Monday 20 July 2020

Ptychoceratodus rectangulus: Lungfish from the Late Triassic of East Greenland.

The study of Late Triassic Dipnoans (Lungfish) has recently undergone renewed interest, improving our understanding of the taxonomy, diversity and palaeobiogeography of this important component of Triassic freshwater and marginal marine ecosystems. Dipnoans are adapted to breathing with both gills and lungs. They are characterised by crushing dental plates instead of true teeth. Despite this specialization to durophagy, extant Lungfish are omnivorous. Dipnoans are an old Sarcopterygian Fish lineage known from the Early Devonian to recent. Their massive and resistant to erosion dental plates are common remains in the Triassic fossil record.

The fossil record of the post-Devonian Lungfish is biased in that bone elements from cranial or postcranial skeleton are relatively rare, mainly preserved as isolated or fragmented elements, and the most common remains of these Fish are their dental plates. Only a few post- Palaeozoic species are known from well preserved cranial or postcranial skeletal remains. This residual fossil record gives remarkably incomplete knowledge about Lungfish palaeoecology and evolution. Consequently, previous phylogenetic analyses failed to offer a robust evolutionary scenario explaining the major changes within the post-Palaeozoic Dipnoans. The Triassic was a golden age for Dipnoans, at least when it comes to the number of genera and species. Such a diversity is probably related to a wide global distribution of warm and semidry continental climate probably preferred by these Fish, similarly to their Recent representatives. Comprehension of the Triassic evolutionary radiation of Dipnoans is crucial for the understanding of their later Mesozoic and younger evolutionary history. The Triassic dipnoans are known from freshwater and marginal marine (brackish) environments. There are two genera, which are particularly common nearly worldwide: Arganodus and Ptychoceratodus. Only two species of Ptychoceratodus are known from a relatively complete cranial skeleton: Ptychoceratodus serratus from the late Ladinian Erfurt Formation in Germany and Ptychoceratodus philippsi from the Cynognathus Zone of the Bergersdorp Formation in the Karoo Basin, dated as late Early Triassic–early Middle Triassic.

In East Greenland, fossiliferous Upper Triassic of Jameson Land Basin, such as the Malmros Klint Member or Ørsted Dal Member, are known for rich faunal assemblages. However, the remains of Dipnoans are still poorly known from the Late Triassic record of Greenland. The oldest record of Lungfishes from Greenland comes from the Middle and Late Devonian. In younger Carboniferous or Permian deposits of Greenland, Dipnoan remains are unknown, but some remains have been collected from the Upper Triassic Fleming Fjord Formation. Recently, dental plates and jaw bones belonging to a new species of Ceratodus (Ceratodus tunuensis) have been described from the upper Carlsberg Fjord Beds.

In a paper published in the journal Acta Palaeontologica Polonica on 15 May 2020, Wojciech Pawlak and Mateusz Tałanda of the Department of Palaeobiology and Evolution at the University of Warsaw, Tomasz Sulej of the Institute of Paleobiology of the Polish Academy of Sciences, and Grzegorz Niedźwiedzki of the Department of Organismal Biology at Uppsala University, present a detailed description of newly collected Ptychoceratodontid skull material and highlight its palaeoecological and palaeobiogeographic significances. They compare it with Ceratodus tunuensis reported from the same lithostratigraphical unit. The new material represents fossils which are much better preserved than the other Dipnoan remains collected so far from the Ørsted Dal Member. For this reason, the study has required a brief re-study of the isolated elements previously described and a comparison with other previously described Ptychoceratodontid fossils. The Fleming Fjord Formation also yielded some aestivation burrows, which carry information about the Late Triassic climate of north Pangea and broadens our knowledge about the distribution of the aestivation behavior among the early Mesozoic Dipnoans. These trace fossils will be a subject of an additional study.

