Spinosaurus thermoregulation

Spinosaurus has been recently revised from a long-legged terrestrial big brother to Baryonyx, to a short-legged aquatic giant that probably found it difficult to walk bipedally (Ibrahim et al. 2014; Fig. 1). As the only quadrupedal theropod, Spinosaurus needs to be considered in terms of its environment.

Figure 1. Aquatic Spinosaurus to scale with contemporary Early Cretaceous giant fish.

Figure 1. Aquatic Spinosaurus to scale with contemporary Early Cretaceous giant fish. Click to enlarge. Spinosaurus may have been so large because its prey was so large. As the only aquatic dinosaur, Spinosaurus may have developed a sail to help regulate body temperature while staying submerged except to lay eggs. It may have never needed to stand bipedally, like its theropod sisters.

As the only aquatic dinosaur (until Hesperornis, ducks and penguins came along), Spinosaurus was unlike its closest sisters in several regards. It was larger. It had shorter hind limbs. And it had that famous sail back. If we put Spinosaurus into it proper environment, shallow waters, then the reason for the sail, the great size and the short hind limbs becomes readily apparent.

Sail for thermoregulation
Most dinosaurs did not live in water. Those that do (like aquatic birds) are covered with insulating feathers that keep them warm. Spinosaurus likely did not have feathers, or enough feathers to keep it warm, but it did have that sail. Exposed above the surface to the warmer air, the sail could have helped Spinosaurus maintain a higher body temperature in cooler waters. Overheating was unlikely surrounded by water. Other theropods with longer dorsal spines, like Acrocanthosaurus, show no aquatic adaptations.

Short legs for walking underwater
The hind limbs on Spinosaurus are so short relative to the body that it is difficult to see how it could have walked bipedally like other theropod dinosaurs. Those heavily clawed arms appear to be ill-suited to support the great weight of its forequarters. In an aquatic environment, however, that great weight essentially disappears. Spinosaurus could have walked along the muddy/sandy bottom. It is not known if the hind feet were webbed, but they look like they were best articulated when they were spread (Fig. 2).

Figure 2. The foot of Spinosaurus with PILs and possible webbing. The joints of the foot on the right appear to be better aligned.

Figure 2. The foot of Spinosaurus in ventral view with PILs and possible webbing. The joints of the foot on the right appear to be better aligned.That’s the vestige of digit 5 below metatarsal 4.

Spinosaurus likely preferred water of a certain depth. Deep enough to cover everything but the sail (floating enough to keep weight off its feet), yet just deep enough to touch the bottom with its clawed feet. After all, Spinosaurus did not have flippers or fins. That’s not to say it didn’t swim in deeper waters, or visit shallower waters. After all, it had to lay eggs on land, but it is likely to have been awkward when not supported by water.

Great size
At the same time and in the same waters as Spinosaurus several different types of giant fish co-existed. Many, no doubt, were on Spinosaurus’ menu. Younger spinosaurs would have eaten younger, smaller fish. The snout of Spinosaurus has many small pits. These are thought to have housed pressure sensors to detect prey in murky waters, as in living crocs.

Spinosaurus has been well studied
and there is little else I can add to the data and hypotheses available online here, here and here. The Spinosaurus in Jurassic Park 3 represents the old long-legged, terrestrial version, so best to forget images of Spino attacking T-rex on land. There is great artwork of the new Spinosaurus here, here, here and here.

And I just ran across this beauty.

References
Ibrahim N, Sereno PC, Dal Sasso C, Maganuco S, Fabbri M, Martill DM, Zouhri S, Myhrvold N, Iurino DA 2014. Semiaquatic adaptations in a giant predatory dinosaur. Science. doi:10.1126/science.1258750.

Jeholopterus wing spread

Updated March 4, 2015 with a new orientation of the Jeholopterus hind limbs and a new tail. 

Yesterday we looked at Jeholopterus, the vampire pterosaur. I also added a dorsal view of Jeholopterus to yesterday’s post to better compare it with the AMNH animated cartoon.

Today we’ll compare long and broad Jeholopterus wings to the likely shorter wings (parts are unknown) of a sister taxon from the same formation, Daohugoupterus (Fig. 1).

