) and postorbital (orange) in non-avian theropods. These elements tend to fuse with size. Phylogenetic miniaturization (= neotony) tends to separate the original elements. When dealing with shrinking taxa, like birds, try to keep this in mind.

Figure 1. Scipionyx skull and overall. The tail and feet are restored.

Figure 4. Scipionyx skull and overall. The tail and feet are restored.

The enantiornithine, Shenqiornis,
will be considered in greater detail In future blogposts.


References
Foth C and OWM Rauhut 2020. Eine kurze Betrachtung der Evolution des Vogelschädels [A short overview on the evolution of the skull of birds]. Jahresbericht 2019 und Mitteilungen 48. ISSN 0942-5845 ISBN 978-3-89937-253-3

Hyomandibular evolution + Introducing the postsquamosal

Revised July 19, 2020
with new bone identities given to several lobefin fish, correcting the mistakes of Thomson 1966.

The hyomandbular is the largest bone
in the entire body of the basal ray-finned fish Trachinocephalus (Fig. 1). Over time and phylogeny it evolves to become the smallest bone in the human body, the stapes (Fig. 3), one of the ultra tiny sound-conducting bones of the middle ear.

Along the way,
the large reptile tree (LRT, 1656+ taxa) presents a new (and heretical) lineage of tetrapod ancestry, distinct from the traditional one that includes Ichthyostega and Acanthostega. Today we go below the surface and formally introduce ‘the lineup.’

Figure 1. Hyomandibular evolution from the first dichotomy of bony fish to Gephyrostegus. The hyomandibular evolves to become the stapes. Note the hyomandibula contact with the intertemporal, quadrate and pterygoid, sometimes fused to these bones. The hyomandibular is poorly ossified in Onychodus, so it restored here. Note how the maxilla splits to produce the quadratojugal.

Figure 1. Hyomandibular evolution from the first dichotomy of bony fish to Gephyrostegus. The hyomandibular evolves to become the stapes. Note the hyomandibula contact with the intertemporal, quadrate and pterygoid, sometimes fused to these bones. The hyomandibular is poorly ossified in Onychodus, so it restored here. Note how the maxilla splits to produce the quadratojugal.

Some bones are relabeled
from the diagram found in Thomson 1966 (modified in Fig. 2), who presented several layers of skull bones (cranial, palatal and dermal) in the the Permian megalichthyid rhipidistian fish, Ectosteorhachis, a late-survivor of an earlier (Mid-Devonian) radiation that ultimately produced tetrapods and humans. Thomson mislabeled the dentary as a maxilla (mx) in his diagram, but all other labels are traditional.

In most fish 
the hyomandibular is roofed over by the otherwise unremarkable intertemporal, which anchors it dorsally.

That brings up a problem in Thomson’s diagram
(Fig. 2). To remedy that problem, here the dorsal rim of the traditional palatoquadrate is relabeled as the hyomandibular fused to the pterygoid and other palatal elements. Second, the labeled hyomandibular (h) is now the preopercular. Third, the traditional preopercular (pop) requires a new name: the postsquamosal. It is not homologous with the preopercular of teleost fish.

The disappearance of the traditional preopercular
Trachinocephalus (Fig. 1) retains a traditional preopercular. Pteronisculus (Fig. 1) has a postsquamosal and lacks a traditional preopercular on the surface. Cheirolepis (Fig. 1) lacks both. The squamosal and postsquamosal appear to be fused or else the tiny postsquamosal is overwhelmed by the advancing squamosal. The traditional rhipidistians have a postsquamosal. The tetrapods (Fig. 1) lack a postsquamosal.

The more derived traditional transitional tetrapods,
Acanthostega and Ichthyostega, have a postsquamosal, but this appears as a reversal, a neotonous trait. These two are secondarily more aquatic than their ancestor, Ossinodus.

Figure 2. Ectosteorhachis skull from Thomson 1966 with layers to show the brain case and palatoquadrate. Some bones are relabeled in the revised view.

Figure 2. Ectosteorhachis skull from Thomson 1966 with layers to show the brain case and palatoquadrate. Some bones are relabeled in the revised view.