The Upper Triassic succession of the Jameson Land is composed of about 200–300 m thick continental sediments of the Fleming Fjord Formation represented mainly by red or greenish mudstone, siltstone, and sandstone beds. The formation is divided into three members: the lowermost Edderfugledal Member, the middle Malmros Klint Member and the uppermost Ørsted Dal Member. Integrated magnetostratigraphical, cyclostratigraphical, and some biostratigraphical studies revealed that the Fleming Fjord Formation age is late Carnian–early Rhaetian, Norian–Rhaetian, or Norian–early Rhaetian. Very rare invertebrate body fossils (e.g. Conchostracans, Bivalves), composition of the Tetrapod faunas/ichnofaunas, and land-derived palynomorphs suggest that the Malmros Klint Member is of late Carnian–middle Norian age, and that the Ørsted Dal Member is of middle Norian-early Rhaetian age. A conspicuous feature of these deposits are sedimentation cycles reflecting climate changes, which are developed slightly differentially in every lithostratigraphical subunit.

Study area map (A) and stratigraphic column (B) with marked outcrops and age of the stratigraphic position of examined material. Pawlak et al. (2020).

The fossiliferous lower part of the Ørsted Dal Member, named the Carlsberg Fjord Beds, is composed of intraformational conglomerate, massive mudstone, mudstone with wave ripple cross-lamination, and sandstone with cross-bedding. Desiccation cracks are common in this interval within both the mudstone and the sandstone beds but there are no signs of pedogenesis. The Ørsted Dal Member contains also two other lithostratigraphic subunits. In the western and central part of the Basin, theCarlsberg Fjord Beds are replaced by fluvial sediments of the Bjergkronerne Beds, and both units are overlaid by carbonate-rich mudstone, dolostone, and clay-rich mudstone ofthe Tait Bjerg Beds.

Both the Malmros Klint Member and Ørsted Dal Member are abundant in skeletal and trace fossils of terrestrial and aquatic Vertebrates. The existing outcrops indicate that the Fleming Fjord Lakeland had about 3000 km², therefore it was a large but predominantly shallow reservoir with frequent desiccation episode. The periodic shallow lake conditions favored bone accumulation, thus the Fleming Fjord Formation infill of the Jameson Land is an interesting taphonomical window of the Late Triassic fauna. These fossiliferous deposits have been searched for Vertebrate remains since the late 1980s. The Vertebrate fauna of the Fleming Fjord Formation is relatively abundant and quite diverse, and includes Actinopterygian and Dipnoan Fish, Temnospondyls, Sauropodomorph and Theropod Dinosaurs, basal Turtles, Phytosaurs, Aetosaurs, Pterosaurs, Cynodonts, and Mammaliaforms. Numerous Tetrapod tracks have been found in several horizons inside the Malmros Klint and Ørsted Dal members.

The examined specimens were collected during the Polish-Danish expedition to Greenland in 2014. Eleven skeletal elements and one imprint were found, which belonged to different individuals of various sizes. All specimens are disarticulated but three-dimensionally preserved without any sings of compaction and additional deformations. Skull roof bones have a characteristic radial pattern of points and lines diverging from the ossification center. Specimens KNK 4154–4161, 4163–4165 were found in the locality 1, and KNK 4162 in the locality 2. Both localities have very similar lithostratigraphic position: section exposed at locality 1 represents transitional layers between the Malmros Klint and the Ørsted Dal members, it is a new fossiliferous spot with poorly exposed bone-bed enriched in remains of freshwater fauna; section exposed at locality 2 shows lowermost part of the Carlsberg Fjord Beds and is rich in disarticulated elements of Temnospondyls. In previous studies this site was named Macknight Bjerg Quarry. There are no signs of postmortal mechanical wearing on the studied elements. Surfaces, cusps, or crests are well preserved. Some observed changes on the bone surfaces are the effects of recent weathering. Almost all of the skeletal elements were enveloped by a slightly heterogeneous mudstone matrix, which was subjected to dissolving and softening in 10% solution of the acetic acid. Some of the specimens were prepared mechanically when the chemical method did not work.

The examined dental plates have typical Ptychoceratodont morphology. They are high crowned, with less than 7 ridges. Ridges originate anteriorly, which results in their fan-like arrangement. Crests meet in the medial margin. Number of the ridges in the examined specimens (5 in the upper and 4 in the lower dental plates) is reduced compared to Ptychoceratodus serratus (6 in the upper and 5 in the lower dental plates). Particularly distinctive feature is the flattening of the last ridge, present in Ptychoceratodus concinnus and Ptychoceratodus rectangulus

The genus Ptychoceratodus is not assigned to any family because of disagreement between researchers about taxonomy of Mesozoic Lungfish. Ptychoceratodontidae diagnosed on dental plates is commonly used by various authors. However, objections to taxonomy based on the dental plates are also present. Pawlak et al. consider cranial features as a better taxonomic tool, however, because most Dipnoan findings are isolated dental plates, there is no other and better possibility to distinguish the species or genera.

