Did the turtle nuchal evolve from cleithra?

Lyson et al.  2013
propose a homology of the turtle nuchal (central anterior roof-like bone of the carapace) with the primitive cleithra (singular: cleithrum, slender, stem-like bone anterior to the scapula). In order to do so, they produced a set of turtle ancestors (or engineering models) that is not validated by the large reptile tree (LRT, 1395 taxa).

Frogs, lepidosaurs, diadectids and para-caseasaurs,
according to Lyson et al., model the ancestry of turtle shoulders and shells (Fig. 1).

Figure 1. On the left, from Lyson et al. 2013 with graphics added. On the right taxa basal to turtles according to the LRT.

Figure 1. On the left, from Lyson et al. 2013 with graphics added. On the right taxa basal to turtles according to the LRT. The right sequence documents a more gradual accumulation of traits. Even so, the gap between Bunostegos and Meiolania includes the complete development of the carapace and plastron… but almost everything else was present. A skull-only taxon, Elginia, nests between the two.

By contrast,
in the LRT Milleretta, is basal to Stephanospondylus, which is basal to diadectids on one branch and pareiasaurs, like Bunostegos, and the basal turtle Meiolania, on the other, documenting a more gradual accumulation of traits without introducing frogs and lepidosaurs. In the LRT, the gap between Bunostegos and Meiolania includes the unchronicled development of the carapace and plastron. Given that issue, almost everything else was present in the skeleton. A skull-only taxon, Elginia (not shown in Fig. 1), nests between the two. There is an online paper on turtle ancestors here.

Taxon exclusion is once again the problem.
Since Lyson et al. used inappropriate and unrelated taxa to demonstrate their hypothesis, it was invalid from the get-go. To my knowledge (let me know if I am wrong):

  1. No one recently suggested that frogs, like Rana, are basal to turtles.
  2. No one recently suggested that Diadectes is basal to turtles.
  3. No one recently suggested that Sphenodon is basal to turtles.
  4. Several authors (many from the Lyson et al. list) have suggested that Eunotosaurus was basal to turtles, but they did not test the above-listed LRT competing candidates when they published.

From Wikipedia Diadectidae
“Paleontologist E.C. Case compared diadectids to turtles in 1907, noting their large pectoral girdles, short, strong limbs, and robust skulls. Case described them as “lowly, sluggish, inoffensive herbivorous reptiles, clad in an armor of plate to protect them from the fiercely carnivorous pelycosaurs.”

The better method
for figuring out anything about turtles is to employ the valid ancestors of turtles, validated by testing against all other published candidates. I know, from testing, that all other candidates, like Eunotosaurus, nest far from turtles.

Getting back to our headline
and the title of the Lyson et al. paper, the genesis of the turtle carapace in hard-shell turtles is not preserved in the fossil record at present. Even so, the rarely preserved cleithrum gives little to no indication that it evolved into an anterior carapace bone… at present. Some day it may.

Lyson et al. note:
“unlike the other midline carapacial elements, the nuchal develops from paired mesenchymal condensations each of which contains a separate ossification center… first observed by Vallén (1942) and led him to conclude the nuchal was homologous with the supracleithra.”

The supracleithrum
by definition, “is a bone of the pectoral girdle situated dorsal to the cleithrum in some fishes and amphibians.”  That definition does not include reptiles.

If we look for a pre-nuchal in pareiasaurs
it is easy to find parasagittal osteoderms (Fig 2). Lyson et al. do not mention the word ‘pareiasaur’ in their paper.

Figure 2. The pareiasaur, Deltavjatia, with osteoderms in orange. Note the anterior set is simple and paired.

Figure 2. The pareiasaur, Deltavjatia, with osteoderms in orange. Note the anterior set is simple and paired, as hoped for by Lyson et al. but not found, except in turtle embryos, by Lyson et al.

Taxon exclusion can ruin a paper.
You can talk about thousands of characters for Eunotosaurus, but if you don’t include one pareiasaur, you’ll in the wrong ballpark on game day. Deltavjatia (Fig. 2) does not preserve a cleithrum. Rather, given its close, but not direct relation to turtles, the turtle nuchal likely arises from the osteoderms that are in place in Deltavjatia. They are the right size, in the correct orientation, and used for the same reason. So the nuchal probably arose from the foremost osteoderms on the torso, while those on the neck became neck armor. Remember, early turtles could not withdraw their neck.

It’s probably worthwhile to remind you of other body parts
that evolve in the ancestry of turtles until they become turtle traits at this time.

Figure 6. Turtle pelvis evolution. Here are the changes in the pelvis of pre-turtles and basal hard-shelled turtles.

Figure 3. Turtle pelvis evolution. Here are the changes in the pelvis of pre-turtles and basal hard-shelled turtles.