The hyomandibular decreases in size
in most later tetrapods (Fig. 3), where it continues to shrink into the auditory channel where it is then known as the stapes.

At the same time, the intertemporal
disappears or fuses to nearby skull bones in several derived basal tetrapods and basal reptiles, all by convergence.

Figure 4. Evolution of the tetrapod mandible and ear bones leading to humans.

Figure 4. Evolution of the tetrapod mandible and ear bones leading to humans in lateral and medial views, first printed in From The Beginning, the Story of Human Evolution (Peters 1991), colors added here.

The quadratojugal first appears in the tetrapod lineage
in Gogonasus (Fig. 1) after the elongate maxilla of Onychodus splits in two.

Finally,
sharks also have a palatoquadrate, but it is composed of a fused lacrimal, jugal and squamosal with a tooth-bearing premaxilla and maxilla fused to the ventral rim. The pseudo- or plesio-palatoquadrate illustrated by Thomson 1966 in Ectosteorhachis and other rhipidistians,is not homologous and is comprised of different bones. 


References
Thomson KS 1966. The evolution of the tetrapod middle ear in the rhipidistian-amphibian transition. American Zoologist 6:379–397.

Helodus: a skull without sutures

Decades prior to PAUP and MacClade,
Professor Moy-Thomas 1936 reasoned that Helodus simplex (Fig. 1; Agassiz 1838; Early Carboniferous, 300mya, 30cm long) ) was close to the ancestry of the clade Holocephalii (ratfish, chimaeras and kin; Fig. 2), which we looked at yesterday. In complete accord, the large reptile tree (LRT, 1641 taxa; subset Fig. 3) fully supports that nesting using modern software: PAUP and MacClade. So… belated well done, Professor Moy-Thomas!

Figure 1. Helodus skull drawings from xxx 1938 show no skull sutures. Colors are applied with blending edges to show were bones are based on tetrapod homologs.

Figure 1. Helodus skull drawings from Moy-Thomas 1936 show no skull sutures. Colors are applied with blending edges to show were bones are based on tetrapod homologs. Gill bars are missing from these diagrams, so were added in light blue here.

In Helodus
the skull bones are all fused together, so suture estimates are provided here (Fig. 1) based on phyogenetic bracketing. Note the tiny premaxillary teeth and complex maxillary teeth. Tabulars appear to be absent. Note the coosified cervicals / anterior dorsals extending to the notochord and first dorsal spine.

Moy-Thomas 1936
considered the anatomy of Helodus in detail. From the abstract:

  1. “The skull is found to be holostylic, and to have many characters in common with the skull of the Holocephali, but in some respects is less specialized.
  2. The pectoral fins, with their long metapterygium, small propterygium, and fused anterior radials, resemble very closely those of the Holocephali.
  3. The pelvic and unpaired fins, and general body shape are found to resemble those of the Holocephali.
  4. It is concluded that the Cochliodonts are almost certainly closely related to the ancestors of the Holocephali, and the relatively unspecialized condition of the teeth gives support to the view that the holostylic condition of the jaws is primitive for the group. It is suggested that all the Bradyodonts were holostylic, that the hyomandibular may never have been suspensory, and that they may have diverged from the true Selachii before the hyomandibular played a part in the jaw suspension.”
FIgure 1. Ratfish (chimaera) and Heterodontus to scale.

FIgure 2. Ratfish (chimaera) and Heterodontus to scale.

 

Taking one phylogenetic step further back from Helodus,
yesterday we looked at Heterodontus (Fig. 2), the Chondrichthyes taxon phylogenetically ancestral to Helodus and the Holocephalii.

Figure 5. Subset of the LRT focusing on basal chordates, vertebrates and bony fish not related to tetrapods. Scomber and Istiophorus are new additions to the gold clade.

Figure 5. Subset of the LRT focusing on basal chordates, vertebrates and bony fish not related to tetrapods. Scomber and Istiophorus are new additions to the gold clade.