Figure 1. Daohugoupterus and Jeholopterus in dorsal view to the same scale. note the much smaller wings of Daohugoupterus with a similar size skull and body. Daohugoupterus may have been the 'bumblebee" of the Daohugou Formation -- technically unable to fly, but able to fly nevertheless. A rapid wingbeat probably sustained it. By contrast, Jeholopterus had broad owl-like wings, probably requiring fewer wingbeats. Note also the placement of the orbits, lateral on the left, more forward facing on the right. Not sure how to handle the hind limb filaments other than in the pattern shown here. Filaments like these may have acted to help silence the flight of Jeholopterus by absorbing sound.

Figure 1. Daohugoupterus and Jeholopterus in dorsal view to the same scale. note the much smaller wings of Daohugoupterus with a similar size skull and body. Daohugoupterus may have been the ‘bumblebee” of the Daohugou Formation — technically unable to fly, but able to fly nevertheless. A rapid wingbeat probably sustained it. By contrast, Jeholopterus had broad owl-like wings, probably requiring fewer wingbeats. Note also the placement of the orbits, lateral on the left, more forward facing on the right. Not sure how to handle the hind limb filaments other than in the pattern shown here. Filaments like these may have acted to help silence the flight of Jeholopterus by absorbing sound.

Jeholopterus had a wing area close to 4x that of the similarly-sized Daohugoupterus. This has ramifications with regard to wing beat amplitude and frequency. In order to fly more wingbeats/second are required for the pterosaur with the smaller wings. Similar difference occur between ducks and gulls.

On the same note, I’d like to direct your attention to a PBS special on owls
available online here. While owls are distinctly different from Jeholopterus, there are some analogies, as I’m sure you’ll see after viewing the program.

Like owls,
the vampire Jeholopterus would have benefited from coming in on its dinosaurian prey quietly before making its presence known. That would have happened the moment it slid those surgically-curved claws and teeth beneath soft parts of the dinosaurian hide.

Figure 2. Wing and other extra dermal membranes surrounding Jeholopterus.

Figure 2. Wing and other extra dermal membranes surrounding Jeholopterus. Note the narrow chord wing membrane preserved on both wings, just like the Zittel wing and the Vienna Pterodactylus. On the right there are some hairy plumes, not unlike those of Longisquama, which had dorsal plumes along the midline. That cannot be demonstrated with the pterosaur crushed in the dorsoventral plane, only imagined.

The extradermal membranes of Jeholopterus (Fig. 2) have been acknowledged, but never traced and modeled to see if the fibers are similar in the front and the back and what their morphology might be. Previously I assumed these were simple fibers, but that no longer seems to be the case in every case.

Here (Fig. 2) the wing membranes have a standard shape duplicated in other pterosaurs that preserved the wing membranes well. If someone else has another take on this specimen, please let me know.

And here’s a hypothesis that can never be proven:
Tianyulong, a small contemporaneous heterodontosaurid dinosaur, had long, elevated filamentous integumentary structures apparent on the back, tail and neck. These would have been ideal passive protection from vampire pterosaurs trying to alight on its back. Relatively naked giant sauropods would have made better landing zones.

References
Cheng X, Wang X, Jiang S and Kellner AWA 2014. Short note on a non-pterodactyloid pterosaur from Upper Jurassic deposits of Inner Mongolia, China. Historical Biology (advance online publication) DOI:10.1080/08912963.2014.974038
Kellner AWA, Wang X, Tischlinger H, Campos DA, Hone DWE and Meng X 2010. The soft tissue of Jeholopterus (Pterosauria, Anurognathidae, Batrachognathinae) and the structure of the pterosaur wing membrane. Proc Royal Soc B 277: 321–329.
strong>Peters D 2003. The Chinese vampire and other overlooked pterosaur ptreasures. Journal of Vertebrate Paleontology 23(3): 87A.
Wang X, Zhou Z, Zhang F and Xu X 2002. A nearly completely articulated rhamphorhynchoid pterosaur with exceptionally well-preserved wing membranes and “hairs” from Inner Mongolia, northeast China. Chinese Science Bulletin 47(3): 226-230.

wiki/Jeholopterus

 

The AMNH animated Jeholopterus

Updated March 3, 2015 with the addition of a dorsal view of Jeholopterus.