The largest specimen of Ptychoceratodus rectangulus in 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. 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. 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. 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 J 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. 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. 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. 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. 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.

Skull roof bones of Dipnoan Ptychoceratodus rectangulus 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. Pawlak et al. (2020).

The operculum (KNK 4160) is small, flattening posteriorly. 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.

Skull roof bones and opercle of Dipnoan Ptychoceratodus rectangulus 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. Pawlak et al. (2020).

One pterygopalatine bone was found with attached upper dental plate. KNK 4154 is an almost complete pterygopalatine missing only the ascending process. 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. 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. 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.

Dental plates and jaw bones of Dipnoan Ptychoceratodus rectangulus 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. Pawlak et al. (2020).

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.

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. This pattern enables occlusion. KNK 4164 represents an incomplete dental plate of a large individual. 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.

The first description of Ptychoceratodus rectangulus, by Otto Linck in 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 Hans-Peter Schultze  transferred it to the genus Ptychoceratodus. Skull bones were also found in the same formation. Otto Linck made biometrical studies, which were repeated in 2015 by Piotr Skrzycki, using analysis according to current standards. Full morphological description or biometric measurements were not possible for some of the dental plates due to preservation. A holotype was never designed for Ptychoceratodus rectangulus until Gilles Cuny, Michel Martin, Raymind Rauscher and Jean-Michel Mazin assigned to this species one minute dental plate from the Norian Marnes de Châlins Formation in eastern France. The tooth plates described by Pawlak et al. 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 Federico Agnolín, Octávio Mateus, Jesper Milàn, Marco Marzola, Oliver Wings, Jan Shulz Adolphssen, and Lars Clemmensen 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 Ceratodus 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 Ceratodus tunuensis, difficult. Other features of Ceratodus tunuensiss (less vulnerable to erosion) are common for Ptychoceratodus serratus. 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 Ceratodus tunuensis holotype (NHMD 115910) show geometrical similarity to Ptychoceratodontids. On these bases Paewlak et al. suppose that Ceratodus tunuensiss is a younger synonym of Ptychoceratodus serratus. 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 ð2 angle in the German specimens) are caused by an intraspecific variability or evolutionary changes related to isolation in two distant basins.

Map of sedimentation environments in north-western Europe and south-eastern Greenland in Norian with marked occurrences of Ptychoceratodus rectangulus. Pawlak et al (2020).

The five original skull bones of Ptychoceratodus serratus, from the same strata of the German  Triassic as the dental plates, were originally interpreted incorrectly by Otto Linck in 1963. They were reinterpreted by Hans-Peter Schultze in 1981 and mentioned by Michel Martin, also in 1981. However, preservation of these skull bones is comparably bad as that of the associated dental  plates. All German specimens are imbedded in a rock matrix and only external surfaces of the bones are accessible for study. Most of them are incomplete, with broken or invisible edges, and one specimen was destroyed during preparation. The set of the Linck’s skull roof bones includes, according to Schultze: the KLMN+3 bones left (SMNS 19705) and right (SMNS  19706), the incomplete right I bone (SMNS 19707), the E bone (destroyed) and an unidentified poorly preserved element (SMNS 19708).

In the German material the best preserved are the KLMN+3 bones, which resemble the Greenland specimens. Supraorbital line turning at roughly right angle, and deep indentation for the medial process of the XYZ on the lateral margin are particular similarities. Clear difference between the German and Greenland KLMN+3 bones is the lack of the indentation for the J bone on the posterior margin in SMNS 19705 and conspicuous indentation in KNK 4165 (in SMNS 19706 and KNK 4158 the posterior margins are not preserved or not visible). It implies that the J bone could be very small, or merged variably with neighboring bones. The J bone in P. serratus is placed between the KLMN+3, I, and AB bones. Both specimens interpreted as the I bone (SMNS 19707 and KNK 4161) are highly fragmentary, however SMNS 19707 is much more complete than KNK 4161. The KLMN+3 imprint (KNK 4165) seems to confirm merging of the I and J bones into the IJ bone because of smooth transition between articulations with the I and J bones.