References
Agassiz L 1838. Recherches Sur Les Poissons Fossiles. Tome III (livr. 11). Imprimérie de Petitpierre, Neuchatel 73-140.
Moy-Thomas JA 1936. On the Structure and Affinities of the Carboniferous Cochliodont Helodus simplex. Cambridge University Press. 73(11):488–503.

wiki/Chondrichthyes
wiki/Chimaera
wiki/Cladoselache
wiki/Chondrosteus
wiki/Symmoriida
wiki/Horn_shark
wiki/Helodus not listed in English

Palatoquadrate evolution

The palatoquadrate
is the upper tooth-bearing jaw bone found in sharks, ratfish and skates. Sometimes marginal teeth are not illustrated on the bone (Figs. 1, 2). Sometimes marginal teeth are indeed absent.

According to Wikipedia
“In some fishes, the palatoquadrate is the dorsal component of the mandibular arch, the ventral one being Meckel’s cartilage. The palatoquadrate forms from splanchnocranium (the portion of the cranium that is derived from pharyngeal arches) in various chordates including placoderms and acanthodians.”

I don’t see a palatoquadrate in any tested placoderm, but then I have no data on the palate in any placoderm. All the dermal jaw bones are homologous with those of tetrapods and their more fish-like ancestors.

Likewise, I don’t see a palatoquadrate in any tested acanthodian. In Ischnacanthus the maxilla + squamosal + jugal look like a palatoquadrate. Perhaps that is the source of this misconception: misidentification.

Previous workers did not realize the jugal, squamosal, lacrimal and quadrate were fused together to form the palatoquadrate in Elasmobranchii (the shark-ish clade, Fig. 1). That you learn by adding taxa and identifying bones based on phylogenetic bracketing. Palatal bones are difficult to ascertain in most fossil taxa, but appear as thin struts in jawless Birkenia (Fig. 1) and the sturgeon Pseudoscaphorhynchus.

On a side note, before we continue:
Rocek 1993 described a putative palatoquadrate in the osteolepiform, Eusthenopteron, but that bone is a large fused quadrate and pterygoid plus a separate palatine and ectopterygoid (according to the diagram in Fig. 4), as in many other bony fish and tetrapods.

Figure 1. Traditional diagram from Richter and Underwood 2018 demonstrating their views on palatoquadrate evolution.

Figure 1. Traditional diagram from Richter and Underwood 2018 demonstrating their views on palatoquadrate evolution.

This palatoquadrate is not found in tetrapods.
Tetrapods, like basal fish, separate the lacrimal, jugal, squamosal, palatine, ectopterygoid, pterygoid, and quadrate. Only in sharks, ratfish and skates do these bones fuse to create a palatoquadrate.

Richter and Underwood 2018
produced a diagram (Fig. 1) intended to demonstrate the origin of the palatoquadrate from a hypothetical autostylistic taxon (ghosted out here) and its purported evolution to known taxa with other mandibular arch supports. In their diagram some of the proximal relatives do not look similar to one another.

Some terms in figure 1
were new to me, and they may be new to you, so here are the definitions:

  1. Autostyly: The mandibular arch is not supported by a hyomandibula (typical  lungfish + tetrapods)
  2. Holostyly: Palatoquadrate fused with the cartilaginous cranium and the second visceral arch entire and free from the cranium.
  3. Hyostyly: Ethmoid articulation between the upper jaw and the cranium, while the hyoid most likely provides vastly more jaw support compared to the anterior ligaments. (typical Chondrichthyes).
  4. Euhyostyly: The hyomandibular cartilages provide the only means of jaw support, while the ceratohyal and basihyal elements articulate with the lower jaw, but are disconnected from the rest of the hyoid.
  5. Amphistyly: The palatoquadrate has a postorbital articulation with the chondrocranium from which ligaments primarily suspend it anteriorly. The hyoid articulates with the mandibular arch posteriorly, but it appears to provide little support to the upper and lower jaws. (typical Actinopterygia).
  6. Orbitostyly: The orbital process hinges with the orbital wall and the hyoid provides the majority of suspensory support.

From the Richter and Unwood summary:
“Much progress has been made in the understanding of the vertebrate skull since pioneering anatomical descriptions made last century. There is still much uncertainty about precise homologies between parts of the skull of distinct groups of fishes, due to the fact that the vertebrate skull shows a remarkable morphological and anatomical plasticity.” 