About a year ago
the American Museum of Natural History (AMNH) in New York City (NYC) put on a pterosaur display, both in their halls and online.

Their animated portrayal
of the Late Jurassic Chinese pterosaur, Jeholopterus, caught my eye (Fig. 1).

Figure 1. Animated GIF created by the AMNH for their web page on Jeholoopterus. Note the complete lack of an airfoil in the wing, the lack of muscles in the limbs, the presence of a uropatagium between the hind limbs, the lack of a tail, eyes set on the sides of a blockhead skull, and no care to reproduce the wide ribcage. In short, there is little that is accurate about this otherwise wonderfully animated pterosaur. And where is all the long hair that should be there?

Figure 1. Animated GIF created by the AMNH for their web page on Jeholoopterus. Note the complete disregard for its preserved anatomy, the  lack of an airfoil in the wing, the lack of muscles in the limbs, the presence of a uropatagium between the hind limbs, the lack of a tail, eyes set on the sides of a blockhead skull, and no care to reproduce the wide ribcage. In short, there is little that is accurate about this otherwise wonderfully animated pterosaur. And where is all the long hair that should be there? The animator was gifted, but the blueprint was largely imaginary.

One wonders what the animators used for reference… certainly not the fossil.
This animation lacks all the traits that make Jeholopterus unique: the up-curved jawline, the forward angled eyes, the very hairy body, the broad ribcage and belly, the deep chest, the low attachment of the wing, the large-boned limbs, the surgically curved claws, the huge feet with a very large digit 5, a longish tail and longer wings. Also lacking here is a wing with a decent airfoil section, a proper trailing edge stretched between the wing tip and elbow, large limb muscles and paired uropatagia behind each hind limb. And where does that box-like skull come from??

This is an old-school pterosaur cartoon,
lacking almost everything we know about this complete and articulated fossil. For comparison, a reconstruction is offered here (Fig. 2) based on precise tracings.

Figure 3. Click to enlarge. The Jeholopterus holotype (left) alongside the referred specimen (right). No doubt they were related, but were likely not conspecific. The one on the right was an insect eater. The one on the left was specialized for drinking dinosaur blood.

Figure 3. Click to enlarge. The Jeholopterus holotype (left) alongside the referred specimen (right). No doubt they were related, but were likely not conspecific. The one on the right was an insect eater. The one on the left was specialized for drinking dinosaur blood.

Jeholopterus had so many traits distinct from those of other anurognathid pterosaurs, that it deserves more respect than the AMNH gave it. Seems they purposely avoided describing it for what it is… a vampire pterosaur (details here). Would have been a bigger draw and a more accurate presentation had they just paid attention to the details.

The genesis of this post
came from an Ask.MetaFilter.com post on binocular vision in pterosaurs posted by Hactar, who wrote: “I am trying to find any information about binocular vision in pterosaurs. This past weekend, I went to the Museum of Natural History’s exhibition on pterosaurs. Their illustrations for Jeholopterus varied greatly in the placement of they eyes from on the sides of the head to facing forward (third picture on the page). (The second image caused me to dub it “freaky monkey pterosaur.”)  So how much binocular vision did pterosaurs have? I have found a couple of scattered references to family Anurognathus (of which Jeholopterus is a genus) having binocular vision, based on the structure of ear [sic] canals. Were these pterosaurs unique in having binocular vision, or did pteranodons and other pterosaurs have vision like a raptors instead of like a tern or pigeon? Links to academic articles are acceptable, I have confederates who can access articles for me. Please nothing by David Peters. From what I can read, his work on pterosaurs is at best somewhat wrong and generally completely inaccurate, which is a shame as he seems to be the only one who has posted anything online about this. (If the site mentions Jeholopterus as a vampire, skip it).”