Take the turtle pelvis, for instance.
Similar precursors can be seen in stem turtle pareiasaurs (Fig. 3). And the skull is interesting. Workers have discussed Elginia with pareiasaurs and Meiolania with turtles, but never Meiolania with pareiasaurs or Elginia with turtles. That you heard here first in a three-part series five years ago.

Figure 2. Hard shell turtle evolution featuring Bunostegos, Elgenia, Meiolania and Proganochelys - NOT to scale.

Figure 4. Hard shell turtle evolution featuring the skulls of  Bunostegos, Elgenia, Meiolania and Proganochelys – NOT to scale. Note the long list of shared traits, longer than in any competing candidate.

If you know one of the seven authors
of Lyson et al. 2013, please make sure they become aware of this critique. A few of them are among those who rejected the submitted manuscript on the origin of turtles. Evidently they prefer the invalid status quo rather than this novel hypothesis for turtle origins.

References
Case EC 1907. Restoration of Diadectes. The Journal of Geology. 15 (6): 556–559.
Lyson TR, Bhullar B-AS, Bever GS, Joyce WG, de Queiroz K, Abzhanov A and Gauthier JA 2013. Homology of the enigmatic nuchal bone reveals novel reorganization of the shoulder girdle in the evolution of the turtle shell. Evolution & Development 15(5):317–325. DOI: 10.1111/ede.12041
Vallén E 1942. Beiträge zur Kenntnis der Ontogenie und der vergleichenden. Anatomie des Schildkrötenpanzers. Acta Zool. Stockholm 23: 1–127.

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Resurrecting extinct taxa: Pareiasauria, Compsognathidae and Ophiacodontidae

Earlier we looked at
four clades thought to be extinct, but are not extinct based on their nesting in the large reptile tree (LRT, 1366 taxa). Today, three more:

Figure 2. Another gap is filled by nesting E. wuyongae between Bunostegos and Elginia at the base of hard shell turtles in the LRT.

Figure 1. Another gap is filled by nesting E. wuyongae between Bunostegos and Elginia at the base of hard shell turtles in the LRT.

Pareiasauria
According to Wikipedia, “Pareiasaurs (meaning “cheek lizards”) are an extinct group of anapsid reptiles classified in the family Pareiasauridae. They were large herbivores that flourished during the Permian period.”

In the LRT two clades of turtles (Fig. 1) are derived in parallel from two small horned pareiasaurs.

Figure 1. Lately the two clades based on two specimens of Compsognathus (one much larger than the other) have merged recently.

Figure 2.  Lately the two clades based on two specimens of Compsognathus (one much larger than the other) have merged recently.

Compsognathidae
According to Holtz 2004, “The most inclusive clade containing Compsognathus longipes but not Passer domesticsus.” Traditionally Compsognathus nests outside the Tyrannoraptora, a clade that traditionally leads to birds.

In the LRT Compsognathus specimens nest at the base of several theropod clades (Fig. 2) including the tyrannosaurs and Mirischia, Ornitholestes and the feathered theropods leading to birds.

Figure 1. Varanosaurus, Ophiacodon, Cutleria and Ictidorhinus. These are taxa at the base of the Therapsida. Ophiacodon did not cross into the Therapsida, but developed a larger size with a primitive morphology. This new reconstruction of Ophiacodon is based on the Field Museum (Chicago) specimen. Click to enlarge.

Figure  3. Varanosaurus, Ophiacodon, Cutleria and Ictidorhinus. These are taxa at the base of the Therapsida. Ophiacodon did not cross into the Therapsida, but developed a larger size with a primitive morphology. This new reconstruction of Ophiacodon is based on the Field Museum (Chicago) specimen. Click to enlarge.

Ophiacodontidae
According to Wikipedia, “Ophiacodontidae is an extinct family of early eupelycosaurs from the Carboniferous and Permian. Ophiacodontids are among the most basal synapsids, an offshoot of the lineage which includes therapsids and their descendants, the mammals. The group became extinct by the Middle Permian.”

In the LRT Ophiacodon (Fig. 3) and Archaeothyris, neither members of the Pelycosauria, are more directly related to basal therapsids, including derived the therapsids: mammals.

References
Holtz TR 2004. Basal tetanurae. PP. 71–110 in The Dinosauria, U of California Press.

/wiki/Pareiasaur
wiki/Ophiacodontidae

 

SVP 2018: Turtle ribs in softshell embryos

Hirasawa et al. 2018
studied the embryonic development of the Chinese soft-shell turtle, Pelodiscus sinensis, seeking clues to the origin of turtles.

Backstory:
The large reptile tree (LRT, 1313) tests a wide gamut of candidates, including all prior turtle ancestor candidates and a thousand more. It recovers a dual origin of turtles (hardshell and soft-shell) from two separate small horned pareiasaurs, Elginia and Sphodrosaurus.

