Comparative cranial morphology of the Late Cretaceous protostegid sea turtle Desmatochelys lowii - PubMed (original) (raw)

Comparative cranial morphology of the Late Cretaceous protostegid sea turtle Desmatochelys lowii

Irena Raselli. PeerJ. 2018.

Abstract

Background: The phylogenetic placement of Cretaceous marine turtles, especially Protostegidae, is still under debate among paleontologists. Whereas protostegids were traditionally thought to be situated within the clade of recent marine turtles (Chelonioidea), some recent morphological and molecular studies suggest placement along the stem of Cryptodira. The main reason why the evolution of marine turtles is still poorly understood, is in part due to a lack of insights into the cranial anatomy of protostegids. However, a general availability of high-quality fossil material, combined with modern analysis techniques, such as X-ray microtomography, provide ample opportunity to improve this situation. The scope of this study is to help resolve its phylogenetic relationships by providing a detailed description of the external and internal cranial morphology of the extinct protostegid sea turtle Desmatochelys lowii Williston, 1894.

Material and methods: This study is based on the well-preserved holotype of Desmatochelys lowii from the Late Cretaceous (middle Cenomanian to early Turonian) Greenhorn Limestone of Jefferson County, Nebraska. The skulls of two recent marine turtles, Eretmochelys imbricata (Linnaeus, 1766) (Cheloniidae) and Dermochelys coriacea Lydekker, 1889 (Dermochelyidae), as well as the snapping turtle Chelydra serpentina (Linnaeus, 1758) (Chelydridae) provide a comparative basis. All skulls were scanned using regular or micro CT scanners and the scans were then processed with the software program Amira to create 3D isosurface models. In total, 81 bones are virtually isolated, figured, and described, including the nature of their contacts. The novel bone contact data is compiled and utilized in a preliminary phenetic study. In addition, an update phylogenetic analysis is conduced that utilizes newly obtained anatomical insights.

Results: The detailed examination of the morphology of the herein used specimens allowed to explore some features of the skull, to refine the scoring of Desmatochelys lowii in the recent global matrix of turtles, and develop five new characters. The alleged pineal foramen in the type skull of Desmatochelys lowii is shown to be the result of damage. Instead, it appears that the pineal gland only approached the skull surface, as it is in Dermochelys coriacea. Whereas the parasphenoid in confirmed to be absent in hard-shelled sea turtles, ist possible presence in Desmatochelys lowii is unclear. The results of the phenetic study show that Desmatochelys lowii is least similar to the other examined taxa in regards to the nature of its bone contacts, and therefore suggests a placement outside Americhelydia for this protostegid sea turtle. The phylogenetic study results in a placement of Protostegidae along the stem of Chelonioidea, which is a novel position for the group.

Keywords: Chelonioidea; Comparative morphology; Cranial anatomy; Cryptodira; Desmatochelys lowii; Late Cretaceous; Protostegidae; Sea turtle evolution; Testudines.

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Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Figure 1

Figure 1. The skull of Eretmochelys imbricata in four types of digital representation (A–D).

Figure 2

Figure 2. Sketches illustrating the categories of observed spatial relations between bones (A–D) and the depth of sutures (E–H).

The spatial relations between bones can be classified into: (A) p, parallel; (B) o, overlapping or u, underlying; (C) c, clasping or cb: clasped by; and (D) t, vertically transverse. The depth of suture can range from (E) s, smooth; (F) if, faintly interfingering; (G) im, moderately interfingering to; and (H) is, strongly interfingering.

Figure 3

Figure 3. Examples of bone contacts based on cross section images in Eretmochelys imbricata.

(A) Parallel and moderately interfingering; (B) underlying/overlapping and faintly interfingering; and (C) clasping and faintly interfingering. The width of each image is about one cm.

Figure 4

Figure 4. Cross section image of D. lowii along the coronal plane.

The lower half of the skull is damaged and obscured by matrix.

Figure 5

Figure 5. The segmented skull of D. lowii.