Several things jump out here: 

  1. The AMNH did not edit their artwork. As noted above, one piece of artwork had lateral eyes. The other had anterior eyes.
  2. Science is a process that can be repeated by anyone. Therefore, Hactar could have taken a skull photo of Jeholopterus (or any other binocular pterosaur, like Batrachognathus), and traced the elements to arrive at his/her own skull reconstruction.
  3. If my work on pterosaurs varies from “somewhat wrong to completely inaccurate,” then I am at a loss as to how to explain the internal consistency of sister taxa that not only nest in complete resolution, but gradually evolve from one to another, apparently modeling the actual evolution of the group with stone cold logic. I also note that no one else is producing accurate tracings AND reconstructions based on those tracings. The alternative, of course, is to accept hopeful monsters, like Bennett’s anurognathid, or Andres’ hypothesis that anurognathids begat pterodactyloids, or Unwin’s uropatagium and other such fanciful hypotheses.

And like I said earlier,
this is Science, so you don’t have to accept anyone’s word for whatever you’re trying to figure out. You can find out for yourself by tracing the specimen and creating your own reconstruction. If my observations and hypotheses cannot be replicated, please send me your interpretations so they can be repaired here.

Figure 4. Jeholopterus in dorsal view. Here the robust hind limbs, broad belly and small skull stand out as distinct from other anurognathids. Click to enlarge.

Figure 4. Jeholopterus in dorsal view. Here the robust hind limbs, broad belly and small skull stand out as distinct from other anurognathids. Click to enlarge.

Don’t just repeat the propaganda ad nauseum.
The data is set in stone. Go get it and you’ll find the process rewarding.

References
Cheng X, Wang X, Jiang S and Kellner AWA 2014. Short note on a non-pterodactyloid pterosaur from Upper Jurassic deposits of Inner Mongolia, China. Historical Biology (advance online publication) DOI:10.1080/08912963.2014.974038
Kellner AWA, Wang X, Tischlinger H, Campos DA, Hone DWE and Meng X 2010. The soft tissue of Jeholopterus (Pterosauria, Anurognathidae, Batrachognathinae) and the structure of the pterosaur wing membrane. Proc Royal Soc B 277: 321–329.
Peters D 2003. The Chinese vampire and other overlooked pterosaur ptreasures. Journal of Vertebrate Paleontology 23(3): 87A.
Wang X, Zhou Z, Zhang F and Xu X 2002. A nearly completely articulated rhamphorhynchoid pterosaur with exceptionally well-preserved wing membranes and “hairs” from Inner Mongolia, northeast China. Chinese Science Bulletin 47(3): 226-230.

wiki/Jeholopterus

Dr. Walter Joyce on turtle origins

A new paper on turtle origins
(Joyce 2015) supports the hypothesis that the Middle Permian Eunotosaurus (Fig. 1) is an intermediate stem turtle (Watson 1914). Joyce sought to clear up a series of misconceptions about amniote systematics and the fossil record, including the remaining ambiguity regarding the phylogenetic position of turtles.

Unfortunately, no phylogenetic analysis was presented.

Fiigure 1. The turtle mimic Eunotosaurus from the Middle Permian was actually closer to Acleistorhinus.

Fiigure 1. The turtle mimic Eunotosaurus from the Middle Permian was actually closer to Acleistorhinus. Note the small number of dorsal vertebrae and the width of the ribs. Note also the ribs are larger  and with more coverage in Eunotosaurus than in the more derived Odontochelys, which is odd considering Eunotosaurus is supposed to be more primitive.

Joyce leads his charge
by undermining Osborn (1903) and Williston (1917) who proposed dividing reptiles into clades based on their temporal fenestrae or lack thereof. Turtles, lacking temporal fenestrae were considered primitive, citing the possible placement of turtles within the Diapsida with secondary loss of the temporal fenestrae (Broom 1924, Goodrich 1930, Rieppel and deBraga 1996, Rieppel 2000a). Note that Eunotosaurus (Fig. 1) has a lateral temporal fenestra lacking a lower temporal bar. It is not a diapsid, but shares this trait with Acleistorhinus and Australothyris, both of which retain the lower temporal bar formed by the jugal + quadratojugal. These are members of a slow, wide, herbivorous clade that also includes Caseasauria (which are not synapsids).

Figure 1. Odontochelys with a newly reconstructed skull.

Figure 2. Odontochelys is a basal turtle close to soft-shelled turtles.