After testing, their ‘uncertainty’ seems to be due to taxon exclusion (Fig. 1). After further testing, adding taxa (Fig. 2) changes the tree topology and sheds new light on the origin of the palatoquadrate.

Figure 2. Modified from Richter and Underwood 2018 with more taxa, and bone colors adjusted to match those in tetrapod taxa in the LRT. Compare to figure 1.

Figure 2. Modified from Richter and Underwood 2018 with more taxa, and bone colors adjusted to match those in tetrapod taxa in the LRT. Compare to figure 1.

Only when marginal teeth appear
(in Falcatus (Fig. 2) for sharks and Polyodon (Fig. 2) for bony fish) do we find the premaxilla and maxilla in chordates. In shark-like taxa with a palatoquadrate the new premaxilla and maxilla that create, support and shed teeth have their genesis as dermal layers fused to the large, supporting lacrimal. Teeth also appear on various palatal bones in various patterns in an assortment of tetrapods.

According to
the large reptile tree (LRT, 1631+ taxa) the last common ancestor of all fish is Birkenia (Fig. 2). For a skeletal system, it has many small dermal splinters that typically fuse together in derived / descendant taxa. The jaw elements remain as distinct bones in tetrapods and bony fish, including Falcatus (Fig. 3). By contrast, in Elasmobranchii (sharks, rays and ratfish) the upper jaw elements fuse to produce a single element, the palatoquadrate.

[Short note]:
The LRT nests finless, jawless Arandaspis, Poraspsis and kin not as traditional fish, but as massively armored lancelets, derived from Branchiostoma.

In short and simple fashion
here are the broad strokes of jaw, gill and palatoquadrate evolution.

  1. Birkenia: mouth anterior and ventral, several gill openings low, eyes dorsal
  2. Osteostraci: mouth anterior and ventral, gill openings ventral, eyes dorsal
  3. Sturgeons: mouth posterior and ventral, single gill openings lateral, eyes lateral… origin #1 of the palatoquadrate
  4. Loganiella (derived from 1. Birkenia): mouth anterior and terminal, several gill openings lateral and covered, eyes lateral
  5. Rhincodon: mouth anterior and terminal, several gill openings lateral and covered, eyes lateral… second origin of the palatoquadrate
  6. Falcatus (derived from 4. Loganiella): mouth anterior and subterminal, several gill openings lateral and covered, eyes lateral…second break up of palatoquadrate
  7. Heterodontiformes: mouth anterior and terminal, several gill openings lateral and covered, eyes lateral… continuation of palatoquadrate
  8. Chimaera: mouth anterior and terminal, single gill opening lateral and covered, eyes lateral
  9. Cladoselache: mouth anterior and terminal, several gill openings lateral and covered, eyes lateral
  10. Hexanchiformes / Chlamydoselache: mouth anterior and terminal, several gill openings lateral and covered, eyes lateral
  11. Squatina: mouth anterior and terminal, several gill openings lateral and covered, eyes lateral
  12. Rays: mouth ventral and subterminal, several gill openings ventral and covered, eyes lateral
  13. Squaliformes: mouth ventral and subterminal, several gill openings lateral and covered, eyes lateral
  14. Skates: mouth posterior and ventral, several gill openings ventral and covered, eyes dorsal
  15. Lamniformes, Lamnidae, Carcarhiniformes: mouth anterior and subterminal, several gill openings lateral and covered, eyes lateral
  16. Polyodon: (derived from 4. Loganiella) mouth anterior and ventral, single gill openings lateral and covered, eyes lateral
  17. Hybodus: mouth anterior and terminal, several gill openings lateral and covered, eyes lateral…break up of palatoquadrate
Figure 3. Falcatus skull. This taxon is close to Polyodon in the LRT.

Figure 3. Falcatus skull. This taxon is close to Polyodon in the LRT. The palatoquadrate is a single unit in the above diagram, but divide into separate elements in the DGS colored image.

Falcatus (Fig. 3) is the last common ancestor in the diagram (Fig. 2) based on the LRT to have separate jaw elements. Descendant taxa all have a palatoquadrate. Falcatus and more primitive taxa are missing from the Richter and Underwood diagram (Fig. 1).