(A) Lateral; (B) medial; (C) anterior and (D) posterior views.

Figure 6

Figure 6. The segmented skull of D. lowii.

(A) Dorsal and (B) ventral views. Gray area represents unknown anatomy. The bar marks 10 mm.

Figure 7

Figure 7. The segmented skull of Eretmochelys imbricata.

(A) Lateral; (B) medial; (C) anterior; and (D) posterior views. The bar marks 10 mm.

Figure 8

Figure 8. The segmented skull of Eretmochelys imbricata.

(A) Dorsal and (B) ventral views. The bar marks 10 mm.

Figure 9

Figure 9. The segmented skull of D. coriacea.

(A) Lateral; (B) medial; (C) anterior and (D) posterior views. The bar marks 10 mm.

Figure 10

Figure 10. The segmented skull of D. coriacea.

(A) Dorsal and (B) ventral views. The bar marks 10 mm.

Figure 11

Figure 11. The segmented skull of Chelydra serpentina.

(A) Lateral; (B) medial; (C) anterior and (D) posterior views. The bar marks 10 mm.

Figure 12

Figure 12. The segmented skull of Chelydra serpentina.

(A) Dorsal and (B) ventral views. The bar marks 10 mm.

Figure 13

Figure 13. The nasal of D. lowii.

(A) 3D model in dorsal view; (B) 3D model in anteroventral view; and (C) illustration in anteromedial view, highlighting the parasagittal processes. The scale marks 10 mm.

Figure 14

Figure 14. The prefrontal and frontal in dorsal, anterior and posterior views.

(A) Desmatochelys lowii, dorsal view; (B) Desmatochelys lowii, anterior view; (C) Desmatochelys lowii, posterior view; (D) Eretmochelys imbricata, dorsal view; (E) Eretmochelys imbricata, anterior view; (F) Eretmochelys imbricata, posterior view; (G) Dermochelys coriacea, dorsal view; (H) Dermochelys coriacea, anterior view; (I) Dermochelys coriacea, posterior view; (J) Chelydra serpentina, dorsal view; (K) Chelydra serpentina, anterior view; and (L) Chelydra serpentina, posterior view. The bar marks 10 mm.

Figure 15

Figure 15. The frontal in anterolateral view of the 3D model and illustration.

(A) Desmatochelys lowii, 3D model; (B) Desmatochelys lowii, illustration; (C) Eretmochelys imbricata, 3D model; (D) Eretmochelys imbricata, illustration; (E) Dermochelys coriacea, 3D model; (F) Dermochelys coriacea, illustration; (G) Chelydra serpentina, 3D model; and (H) Chelydra serpentina, illustration. Light blue surface indicates the suture area with the prefrontal. The bar marks 10 mm.

Figure 16

Figure 16. Medial view of the parietal.

(A) Desmatochelys lowii; (B) Eretmochelys imbricata; (C) Dermochelys coriacea; and (D) Chelydra serpentina. The bar marks 10 mm.

Figure 17

Figure 17. Ventral view of the prefrontal, frontal, parietal, and postorbital.

(A) Desmatochelys lowii; (B) Eretmochelys imbricata; (C) Dermochelys coriacea; and (D) Chelydra serpentina. The bar marks 10 mm.

Figure 18

Figure 18. Lateral view of the postorbital, squamosal, quadratojugal, and jugal.

(A) Desmatochelys lowii; (B) Eretmochelys imbricata; (C) Dermochelys coriacea; and (D) Chelydra serpentina. The bar marks 10 mm.

Figure 19

Figure 19. Frontal view of the jugal.

(A) Desmatochelys lowii; (B) Eretmochelys imbricata; (C) Dermochelys coriacea; and (D) Chelydra serpentina. The bar marks 10 mm.

Figure 20

Figure 20. Quadrate and quadratojugal in posterolateral view.

(A) Desmatochelys lowii; (B) Eretmochelys imbricata; (C) Dermochelys coriacea; and (D) Chelydra serpentina. The bar marks 10 mm.