Joyce reviews the DNA evidence
for turtles in the first decade of research as sisters to Aves, Crocodilia, Diapsida, Lepidosauria, Mammals + Archosaurs, or Archosaurs (refs in Joyce 2015). The second decade of research supported a sisterhood to archosaurs with Lyson et al. (2012) as the major exception to this trend. Lyson et al. linked turtles with lepidosaurs. HJoyce notes an expansion of the dataset by subsequent workers retrieved archosaurian affinities.

Joyce notes, “Using one of the largest data sets assembled to date, Lu et al. (2013) recently evaluated the phylogenetic signal contained in 4,584 orthologous genes  separately and arrived at the surprising conclusion that the three primary placements of turtles (i.e., as sister to Diapsida, Lepidosauria, or Archosauria) are supported by roughly the same number of genes, and that an archosaurian signal emerges only through the concatenation of the data. There only is one tree of life and it is apparent that either the molecular or the morphological signal is wrong.”

Joyce then notes that morphology does not support an archosaur relationship to turtles, an observations supported by several studies including the large reptile tree. Citing Lee (2013) Joyce notes “The morphological signal is therefore not able to resolve the placement of turtles withinamniotes for the moment, just like the molecular data, perhaps because of a sampling bias, or perhaps because the three major reptilian lineages diverged from one another very rapidly during the Late Paleozoic, the same two biases effecting molecular data.” Joyce agrees with Lee (2013) that “the interpretation of Eunotosaurus as an intermediate stem turtle is highly robust, relatively immune to perturbances and is independent from the placement of turtles within Amniota.”

Eunotosaurus does have an impressive number of traits otherwise shared only with turtles. Unfortunately, other taxa not listed by Lee or Joyce have more similar traits and the addition of these taxa and traits result in a separation of Eunotosaurus from turtles, as shown in the large reptile tree. Joyce notes that Odontochelys bridges the apparent morphological gap between Eunotosaurus and turtles and discusses their respective anatomies point by point. This is always a problem because Joyce lists no more than a half dozen traits when phylogenetic analysis  examines well over 200 — and a broader range of taxa.

Joynce notes, “In the older literature, turtles were variously allied with Eunotosaurus africanus (Watson, 1914), plesiosaurs (Moodie, 1908), placodonts (Jaeckel, 1902), “cotylosaurs” (Cope, 1896), temnospondyls (Vallén, 1942), pareiasaurs (Gregory, 1946), diadectids (Olson, 1947), and captorhinomorphs (Carroll, 1969), but most of these hypotheses were support by isolated characters and lacked a global perspective. Computer-assisted analyses hypothesized a sister group relationship to “anapsid” procolophonids (Reisz and Laurin, 1991; Laurin and Reisz, 1995) or pareiasaurs (Lee, 1993, 1997), “parapsid” sauropterygians (Rieppel and deBragga, 1996; deBraga and Rieppel, 1997), or “parapsid/diapsid” lepidosaurs (Müller, 2004). Interestingly, all modern analysis excluded E. africanus a priori, even though this taxon had consistently been listed as sister to turtles in standard paleontological textbooks and classifications throughout much of the century (e.g., Huene, 1956; Romer, 1956; Carroll, 1988). The Eunotosaurus hypothesis is different from all previous hypotheses regarding the origin of turtles because it is based on an impressive set of characters that were previously thought to be unique to turtles that originated one by one over the course of tens of millions of years. The most convincing aspect of the Eunotosaurus hypothesis is that these characters do not originate in parallel with the turtle shell, but rather in sequence as exaptations (Gould and Vrba, 1982) millions of years prior to the formation of a full shell and for reasons not related to the formation of the shell.” See Joyce 2015 for refs.

Joyce concludes with, “Future work will therefore have to focus on better understanding the anatomy of E. africanus, in particular its cranial anatomy, and in revising the phylogenetic relationships of Late Paleozoic amniotes.” Indeed, since so many Eunotosaurus sisters in the large reptile tree are known from skulls only, this is the prudent way to go. Also look at the feet, the pectoral and pelvic girdles. Look at everything and add taxa. Convergence will separate from homology as more data flows in. And don’t forget to add Stephanospondylus, Sclerosaurus and Elginia, three taxa typically omitted from turtle studies, but nest close to turtles in the large reptile tree (now up to 504 taxa).