Figure 4. The purported palatoquadrate in Eusthenopteron (Fig. 5) is actually a fused pterygoid + quadrate. The palatine and ectopterygoid are separated by sutures. The dermal skull bones are shown in figure 5.

Figure 4. The purported palatoquadrate in Eusthenopteron (Fig. 5) is actually a fused pterygoid + quadrate. The palatine and ectopterygoid are separated by sutures. The dermal skull bones are shown in figure 5.

Eusthenopteron
While overall similar to the shark palatoquadrate (Figs. 1, 2), the fused pterygoid + quadrate of Eusthenopteron (Fig. 4) is not homologous, except that both include the quadrate. This diagram, which indicates a fused quadrate + pterygoid, may in error, or an exception. Most tetrapods and their fish ancestors don’t fuse these bones.

Figure 2. Eusthenopteron skull showing some changes from the Cheirolepis skull.

Figure 5. Eusthenopteron. The interior of the skull, including the palatal bones are shown in figure 4.

Figure F. Basal tetrapods 2020.

Figure F. Basal tetrapods 2020.

References
Coates M, Gess R, Finarelli J, Criswell, K and Tietjen K 2016. A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes. Nature. doi: 10.1038/nature20806
Janvier P 1996. Early Vertebrates. Oxford: Claredon Press.
Richter M and Underwood C 2018, 2109. Origin, Development and Evolution of the Fish Skull, pp 144-159 in Johanson Z, Underwood C and Richter M Eds. Evolution and Development of Fishes. Cambridge University Press.
DOI: https://doi.org/10.1017/9781316832172.009
Rocek Z 1993. Palatoquadrate in a Devonian fish Eusthenopteron Evidence of its dual origin. Journal of Zoological Systematics and Evolutionary Research 31(1):38–46.. https://doi.org/10.1111/j.1439-0469.1993.tb00177.x

https://blogs.scientificamerican.com/laelaps/fossil-rewrites-ratfish-history/

Desmatochelys enters the LRT after bone reinterpretation

Certain aspects
of certain turtle skulls have been traditionally misinterpreted, as reported earlier.

Figure 1. The skull of the Cretaceous sea turtle, Desmatochelys, is relabeled here with the addition of color.

Figure 1. The skull of the Cretaceous sea turtle, Desmatochelys, is relabeled here with the addition of color.

A not so recent paper on the sea turtle, Desmatochelys
(Fig. 1), by Cadena and Parham 2015 misidentified several skull bones, here corrected.


References
Cadena EA and Parham. JF 2015. Oldest Known Marine Turtle? A New protostegid from the Lower Cretaceous of Colombia. PaleoBios. 32(1).

The origin of the tetrapod quadratojugal

As we’ve seen
over the past several dozen fish additions to the large reptile tree (LRT, 1583 taxa) facial bones homologous with those of tetrapods often divide and fuse. We’ve already seen multipart jugals, lacrimals, nasals and squamosals in fish. This can be confusing and is probably the reason why fish facial bones are traditionally not labeled with tetrapod nomenclature. Expect homology arguments to last for decades, but here all fish facial bones are colored with tetrapod homologs.

Figure 1. Gogonasus skull demonstrating the genesis of the split between the toothy maxilla and the toothless quadratojugal.

Figure 1. Gogonasus skull demonstrating the genesis of the split between the toothy maxilla and the toothless quadratojugal.

The one bone that first appears by a split
of the maxilla into anterior toothy and posterior toothless portions is the quadratojugal. Phylogenetically the quadratojugal first appears on Gogonasus (Fig. 1). Prior taxa lack it. Later taxa have it. Even so, until the cladogram got figured out, this was puzzling.

Figure 1. Subset of the LRT alongside the definitions published in Laurin, Girondot and de Ricqles 2000.

Figure 2. Subset of the LRT alongside the definitions published in Laurin, Girondot and de Ricqles 2000. Note the Gogonasus node, where the quadratojugal first appears.

