Ptychoceratodus rectangulus ( Linck, 1936 )

Pawlak, Wojciech, Tałanda, Mateusz, Sulej, Tomasz & Niedźwiedzki, Grzegorz, 2020, Dipnoan from the Upper Triassic of East Greenland and remarks about palaeobiogeography of Ptychoceratodus, Acta Palaeontologica Polonica 65 (3), pp. 561-574 : 565-570

Ptychoceratodus rectangulus ( Linck, 1936)

Figs. 3–5 , 7 .

1936 Ceratodus rectangulus sp. nov.; Linck 1936: 1–23, pl. 4: 1–5, pl. 5: 1–4.

1938 Ceratodus rectangulus Linck, 1936 ; Linck 1938: 1–11, text-figs. 1–3, 5.

1963 Ceratodus rectangulus Linck, 1936 ; Linck 1963: 1–9, pl. 20, text-fig. 1.

1981 Ceratodus rectangulus Linck, 1936 ; Martin 1981: 11, text-fig. 5a.

1981 Ptychoceratodus rectangulus ( Linck, 1936) ; Schultze 1981: 21, text-fig. 13.

1998 Ptychoceratodus rectangulus ( Linck, 1936) ; Cuny et al. 1998:7, text-fig. 5j.

2018 Ceratodus tunuensis sp. nov.; Agnolín et al. 2018: 1–6, text-figs. 3–5.

2

Component

2

Component

Component 1

Syntypes: Upper dental plates: SMNS 17963–17965 , 17970 ; lower dental plates: SMNS 17966 , 17967 , 17971 , 17973 .

Type locality: Ochsenbach region, Baden-Württemberg, Germany.

Type horizon: Löwenstein Formation (Stubensandstein), middle Keuper, middle–upper Norian.

Material.—Right upper dental plate with attached almost complete pterygopalatine bone: KNK 4154; right upper dental plate: KNK 4155; right lower dental plate: KNK 4156; right lower dental plate: KNK 4157; right KLMN+3 bone: KNK 4158; right XYZ bone: KNK 4159; right operculum: KNK 4160; incomplete IJ? bone: KNK 4161; E bone: KNK 4162; left XYZ bone: KNK 4163; incomplete dental plate: KNK 4164; KLMN+3 bone imprint: KNK 4165. All from middle– upper Norian, locality 1, despite KNK 4162 from locality 2: Macknight Bjerg, Greenland. Upper dental plates: NHMD 115910–115912; lower dental plates: NHMD 115913–115916; prearticular bone: NHMD 115917. All from upper Norian, Lepidopteriselv, Greenland. Unidentified dental plates: BRSUG 26352, from Norian, Grozon, France. KLMN+3 bones: SMNS 19705, 19706; XYZ bone: SMNS 17971; incomplete IJ? bones:SMNS 19707, E? bone:SMNS 19708.All from middle–upper Norian, Ochsenbach region, Germany.

Diagnosis.—Dental plates high-crowned, lingual margin about two times longer than the mesial. Angle between the medial and lingual margin about 90° or more. Upper dental plates with five ridges, the first ridge initially curved anteriorly anterolaterally, then slightly twisted posteriorly, the last ridge flattened. Lower dental plates with four ridges, the last ridge flattened on the lingual side. Prearticular sulcus is double (like in other ptychoceratodontids except P. serratus ), but the posterior sulcus is deeper than the anterior. Pterygopalatine ascending process above the second ridge, slightly shifted toward the third ridge, similarly to P. serratus . Thick descending process of KLMN+3 bone with wide basement, confluent with relatively high and wide anterior crest.