References
Joyce W 2015. The origin of turtles: a paleontological perspective. Journal of Experimental Zoology Part B Molecular and Developmental Evolution. 00B:1–13.
Lyson TR, Sperling EA, Heimberg AM, et al. 2012. MicroRNAs support a turtle + lizard clade. Biol Lett 8:104–107.
Watson DMS. 1914. Eunotosaurus africanus Seeley, and the ancestry of the Chelonia. Proc Zool Soc Lond 1914:1011–1020.

What is Agnosphitys? A kind of basal theropod, most likely.

Figure 1. To scale compared to Marasuchus, Agnosphitys cromhallensis (Fraser et al. 2002) is known from a selection of uncrushed bones, all of which resemble those from Marasuchus, but slightly larger with a relatively longer rostrum and shorter arms. These two represent a separate and distinct lineage of theropods.

Figure 1. To scale compared to Marasuchus, Agnosphitys cromhallensis (Fraser et al. 2002) is known from a selection of uncrushed bones, all of which resemble those from Marasuchus, but slightly larger with a relatively longer rostrum and shorter arms. These two represent a separate and distinct lineage of theropods.  Click to enlarge.

Agnosphitys cromhallensis (Fraser et al. 2002, Late Triassic, 25 cm snout/vent length estimated) is a small dinosaur enigma known from a half dozen perfectly preserved bones, none of which would articulate with each other in vivo.

Fraser et al. considered Agnosphitys a dinosauriform, closer to true dinosaurs than either Eoraptor (here considered a basal phytodinosaur) and Herrerasaurus. The ilium indicates that only two sacrals were present. Unfortunately Fraser et al. considered three sacrals to be a dinosaur minimum, which is not true.

One solution is to pretend
the rest of the bones are present and create a reasonable restoration (Fig. 1). The pelvis has been compared to that of Marasuchus, only larger. So with that start, if we add the bones to a Marasuchus blueprint we find that the maxilla of Agnosphitys is relatively longer, the humerus is relatively shorter and the astragalus is similar in size.

Basal dinosaurs with a shorter rostrum are generally phytodinosaurs. Those with a longer rostrum and sharp teeth are generally theropods. Those with a short and gracile humerus generally have short forelimbs, another theropod clue, also found in ornithischians. Those with a rostrum as long as the humerus generally remove basal ornithischian candidates, but retain basal theropods like Tawa, which has a typical ilium shape, unlike that of Agnosphitys.

Former dinosauriform candidates, like Lagerpeton, no longer nest near basal dinosaurs. New pro to-dinosaurs like Lewisuchus, Saltoposuchus, Pseudhesperosuchus and Trialestes are better candidates to compare to Agnosphitys, but none share a similar ilium shape.

The lean of the anterior maxilla appears to favor a long low rostrum, like that of Tawa, rather than the more robust rostrum of Herrerasaurus.

Most workers do not nest Marasuchus with theropod dinosaurs.
Here it nests with a branch of theropods that are not usually included in phylogenetic analysis. That branch also includes Procompsognathus and Segisaurus. These three are not in the main line of theropods that led to T-rex and birds.

References
Fraser NC, Padian K, Walkden GM and Davis LM 2002. Basal dinosauriform remains from Britain and the diagnosis of the Dinosauria. Palaeontology. 45(1), 79-95.

Notes on lizard hands and feet with comparisons to the bird finger issue

Chalcides ocellatus is a basal extant skink. It has a basic (plesiomorphic) hand and foot, each with five digits and very few derived traits (Fig. 1, like the fusion of m4.3 + m4.4 and the fusion of mc1 + c1).

By contrast, another species, C. chalcides, the Italian three-toed skink, has but three toes on both the manus and the pes, which are, at best, tiny vestiges on tiny vestigial limbs.

Figure 1. The manus and pes of Chalcides ocellatus from Young et al. 2009, cleared and stained to show the bones in natural articulation.

Figure 1. The manus and pes of Chalcides ocellatus from Young et al. 2009, cleared and stained to show the bones in natural articulation at left. Ghosted with PILs added at right. Click to enlarge.