Figure 2. Another gap is filled by nesting E. wuyongae between Bunostegos and Elginia at the base of hard shell turtles in the LRT.

Figure 1. Another gap is filled by nesting E. wuyongae between Bunostegos and Elginia at the base of hard shell turtles in the LRT.

Hirasawa et al. report, “The turtles’ body plan differs from that of the other tetrapods in the solid dorsal and ventral shells (carapace and plastron).” Actually, no. The placodont Henodus has a similar carapace and plastron.

Hirasawa et al. report, “It has been widely accepted that the carapace and plastron evolved from the ribs and clavicular girdle plus gastralia, respectively, but the evolutionarytransition was poorly preserved in the fossil record.” In the LRT turtle ancestors did not have gastralia until the recent discovery that Sphodrosaurus was ancestral to soft-shell turtles and lateral (not medial) ‘gastralia’ (actually plastron progenitors) first appear.

FIgure 1. Partial reconstruction of Sphodrosaurus based on tracings in figure 2.

FIgure 2. Partial reconstruction of Sphodrosaurus based on tracings in figure 2. Plastron primordia appear in cyan (and so does the humerus and two sacrals, sorry!)

Hirasawa et al. report, “In the turtle embryo, the rib primordia are not extended to the lateral body wall unlike those of the other amniotes, and only the deep layer of the body wall muscle develops in the lateral body wall. Concomitantly, the sternum, which develops adjacent to the ventral ends of the ribs in the other amniotes, does not develop in the turtle embryo.” Actually the sternum is a rare ossification in tetrapods, not found in turtles, or LRT turtle ancestors. However the interclavicle is present in the turtle plastron.

Hirasawa et al. report, “Among fossil taxa, sauropterygians have repeatedly been placed in the closest position to turtles by phylogenetic analyses in previous studies.” In the LRT sauropterygians are not related to turtles except at the origin of the amnion in Viséan amphibian-like reptiles like Silvanerpeton.

The LRT is online for anyone to test taxa
relevant to the ancestry of turtles or any other included taxon or clade.

References
Hirasawa R et al. 2018. Developmental biological inferno on the evolution from the ribcage to the turtle shell. SVP abstract.

SVP 2018: Basal hardshell turtle Meiolania aquatic

Lichtig and Lucas 2018
discuss the giant, horned, club-tailed basal turtle, Meiolania (Fig. 1).They report, “Analyzing the habitus of Meiolania based on limb proportions, shell shape, and femoral morphology suggests that it was an aquatic turtle similar in all these morphologies to Chelydra [the smaller genus of snapping turtle].”

Figure 5. Meiolania, the most primitive of known turtles, has lateral forelimbs, like non turtles.

Figure 1. Meiolania, the most primitive of known turtles, has lateral forelimbs, like non turtles.

The authors continue:
“The round shape of the femoral head indicates a walking mode of locomotion,
as is seen in the bottom walking of Chelydra. Furthermore, the plastral fontanelle is not known in any turtle suggested to be terrestrial. A “tail club” has been suggested to indicate a terrestrial habitus, as swinging this as a weapon would be difficult to impossible in the water. This ignores the fact that the tail “clubs” of Meiolania and Proganochelys are too narrow to be used as a club. The body of the tail would contact a target before the “club’s”
impact, reducing the effect of any strike. In addition, no extant terrestrial turtle has such a large tail, but this is exactly what is seen in the bottom walking Chelydra and Platysternon.” Some good hypotheses there.

Maybe amphibious?
At home either in a pond or between ponds?

Ancestral pareiasaurs were also likely amphibious
according to bone micro anatomy studies (Kriloff et al. 2008).

References
Lichtig AJ and Lucas SG 2018. The late Cenozoic turtle Meiolania platyceps was aquatic. SVP abstracts.
Kriloff A, Germain D, Canoville A, Vincent P,  Sache M and Laurin M 2008.
Evolution of bone microanatomy of the tetrapod tibia and its use in palaeobiological inference. Journal of Evolutionary Biology. 21 (3): 807–826. doi:10.1111/j.1420-9101.2008.01512.x

 

SVP 2018: New data on Chinlechelys (Triassic turtle)

Lucas and Licthtig 2018
present new details on the morphology of Chinlechelys, a Late Triassic turtle.

Figure 1. Chinlechelys compared to Proganochelys. The cervical armor is relatively larger, the humerus is relatively smaller.

Figure 1. Chinlechelys compared to Proganochelys. The cervical armor is relatively larger, the humerus is relatively smaller. Evidently more is known now.

The authors report, “a significant portion of the carapace. This new material includes thoracic vertebrae 1–3 and 5–8 as well as the isolated axillary notch and a small carapace fragment. A fragment originally described as cervical armor on further inspection is found to be a portion of the posterior carapace margin. This fragment shows a distinct radial sculpture like that of the posterior marginals of Proganochelys.  The peripheral previously published is not from the posterior of the carapace, but rather from its lateral margin.  