Description.— Skull roof: The largest specimen of the Greenland material is KNK 4158, interpreted as an incomplete right KLMN+3 bone because of characteristic lateral line grooves on the dorsal side, and descending process and crests on the ventral side ( Fig. 4A ). The bone is 36 mm long and 44 mm wide. The posterior part is damaged, whereas other edges are natural. The lateral part of the bone is clearly downturned. The lateral edge is concave. Posteriorly, the bone expands laterally and forms conspicuous lateral process, which gives KNK 4158 trapeze-like outline from dorsal view. The dorsal surface of the bone is relatively smooth, whereas the lateral and anterior margins display cancellous structure. The supraorbital sensory canal groove angle is not preserved. The anterior part of the groove is shallow, whereas the lateral part is deep and widens toward the bone’s margin. The indentation for the medial process of the XYZ bone is visible posterior to the lateral process. The ventral surface is predominantly cancellous, being smooth only posteriorly. Two crests are visible in the central region of the bone: anterior and posterior ( Fig. 4A 3 , A 4 ). The posterior crest is directed straight to the lateral margin of the bone, and forms a slight bulge, more conspicuous laterally. The anterior crest is higher and slenderer than the posterior one, and directed anteriorly. The angle between the crests is roughly right. The anterior crest passes smoothly to the descending process of the KLMN+3 bone. The process is partially broken. However, it is clearly leaned towards the medial side. The groove for the ascending process of pterygoid appears on the medial side of the descending process base. The protrusion for the E bone is visible on the medial margin of the KLMN+3 bone. The morphology of the posterior part of the KLMN+3 bone is visible on the imprint of the same individual KNK 4165 ( Fig. 5A ). The posterior margin has an indentation for the J bone, however, there are no visible differences between the indentation and rest of the posterior margin. It suggests that the I bone and the bone were merged into a single IJ bone. Three shallow grooves begin from the inclined openings on the dorsal surface of both specimens. They probably represent blood vessel ducts. Shape of the complete KLMN+3 was roughly rectangular, which differs it from anteroposteriorly elongated KLMN+3 of Arganodus atlantis (see Kemp 1998). Two complete XYZ bones were found. KNK 4159 is 52 mm long and belonged to a large fish, whereas KNK 4163 is only 29 mm and represents evidently smaller individual. KNK 4159 is a flat bone, geometrically composed of three distinct regions: a flat posterior process, slightly convex but still relatively flat body, and curved articular process ( Fig. 3B ). The dorsal margin of XYZ has elongated protrusion for the KLMN+3 bone along the edge. The outer surface of the articular process differs from the other bone parts by its cancellous character. Radial marks cover the outer surface of the bone body and the posterior process, converging at the center of ossification. A temporal sensory line groove goes from the posterior to the anterior margin and is shallowing posteriorly. The split of the temporal line is absent. Thus, probably it was located among soft tissues. Five conspicuous elliptical openings appear in the equal distances inside the temporal line groove. The dorsal and anterodorsal margins articulated the XYZ with the KLMN+3 and IJ bones. The anterodorsal suture is relatively short, what is different from YZ bone in the skull of Ceratodus sturii (see Kemp 1998). The posteriormost part of the dorsal margin and the posterior process were joined with the IJ bone. The articular process, which was joined with the lower jaw is clearly thicker than other parts of the XYZ bone. The specimen KNK 4163 is slightly incomplete. Morphology of the bone surface is similar to the larger KNK 4159, but the distinctive feature of this specimen is bone geometry. Whole KNK 4163 is relatively flat, there is no curvature between the bone body and the articular process, unlike in KNK 4159. The delta-shaped bone (KNK 4162) is surrounded by hard sediment impossible to remove without damaging the specimen, thus only one side of the bone is visible ( Fig. 5C ). The bone is asymmetrical. A conspicuous convex outer side of the bone is probably its dorsal surface. There are visible openings for the blood vessels and a damaged, anterioposteriorly oriented crest along the entire bone. The inner surface is only partially exposed. There is visible an indentation matching the KLMN+3 protrusions. This is why KNK 4162 is interpreted as the E bone.

Operculum: The operculum (KNK 4160) is small, flattening posteriorly ( Fig. 5B ). Only the ventral half of the operculum is preserved. The posterior edge is very thin. Outer surface of KNK 4160 is significantly eroded, but the original surface was probably smooth. In contrast, the inner surface is cancellous. A conspicuous articulation with the XYZ bone is visible on the anterior margin. The cross section revealed a blood vessel canal that goes along the anterior margin.