From the Young et al. (2009) abstract:
“Digit identity in the avian wing is a classical example of conflicting anatomical and embryological evidence regarding digit homology. In recent years, gene expression as well as experimental evidence was published that supports the hypothesis that this discrepancy arose from a digit identity shift in the evolution of the bird wing. A similar but less well-known controversy has been ongoing since the late 19th century regarding the identity of the digits of the three-toed Italian skink, Chalcides chalcides.The data confirm that the adult and the embryological evidence for digit identity are in conflict, and the expression of Hoxd11 suggests that digits 1, 2, and 3 develop in positions 2, 3, and 4. We conclude that in C. chalcides, and likely in its close relatives, a digit identity frame shift has occurred, similar to the one in avian evolution.” 

The three-toed skink (Chalcides chalcides, Fig. 2) is more derived than the five-toed skink. Digits 4 and 5 are outwardly absent from both the manus and pes. But note the buried vestige of digit 4 in both the manus and pes. The phase shift described by Young et al. happens during embryology (not shown here). In the adult basic homologies are maintained. The medial digit is #1.

Figure 2. Manus and pes of 3-toed Chalcides chalices. Note the broad base of mc1, similar to that in the figure 1.

Figure 2. Manus and pes of 3-toed Chalcides chalices. Note the broad base of mc1, similar to that in the figure 1. A vestige of digit 4 is present in both the manus and pes.

Here’s a possible explanation for apparent “Phase Shift” during embryogenesis.
Looking at the entire family tree of amniotes and tetrapods, it appears that this so-called “phase shift” may have its roots in basal tetrapods which had more than 5 fingers and toes (Fig. 3) and our genes remember that part of our ancestry. Remember there are gills in tetrapod embryos, too~! And gills goes back in further in our genetic ancestry.

As in Acanthostega,
that little bud developing medially on the embryo bird manus (Fig. 3) is probably not digit 1. Rather, it appears to be homologous to digit “pre-1″ on Acanthostega (Fig. 3), making a brief appearance during embryo development before disappearing as embryo growth continues. I don’t think the proper term for this is “phase shift”. Rather it is an ephemeral and short-lived appearance of digit pre-1 that probably occurs in most tetrapods.

Figure 3. Is this the source of the phase shift? At left, an embryo bird wing. Center an right, manus and pes of Acanthostega, a stem tetrapod with more than five digits. Orange dots identify homologies with five digit tetrapods.

Figure 3. Is this the source of the phase shift? At left, an embryo bird wing. Center an right, manus and pes of Acanthostega, a stem tetrapod with more than five digits. Orange dots identify homologies with five digit tetrapods. Click to enlarge.

Has this been considered before in academic publications? If so, it’s a convergent hypothesis.

References
Young RL, Caputo V, Giovannotti M, Kohlsdorf T, Vargas AO, May GE ,a and Wagner GP 2009. Evolution of digit identity in the three-toed Italian skink Chalcides chalcides: a new case of digit identity frame shift. Evolution and Development 11:6, 647–658.

Sirenoscincus mobydick: the only terrestrial tetrapod with ‘flippers’

Sakata and Hikida 2003
introduced us to a new and extant fossorial (burrowing) lizard (Sirenoscincus yamagishii. Fig.1). The authors described having “an elongated body and eyes covered by scales, lacking external ear openings and pigmentation through- out the body, resembles Cryptoscincus and Voeltzkowia. However it differs from these or any other scincid genera known to the present in having small but distinct forelimbs, each with four stout claws, and complete lack of hind limbs.”

Figure 1. Sirenoscincus-yamagishii, a new skink with forelimbs and no hind limbs. Note the four fingers.

Figure 1. Sirenoscincus-yamagishii, a new skink with forelimbs and no hind limbs. Note the four fingers.

Sirenoscincus is a very tiny lizard
with 53 presacral vertebrae and a tail longer than the snout vent length. The snout is pointed and the lower jaw is countersunk, like a shark’s mouth. The forelimbs are tiny with indistinct fingers and four stout claws. An outgroup taxon, Gymnophthalmus, also has tiny fingers and the medial one is a vestige.