Figure 21

Figure 21. Posterior (left) and ventral (right) view of the squamosal.

(A) Desmatochelys lowii; (B) Eretmochelys imbricata; (C) Dermochelys coriacea; and (D) Chelydra serpentina. The bar marks 10 mm.

Figure 22

Figure 22. Premaxilla, maxilla, vomer, palatine, and jugal in dorsal view.

(A) Desmatochelys lowii; (B) Eretmochelys imbricata; (C) Dermochelys coriacea; and (D) Chelydra serpentina. The bar marks 10 mm.

Figure 23

Figure 23. Lateral and anterior view of thepremaxilla.

(A) Desmatochelys lowii, lateral view; (B) Desmatochelys lowii, anterior view; (C) Eretmochelys imbricata, lateral view; (D) Eretmochelys imbricata, anterior view; (E) Dermochelys coriacea, lateral view; (F) Dermochelys coriacea, anterior view; (G) Chelydra serpentina, lateral view; and (H) Chelydra serpentina, anterior view. The bar marks 10 mm.

Figure 24

Figure 24. Vomer and vomer with palatine in lateralview.

(A) Desmatochelys lowii, lateral view of vomer; (B) Desmatochelys lowii, anterolateral view of vomer and palatine; (C) Eretmochelys imbricata, lateral view of vomer; (D) Eretmochelys imbricata, anterolateral view of vomer and palatine; (E) Dermochelys coriacea, lateral view of vomer; (F) Dermochelys coriacea, anterolateral view of vomer and palatine; (G) Chelydra serpentina, lateral view of vomer; and (H) Chelydra serpentina, anterolateral view of vomer and palatine. The bar marks 10 mm.

Figure 25

Figure 25. Palatine in dorsolateral view.

(A) Desmatochelys lowii; (B) Eretmochelys imbricata; (C) Dermochelys coriacea; and (D) Chelydra serpentina. The bar marks 10 mm.

Figure 26

Figure 26. Pterygoid (A–D) and pterygoid, basisphenoid, and basioccipital (E–H) in dorsal view.

(A) Desmatochelys lowii; (B) Eretmochelys imbricata; (C) Dermochelys coriacea; (D) Chelydra serpentina; (E) Desmatochelys lowii; (F) Eretmochelys imbricata; (G) Dermochelys coriacea; and (H) Chelydra serpentina. The bar marks 10 mm.

Figure 27

Figure 27. Epipterygoid, parietal, and pterygoid.

(A) Desmatochelys lowii and (B) Chelydra serpentina. The bar marks 10 mm.

Figure 28

Figure 28. Basisphenoid and parasphenoid in anterolateral view.

(A) Desmatochelys lowii; (B) Eretmochelys imbricata; (C) Dermochelys coriacea; and (D) Chelydra serpentina. The bar marks 10 mm.

Figure 29

Figure 29. Parasphenoid of Dermochelys coriacea.

(A) Lateral; (B) posterior; and (C) dorsal view. The bar marks 10 mm.

Figure 30

Figure 30. Parasphenoid of Chelydra serpentine.

(A) Ventral view on the pterygoid, basisphenoid, and parasphenoid and (B) cross section image highlighting the parasphenoid in red. The bar marks 10 mm.

Figure 31

Figure 31. Medial and lateral view of theopisthotic and prootic.

(A) Eretmochelys imbricata, medial view; (B) Eretmochelys imbricata, lateral view; (C) Dermochelys coriacea, medial view; (D) Dermochelys coriacea, lateral view; (E) Chelydra serpentina, medial view; and (F) Chelydra serpentina, lateral view. The bar marks 10 mm.

Figure 32

Figure 32. Quadrate, prootic, opisthotic, and supraoccipital in dorsal view.

(A) Desmatochelys lowii; (B) Eretmochelys imbricata; (C) Dermochelys coriacea; and (D) Chelydra serpentina. The bar marks 10 mm.