“Four bones concreted together tentatively considered the posterior left corner of a skull are unusual in their morphology relative to other turtles. The long quadratojugal is similar to that of dwarf pareiasaurs in its long length and the presence of a ventral ridge parallel to the lateral margin. The squamosal has a distinct ventral conch similar to Anthodon and more anterior and ventral than the otic notch of Proganochelys. Overall, this makes a very unusual turtle, but some of this may not be as odd as it seems as many of these features are unclear in other Triassic turtles like Proganochelys. The preserved ribs and overlying neutrals and costals support the idea that the costals and ribs were originally two separate ossifications that were fused latter in evolution.”

I’m sure we’ll be hearing more later.
All of this appears to support the dual origin of turtles from pareiasaurs as recovered by the large reptile tree (LRT, 1311 taxa).

References
Lucas SG and Lichtig AJ 2018. New morphology of Chinlechelys, a Late Triassic turtle from New Mexico, U.S.A. SVP abstract.

SVP 2018: New elginiid parieasaur skull from China

Liu and Bever 2018
describe a complete pareiasaur skull close to Eliginia (Fig. 1), the proximal outgroup to hardshell turtles in the large reptile tree, and no where else. The new specimen is significantly larger than Elginia. The authors refer the specimen to Sanchuansaurus, a taxon apparently known from far less material. Evidently this abstract does not represent the earlier Eliginia wuyongae (Fig. 1), which is not significantly larger than Elginia. Liu and Bever make no mention of the eliginiid relationship to basal hard-shell turtles.

Figure 2. Another gap is filled by nesting E. wuyongae between Bunostegos and Elginia at the base of hard shell turtles in the LRT.

Figure 1. Another gap is filled by nesting E. wuyongae between Bunostegos and Elginia at the base of hard shell turtles in the LRT. The new specimen is larger than Eliginia.

References
Liu J and Bever GS 2018. The first complete pareiasaaur skull from China and its implications for the taxonomy of Chinese pareiasaurs.

wiki/Shihtienfenia

First evidence for elginiid pareiasaurs in the Karoo (South Africa)

In the large reptile tree (LRT, 1308 taxa) pareisaurs split after Stephanospondylus into two clades: 1) traditional pareiasaurs and 2) turtle-ancestor pareiasaurs (Fig. 1). Only the latter clade develop distinct supratemporal horns.

Figure 3. Dorsal views of bolosaur, diadectid, pareiasaur, turtle and lanthanosuchian skulls. The disappearance of the turtle orbit in lateral view occurs only in hard shell turtles.

Figure 1. Dorsal views of bolosaur, diadectid, pareiasaur, turtle and lanthanosuchian skulls. The disappearance of the turtle orbit in lateral view occurs only in hard shell turtles. Horns only appear in elginid and sclerosaurid pareiasaurs.

On a recent trip
to the Sam Noble Oklahoma Museum of Natural History in Norman, Oklahoma, USA, I studied a pareiasaur horn, OMNH 708 (Fig. 2). For over a century elginiid pareiasaurs were only known from Scotland. This year other elginids were reported from China (Liu and Bever 2018), and others were reported in 2005 from Eastern Europe (Bulanov and Yashina 2005). OMNH 708 represents yet another specimen, perhaps the first from the Late Permian Karoo beds of South Africa. (Please, let me know of not so.)

Figure 1. OMNH 708, a Permian pareiasaur horn form the Karoo, South Africa.

Figure 1. OMNH 708, a Permian pareiasaur horn form the Karoo, South Africa.

The OMNH specimen
is much larger than the Scottish and Chinese specimens (Fig. 2), more in the size range of ancestral pareiasaurs and Stephanospondylus.

Figure 3. Elginia and OMNH 708 at two scales.

Figure 3. Elginia and OMNH 708 at two scales.

The only question that remains is
is this really a pareiasaur horn? Or has everyone misinterpreted it? It really looks like a cow or bison horn (Fig. 4), but its origin in Permian strata prohibits that.

Figure 4. A modern cow skull horn most closely resembles the Permian pareiasaur horn, by convergence, of course.

Figure 4. A modern cow skull horn most closely resembles the Permian pareiasaur horn, by convergence, of course.

Say, ‘hello’, to convergence, once again.

References
Bulanov VV and Yashina OV 2005. Elginiid pareisaurs of Eastern Europe. Paleontological Journal 39(4):428–432.

Liu J and Bever GS 2018. The tetrapod fauna of the upper Permian Naobaogou formation of China: A new species of Eliginia (Parareptilia, Pareiasauria). Papers in Paleontology 2018: 1-13.
Newton ET 1893. On some new reptiles from the Elgin Sandstone: Philosophical Transactions of the Royal Society of London, series B 184:473-489.

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