Gogonasus andrewsae (Long 1985, Long et al. 1997; Late Devonian, 380 mya; NMV P221807; 30-40cm in length) is the best preserved specimen of its type. This is the crossopterygian transitional between rhizodontids, coelocanths and higher tetrapodomorphs. The maxilla splits in two creating the tetrapod quadratojugal. The squamosal splits in two creating the tetrapodomorph preopercular. A pineal opening appears, so does the tetrapodomorph choana. The lacrimal contacts the external naris. This is the crossopterygian that gave rise to tetrapodomorphs and tetrapods, rhizodontids and gulper eels.


References
Long JA 1985. A new osteolepidid fish from the Upper Devonian Gogo Formation of Western Australia, Recs. Western Australia Mueum 12: 361–377.
Long JA et al. 1997. Osteology and functional morphology of the osteolepiform fish Gogonasus Long, 1985, from the Upper Devonian Gogo Formation, Western Australia. Recs. W. A. Mus. Suppl. 57, 1–89.

wiki/Gogonasus

Where do sea horses come from?

A little off topic,
but I was curious to see how the odd morphology of the sea horse came to be, who its ancestors were and what transitional taxa went through on their evolutionary journey through deep time. Hope you find this interesting.

The relationship between sticklebacks and sea horses
has been known for many decades. Both are members of the clade Gasterosteiformes, which is in the clade of spiny finned fish, Acanthopterygii, which is in the clade of bony ray-finned fish, Actinopterygii.

A helpful guide
is Gregory 1933, available online as a PDF. Most of the images below come from that book.

No phylogenetic analysis was performed here,
so think of the following images as broad evolutionary brush strokes, not a narrow ladder of succession. Few details are offered because most are apparent at first glance. Precise last common ancestors remain unknown. These are rare derived representatives of deep time radiations.

Even so,
the early appearance of body armor in the stickleback, G. aculeatus; the diminution of the tail (except in the pipefish/fantails, Dunckerocampus and Solenostomus); the gradual loss of the fusiform shape; the elongation of the rostrum and the reduction of the mouth are all apparent in this series of illustrations.

Figure 1. Stickleback to sea horse evolution through pipefish. Sticklebacks have some of the body armor that overall encases and stiffens sea horses and sea dragons like Hippocampus and two species of Phyllopteryx. The ghost pipeish Solenostomus, is distinct from the more slender, elongate types of pipefish.

Figure 1. Stickleback to sea horse evolution through pipefish. Sticklebacks have some of the body armor that overall encases and stiffens sea horses and sea dragons like Hippocampus and two species of Phyllopteryx. The ghost pipeish Solenostomus, is distinct from the more slender, elongate types of pipefish.

The skulls of the taxa shown above
(Fig. 2) detail other changes, such as how far anteriorly the quadrate and palate bones shift on these fish with an ever longer rostrum and ever smaller mouth losing tiny teeth. The hyomandibular (hy) is the stapes in tetrapods. Not sure about the homology of the squamosal and the labeled preopercular, but the following is offered. Sometimes fish and tetrapods have different names for the same bones, as we learned earlier here.

Figure 2. Most of the bones of the tetrapod skull are also found in sea horses with some odd changes, like the placement of the quadrate well anterior to the orbit. Hippocampus illustration from Franz-Odendaal and Adriaens 2014. Not the parasphenoid passing midway through the orbit, while in tetrapods the orbit is typically raised above this anteriorly-directed splint-like bone arising from the basicranium. Everything below it is mouth cavity.

Figure 2. Most of the bones of the tetrapod skull are also found in sea horses with some odd changes, like the placement of the quadrate well anterior to the orbit. Hippocampus illustration from Franz-Odendaal and Adriaens 2014. Not the parasphenoid passing midway through the orbit, while in tetrapods the orbit is typically raised above this anteriorly-directed splint-like bone arising from the basicranium. Everything below it is mouth cavity.

So many bones
are displaced or lost in sea horses distinct from their basal vertebrate locations (e.g. Cheirolepis, Fig. 3) that an evolutionary series illustration (Fig. 2) proves helpful in understanding the lumping and splitting of clade members. Sarcopterygians, like Osteolepis (Fig. 3), split off early from other ray-finned fish, which is why they appear share more traits and proportions with Cheirolepis. Note the jaw hinge remains posterior to the orbit in these two.