Figure 33

Figure 33. Opisthotic in medial view.

(A) Desmatochelys lowii; (B) Eretmochelys imbricata; (C) Dermochelys coriacea; and (D) Chelydra serpentina. The bar marks 10 mm.

Figure 34

Figure 34. Medial and lateral view of the quadrate.

(A) Desmatochelys lowii, medial view; (B) Desmatochelys lowii, lateral view; (C) Eretmochelys imbricata, medial view; (D) Eretmochelys imbricata, lateral view; (E) Dermochelys coriacea, medial view; (F) Dermochelys coriacea, lateral view; (G) Chelydra serpentina, medial view; and (H) Chelydra serpentine, lateral view. The bar marks 10 mm.

Figure 35

Figure 35. Posterior view of KUVP 1200, type specimen of D. lowii, highlighting the remains of the columella auris visible through the fenestra postotica.

The bar marks 10 mm.

Figure 36

Figure 36. Columella of Chelydra serpentina.

(A) “Side view”; (B) proximal view; and (C) cross section through the basis columellae in CT scan marking two foramina. The bar marks 10 mm, unless noted otherwise.

Figure 37

Figure 37. Basioccipital in anterolateral view.

(A) Eretmochelys imbricata; (B) Dermochelys coriacea; and (C) Chelydra serpentine.

Figure 38

Figure 38. Exoccipital in medial view.

(A) Eretmochelys imbricata; (B) Dermochelys coriacea; and (C) Chelydra serpentina.

Figure 39

Figure 39. Exoccipital, opisthotic, prootic, basisphenoid, and an unidentifiable posterior portion of the basicranium of D. lowii in medial view.

The bar marks 10 mm.

Figure 40

Figure 40. Exoccipital, opisthotic, prootic, and basisphenoid of Eretmochelys imbricata in medial view.

The bar marks 10 mm.

Figure 41

Figure 41. Exoccipital, opisthotic, prootic, basisphenoid, and parasphenoid of Dermochelys coriacea in medial view.

The bar marks 10 mm.

Figure 42

Figure 42. Exoccipital, opisthotic, prootic, basisphenoid, and parasphenoid of Chelydra serpentina in medial view.

The bar marks 10 mm.

Figure 43

Figure 43. Time calibrated, most parsimonious tree (reduced version) resulting from the phylogenetic analysis.

CI, consistency index; RI, retention index. The full tree is provided in Figs. S7.1–S7.3.

Figure 44

Figure 44. Basioccipital and exoccipital in anterior view.

(A) Eretmochelys imbricata; (B) Dermochelys coriacea; and (C) Chelydra serpentina. The bar marks 10 mm.

Figure 45

Figure 45. Comparison historical illustrations of KUVP 1200, holotype of D. lowii, in dorsal view.

(A) Illustration of current condition of specimen; (B) redrawn from Zangerl & Sloan (1960); and (C) reproduced from Williston (1894).

Figure 46

Figure 46. Illustration of KUVP 1200, holotype of D. lowii, in lateral view, redrawn from Zangerl & Sloan (1960).

Note that crushed basicranium and broken tip of the supraoccipital are marked, but no damage indicated to the dorsal bulge.

Figure 47

Figure 47. Pictures of the “pink spot” of D. coriacea.

(A) “Pink spot” marked with a red arrow on a living D. coriacea (picture by Silvia Bonizzoni, Dolphin Biology and Conservation) and (B) skull of a D. coriacea with light shining from the interior through the thinnest part of the parietal’s dorsal plate.

Figure 48

Figure 48. Cross section image of the basisphenoid of Eretmochelys imbricata.

The yellow line marks the outline of the basisphenoid.

Figure 49

Figure 49. Photography (A) and illustration (B) of the sulcus pro-epipterygoidei.

In the illustration, the dark red area marks the epipterygoid, while the anterior to it light red area highlights the extent of the sulcus epipterygoidei.

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Grants and funding

This study was financially supported by funds from the Department of Geosciences of the University of Fribourg. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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