Then a second Sirenoscincus species was discovered
S. mobydick (Miralles et al. 2012, Fig. 2; see online interview here). “The specicific epithet refers to Moby Dick, the famous albino sperm whale imagined by Herman Melville (1851), with whom the new species shares several uncommon characteristics, such as the lack of hind limbs, the presence of fipper-like forelimbs, highly reduced eyes, and the complete absence of pigmentation.”

Figure 3. Sirenoscincus mobydick.

Figure 2. Sirenoscincus mobydick.

S. mobydick has only five scleral ring bones, the lowest of any lizard. The authors reinterpreted several scale patterns on the holotype species. So, mistakes do happen, even at a professional level. Those mistakes get corrected and no one gets upset (hopefully unlike the blogosphere!).

Figure 2. Sireonscincus mobydick, named for its flippers, unique for any terrestrial tetrapod.

Figure 3. Sireonscincus mobydick, named for its flippers, unique for any terrestrial tetrapod. Colors added.

Fossorial skinks are often described by their scale patterns.
Unfortunately that doesn’t work with prehistoric skeletons, so I was only able to add only the bone traits of Sirenoscincus mobydick to the large reptile tree (subset shown in Fig. 7). The skeletal traits nested S. mobydick between two skinks Gymnophthalmus and Sineoamphisbaena, another taxon with forelimbs only (granted, the posterior half is not known). Like Sineoamphisbaena, Sirenoscincus prefrontals contact the postfrontals, unlike those of most lizards. In derived taxa the quadrate leans almost horizontally. That’s not the case with Sirenoscincus, which has a vertical but bent quadrate.

Figure 4. Sirenoscincus mobydick pectoral and pelvic girdles. Colors added.

Figure 4. Sirenoscincus mobydick pectoral and pelvic girdles. Colors (other then the original red) are added here.

Miralles et al. (2012) reported,  “Due to the absence of molecular data the phylogenetic position of the genus Sirenoscincus is still an enigma, even if we can reasonably claim it belongs to the Malagasy scincine clade.” In the last few days author, A. Miralles reported via email that molecular data have recently nested S. mobydick with skinks. 

Figure x. Chalcides guentheri and C. occellatus, two skinks were morphology quite similar to that of Sirenoscincus.

Figure 5. Chalcides guentheri and C. occellatus, two skinks with morphologies quite similar to that of Sirenoscincus. C. oscellatus has longer legs. Note the wrapping of the maxilla over the premaxilla which is continued in Sirenoscincus mobydick which has a smaller orbit. Also note the prefrontal and postfrontal are closer to contact in C. ocellatus.

An outgroup taxon is Chalcides (Fig. 5) where you’ll note the same long overlap of the maxilla over the premaxilla. A sister, Sineoamphisbaena also has an underslung mandible, but much more robust forelimbs (only the humerus is known). Could this be a redevelopment? Or has the cladogram missed something, needing more taxa perhaps, to fill this gap? No doubt new taxa will fill these various morphological gaps.

Figure 6. Sineoamphisbaena is a sister to Sirenoscincus in which the prefrontal contacts the postfrontal.

Figure 6. Sineoamphisbaena is a sister to Sirenoscincus in which the prefrontal contacts the postfrontal. The lower jaw is countersunk and the upper teeth don’t point down, they point in (medially).

New data has revised the relationship of skinks to reptiles in the large reptile tree (Fig. 7). Some to most of the confusion (here or earlier) likely results from the massive convergence in burrowing lizards. And some portion is also due to having good data (old line drawings) replaced by better data (rotating online images), often thanks to the good scientists over at Digimorph.org.

Figure 7. Here's where Sirenoscincus nests in the lizard family tree.

Figure 7. Here’s where Sirenoscincus nests in the lizard family tree.

References
Miralles A et al. 2012. Variations on a bauplan: description of a new Malagasy “mermaid skink” with flipper-like forelimbs only (Scincidae, Sirenoscincus Sakata & Hikida, 2003). Zoosystema 34(4):701-719.
Sakata S and Hikida T 2003. A fossorial lizard with forelimbs only: description of a new genus and species of Malagasy skink (Reptilia: Squamata: Scincidae). Current Herpetology 22:9-15.