Figure 2. Cheirolepis skull (left) with skull bones colorized as in Osteolepis (right) and Enteognathus, figure 1. Colors make bone identification much easier. Note the post opercular bone differences between Osteolepis and Cheirolepis indicating separate and convergent derivation, based on present data.

Figure 3. Cheirolepis skull (left) with skull bones colorized as in Osteolepis (right) and Enteognathus, figure 1. Colors make bone identification much easier. Note the post opercular bone differences between Osteolepis and Cheirolepis indicating separate and convergent derivation, based on present data.

Understanding where we came from,
and where our cousins went in their evolutionary journeys are the twin missions of this blogpost in support of ReptileEvolution.com.


References
Franz-Odendaal TA and Adriaens D 2014. Comparative developmental osteology of the seahorse skeleton reveals heterochrony amongst Hippocampus sp. and progressive caudal fin loss. EvoDevo 2014, 5:45
Gregory WK 1933. Fish skulls: a study of the evolution of natural mechanisms. American Philosophical Society. ISBN-13: 978-1575242149 PDF

wiki/Seahorse
diverosa.com/Syngnathidae

Acostasaurus enters the LRT

Pérez and Noé 2017 described
a near complete 3D skull, a complete hindlimb and several vertebrae of a eusauropterygian, Acostasaurus (Fig. 1), they considered it a 4-5m long, snort-snouted pliosaur, one of many ‘pliosaurs’ found in Barremian (Early Cretaceous) Columbia. 

Figure 1. Acostasaurus skull from Pérez and Noé 2017, colors added.

Figure 1. Acostasaurus skull from Pérez and Noé 2017, colors added.

Some of those purported Columbian ‘pliosaurs’
turned out to be giant sisters to more basal eusauropterygians in the large reptile tree (LRT, 1430 taxa). You might remember (here) the giant Sachicasaurus nested with Nothosaurus and (here) the very large Bobosaurus nested with the smaller Corosaurus.

In the LRT
Acostasaurus nests with Anningsaura (Fig. 2) apart from the pliosaurs in the LRT.

Figure 6. Anningasaura colorized from an old engraving. No other aquatic taxon has such bizarrely curved teeth. This taxon is closely related to Hauffiosaurus.

Figure 6. Anningasaura colorized from an old engraving. No other aquatic taxon has such bizarrely curved teeth. This taxon is closely related to Hauffiosaurus.

The authors compared Acostasaurus
with Simolestes a taxon not yet added to the LRT. Look for it soon.

Figure 4. Subset of the LRT focusing on Eusauropterygians (pachypleurosaurs, nothosaurs, plesiosaurs and kin).

Figure 4. Subset of the LRT focusing on Eusauropterygians (pachypleurosaurs, nothosaurs, plesiosaurs and kin).


References
Gómez Pérez M and Noè LF 2017. Cranial anatomy of a new pliosaurid Acostasaurus pavachoquensis from the Lower Cretaceous of Colombia, South America. Palaeontographica Abteilung A. 310 (1–2): 5–42. doi:10.1127/pala/2017/0068.

wiki/Acostasaurus

Splitting the frontals in pliosaurs

Pliosaurs are like derived pterosaurs in very few ways,
but this one stands out: The premaxilla extends all the way back to the parietal (Fig. 1) in both clades.

Figure 1. Kronosaurus dorsal skull colorized from originals in McHenry 2009.

Figure 1. Kronosaurus dorsal skull colorized from originals in McHenry 2009. Note the premaxilla/parietal contact. Here the nasals appear to fuse to the maxillae, something added here in pink that was not intended by McHenry 2009. n = naris. o = orbit.

Other sauropterygian taxa split the nasals
as the premaxilla extends to the frontals. Pliosaurs also split the frontals with the invading premaxilla (Fig. 1).

By contrast,
in pterosaurs the premaxillae more or less sits on top of the nasals and frontals whenever the premaxillae extend posteriorly.

If you’ve ever had an interest in the giant pliosaur, Kronosaurus
you will be thrilled to read McHenry 2009. It’s chock full of details like this (Fig. 1).