Jaw bones and dental plates: One pterygopalatine bone was found with attached upper dental plate. KNK 4154 is an almost complete pterygopalatine missing only the ascending process ( Fig. 6A ). Surface of the pterygopalatine is fibrous above the dental plate, and smooth in the posterior part. The basis of the pterygopalatine process is located between the second and third ridge of the upper dental plate. Prearticular bones are represented only by fragments directly attached to the lower dental plates (KNK 4156, 4157). The ventral surface of the KNK 4156 prearticular bone is fibrous and has a distinctive double sulcus ( Fig. 6B 3 , B 4 ). The anterior sulcus is isometric and significantly smaller and shallower than the posterior, which is posteriorly elongated and deeper. Five dental plates were found. KNK 4154 and KNK 4155 are palatal plates of moderate size (31.3 and 22.1 mm long, respectively), whereas KNK 4156 and KNK 4157 are smaller prearticular plates (23.9 and 15.1 mm long, respectively). The dental plates are slightly elongated and high crowned with fan-like pattern of ridges ( Fig. 6A–C ). Crests of the three anterior ridges are meeting at the mediolingual junction. The specific feature of the dental plates is flattening of the last ridges.

The upper dental plates have five ridges, and slightly convex occlusal surfaces in the posterior part. The last ridges are almost completely flat. KNK 4155 has conspicuous cusps on the three last ridges. The fifth ridge of the smaller specimen (KNK4154) is more confluent with the fourth ridge than in the bigger (KNK4155).

The lower dental plates have four ridges and the occlusal surface is about flat. The last ridges of lower dental plates are composed of medial crest and flattened lingual part. Cusps are visible on all preserved ridges. KNK 4157 has slightly separated fifth ridge with low crest ( Fig. 6C ).

First crest is differently oriented in the upper and lower plates. In the upper plates it follows mesial margin, but in the lower it is in distance from the margin ( Fig. 6B 1 , B 2 , C).

A1

This pattern enables occlusion. KNK 4164 represents an incomplete dental plate of a large individual ( Fig. 6D ). Only first two ridges are preserved. The surface is heavily worn and the ratio of thickness to length is relatively small. It is difficult to establish if it is a palatal or prearticular dental plate because no identifiable bone part is attached to the specimen.

Remarks.—The first description of Ptychoceratodus rectangulus in Linck (1936) was based on eight poorly preserved dental plates from the middle Löwenstein Formation of Ochsenbach (middle Norian, Germany). It was initially assigned to genus Ceratodus , until Schultze (1981) transferred it to the genus Ptychoceratodus . Skull bones were also found in the same formation. Linck (1936, 1938, 1963) made biometrical studies, which were repeated in Skrzycki’s (2015) analysis according to current standards. Full morphological description or biometric measurements were not possible for some of the dental plates due to preservation ( Skrzycki 2015). A holotype was never designed for P. rectangulus . Cuny et al. (1998) assigned to this species one minute dental plate from the Norian Marnes de Châlins Formation in eastern France. The tooth plates described here are not as numerous as the material from Ochsenbach but are better preserved, what makes more detailed morphological or histological studies possible.