References
McHenry CR 2009. ‘Devourer of Gods’ The palaecology of the Cretaceous pliosaur Kronosaurus queenslandicus. PhD dissertation, U of Newcastle.

Third eye distribution in 1413 LRT tetrapods

The mysterious pineal/parietal eye/opening.
Some tetrapods have one between the parietals (Fig. 1). Others seal off that opening. Some either greatly expand the pineal or reduce the length of the parietal creating a ‘large’ opening relative to the length of the parietal. The pineal body varies greatly in size. In humans it is tucked in below the cerebrum and cerebellum. By contrast, in lampreys it extends high above the brain and is light sensitive (Fig. 2). According to Wikipedia, “The [third] eye is photoreceptive and is associated with the pineal gland, regulating circadian rhythmicity and hormone production for thermoregulation.

Wikipedia reports, 
“The tuatara has a third eye on the top of its head called the parietal eye. It has its own lens, a parietal plug which resembles a cornea, retina with rod-like structures, and degenerated nerve connection to the brain. The parietal eye is only visible in hatchlings, which have a translucent patch at the top centre of the skull. After four to six months, it becomes covered with opaque scales and pigment. Its purpose is unknown, but it may be useful in absorbing ultraviolet rays to produce vitamin D, as well as to determine light/dark cycles, and help with thermoregulationOf all extant tetrapods, the parietal eye is most pronounced in the tuatara. It is part of the pineal complex, another part of which is the pineal gland, which in tuatara secretes melatonin at night. Some salamanders have been shown to use their pineal bodies to perceive polarised light, and thus determine the position of the sun, even under cloud cover, aiding navigation.

The distribution pattern in extinct tetrapods
is readily apparent in a broad sense ( Fig. 1). Even so, exceptions appear often. Reversals appear rarely. It is interesting to note the last time a third eye opening appeared in the skulls of various lineages. The large reptile tree (LRT, 1413 taxa) scores for 231 traits, one of which is #39, the pineal foramen.  Scoring choices include:

  1. Present and tiny <.20 parietal length
  2. Absent
  3. Present and large ≥ .20 parietal length
  4. Between the frontals (= anterior to the parietals)
Figure 1. Click to enlarge. This is the complete LRT highlighting the distribution of the pineal opening.

Figure 1. Click to enlarge. This is the complete LRT highlighting the distribution of the pineal opening.

The primitive state is: ‘between the frontals’.
This occurs in basal fish, prior to sarcopterygians. This state also occurs by reversal in several iguanid squamates related to Chlamydosaurus.

The derived state is ‘absent’.
This occurs in a wide variety of taxa from derived eryopids to mammals, several squamates, macrocnemids (including fenestrasaurs) and euarchosauriformes among others.

Most basal tetrapods
have a ‘tiny’ pineal opening.

Most basal reptiles
have a large pineal opening.

Figure 3. Pineal body in a primitive jawless fish, like the lamprey.

Figure 3. Pineal body in a primitive jawless fish, like the lamprey.

According to Wikipedia
“The pineal gland is a small endocrine gland in the brain of animals with backbones. The pineal gland produces melatonin, a serotonin-derived hormone which modulates sleep patterns in both circadian and seasonal cycles.”

Figure 1. The basal synapsid, Vaughnictis, and the basal caseasaur, Eothyris. For starters, synapsids have a taller than wide skull and caseasaurs have a wider skull. See text for other details.

Figure 3. The basal synapsid, Vaughnictis, and the basal caseasaur, Eothyris. Both branches of basal reptilesw (Fig. 1) retained a pineal/parietal opening for the third eye.

“The results of various scientific research in evolutionary biology, comparative neuroanatomy and neurophysiology, have explained the phylogeny of the pineal gland in different vertebrate species. From the point of view of biological evolution, the pineal gland represents a kind of atrophied photoreceptor. In the epithalamus of some species of amphibians and reptiles, it is linked to a light-sensing organ, known as the parietal eye, which is also called the pineal eye or third eye.”


References
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.

wiki/Parietal_eye
wiki/Pineal_gland
wiki/Tuatara