The dental plates described by Agnolín et al. (2018) as Ceratodus tunuensis came also from the Carlsberg Fjord Beds. They were classified as ceratodontid due to robust and short ridges having broad occlusal surface with rounded outline. However, all described specimens are worn and incomplete (except the complete but worn NHMD 115912). Ptychoceratodus rectangulus specimens from Ochsenbach show similar degree of erosion, however, they are mostly complete. High degree of incompleteness of the C. tunuensis dental plates may indicate that they were subject of transport and their wear is partially result of post-mortem erosion. This makes the proper evaluation of features like roundness of the occlusal surface, ridges’ and crests’ length, or depth of the inter-ridge furrows, indicated as diagnostic features of C. tunuensis , difficult. Other features of C. tunuensis (less vulnerable to erosion) are common for P. rectangulus . These features are: the α angle dimension, curvature of the first ridge, dental plates elongation, number of ridges and prearticular sulcus shape. Measurements of the C. tunuensis holotype (NHMD 115910) show geometrical similarity to ptychoceratodontids ( Fig. 3 ). On these bases we suppose that C. tunuensis is a younger synonym of P. rectangulus . It is impossible to determine if the differences in dimensions of the upper dental plates from Greenland and Germany (e.g., wider α angle and narrower 2 angle in the German specimens) are caused by an intraspecific variability or evolutionary changes related to isolation in two distant basins ( Fig. 7 ).

Stratigraphic and geographic range.— Germany, Ochsenbach region; Löwenstein Formation (Stubensandstein), Middle Keuper, middle–upper Norian. Greenland, Macknight Bjerg Quarry; lower part of the Carlsberg Fjord Beds

Ørsted Dal Member), Fleming Fjord Formation, middle– upper Norian. France, Grozon region; Marnes de Châlins Formation, Norian.

Figures

Fig. 3. Principal Component Analysis of species of Ptychoceratodus dental plates. A. Lower dental plates. B. Upper tooth plates. In the analysis we used α, δ1, δ2, δ3, BCp, C1C2, and C1Cp measurements sensu Vorobyeva and Minikh (1968). White rectangles mean specimens from Macknight Bjerg locality (East Greenland) described herein.

Fig. 4. Skull roof bones of dipnoan Ptychoceratodus rectangulus (Linck, 1936) from the Carlsberg Fjord Beds, Fleming Fjord Formation (Upper Triassic), Macknight Bjerg, East Greenland. A. KNK 4158, KLMN+3 bone in dorsal (A1, A2) and ventral (A3, A4) views. B. KNK 4159, XYZ bone in lateral B1, B2) and medial (B3, B4) views. Arrows indicate sutures for corresponding bones.

Fig. 5. Skull roof bones and opercle of dipnoan Ptychoceratodus rectangulus (Linck, 1936) from the Carlsberg Fjord Beds, Fleming Fjord Formation (Upper Triassic), Macknight Bjerg, East Greenland. A. KNK 4165, KLMN+3 bone imprint, in dorsal view. B. KNK 4160, opercle in lateral (B1) and medial (B2) views. C. KNK 4162, E bone in dorsal view. Arrows indicate sutures for corresponding bones.

Fig. 7. Map of sedimentation environments in north-western Europe and south-eastern Greenland in Norian with marked occurrences of Ptychoceratodus rectangulus. Based on McKie and Williams (2009) and Ziegler (1998).

Fig. 6. Dental plates and jaw bones of dipnoan Ptychoceratodus rectangulus (Linck, 1936) from the Carlsberg Fjord Beds, Fleming Fjord Formation (Upper Triassic), Macknight Bjerg, East Greenland. A. KNK 4154, upper dental plate with pterygopalatine bone in occlusal view. B. KNK 4156, lower dental plate in occlusal (B1, B2) and ventral (B3, B4) views.C. KNK 4157, lower dental plate with fragmentary prearticular bone in occlusal view. D. KNK 4164, incomplete dental plate in occlusal view.

Fig. 1. Study area map (A) and stratigraphic column (B) with marked outcrops and age of the stratigraphic position of examined material. Based on Bengaard and Henriksen (1984) and Clemmensen et al. (1998).

Fig. 2. Scheme of the measured dimensions of dental plates of Triassic dipnoans. Abbreviations: B, mediolingual junction; C1, C2, C3, C4, ends of corresponding ridges; Cp, end of the last ridge; α, angle between medial and lingual margin; ð1, ð2, ð3, angles between last ridge and corresponding ridges; BCp, BC1, BC2, C1C2, C2C3, C1Cp, C2Cp, distances between corresponding points. Based on Frederickson and Cifelli (2017).