Skull roof of the Dipnoan Ptychoceratodus rectangulus. (A) Hypothetical Fish silhouette with marked position of the skull roof. (B) Skull roof reconstruction, originally preserved structures on the left side. Based on: SMNS 19707, KNK 4158, 4159, 4162, 4165. Arrows indicate sutures for corresponding bones. Pawlak et al. (2020).

The delta-shaped KNK 4162 is interpreted as the E bone because its posterolateral edges match the KLMN+3 anteromedial margin. Badly preserved SMNS 19708 can represent the same skull element. In Schultze’s reconstruction, the destroyed bone was interpreted as the E bone on the basis of ragged edge in the posterior part, which matches the anteromedial margin the of the KLMN+3 (SMNS 19705, 19706). These interpretations are not contradictory because the destroyed specimen revealed only the ventral surface, and KNK 4162 only the dorsal. Both specimens had similar size, therefore the shape differences are not an effect of allometric growth, but according to Schultze the E bone shape can be variable, or specimens are just incomplete. Pawlak et al. used the delta-shaped bone as the E bone in the reconstruction, however, it needs confirmation by new material. Morphology of the minute bone fragment, described by Ottp Linck in 1936 as the operculum (SMNS 17973), indicates that it is rather the descending process of the XYZ bone.

The skull roof bone pattern of Ptychoceratodus rectangulus is generally similar to other Ptychoceratodontids with known skull anatomy: Ptychoceratodus serratus, Ptychoceratodus philipsi, Ferganoceratodus jurassicus, and Ferganoceratodus martini. Unique features of the Ptychoceratodus rectangulus skull among other Ptychoceratodontids are robust ventral crests of the KLMN+3 bone, similar rather to Arganodus atlantis, and possibly the delta-shaped E bone with dorsal ridge. Right angle curvature of the supraorbital sensory line is similar rather to Ferganoceratodus spp. than Ptychoceratodus spp. Ferganoceratodus martini and Ferganoceratodus jurassicus have reduced number of bones in the medial and mediolateral series in comparison with Ptychoceratodus serratus,. However, number of bones in the medial and mediolateral series in Ptychoceratodus rectangulus is uncertain. Presence of only two bones at least in the mediolateral series is highly probable.

Ptychoceratodus rectangulus has regularly spaced, relatively large openings in the grooves of temporal line. They correspond to localisation of sensory canal pits in Recent Lungfish. The extant Protopterus has similarly developed pattern of these openings. Importance of mechanoreception or electroreception for Protopterus suggest similar mode of life for Ptychoceratodus rectangulus.

Ptychoceratodus rectangulus coexisted in the Jameson Land Basin with large Temnospondyls like the Capitosaurid Cyclotosaurus naraserluki and especially the common Plagiosaurid Gerrothorax pulcherrimus. Occurrence of the same Dipnoan species in the middle–upper Norian sediments of Germanic and Jameson Land basins indicates that at least then the areas were connected. In the Late Triassic both basins had been situated relatively close, until the opening of the northernmost part of Atlantic Ocean in the Cretaceous, which split Europe and Greenland. The Mesozoic rifting favored origin of sedimentation basins, which could have been temporally connected with the Germanic Basin and enabled freshwater fauna exchange.

The record of Ptychoceratodus from the Fleming Fjord Formation in East Greenland shows a close correlation between the Late Triassic Jameson Land Basin Fish fauna and the fauna of the coeval Germanic Basin. The studied Ptychoceratodus rectangulus remains were found in the transitional layers between the Malmros Klint and the Ørsted Dal members and in the lower part of the Carlsberg Fjord Beds (middle–upper Norian). The species was previously known from the Löwenstein Formation (middle–upper Norian) in Germany. Geographical distribution of Ptychoceratodus rectangulus shows at least temporary connection between the two basins during middle–late Norian. Recently described Ceratodus tunuensis from the uppermost part of the Carlsberg Fjord Beds is probably conspecific with Ptychoceratodus rectangulus. Ptychoceratodus rectangulus had sensory line pit pattern at least similar to that of the extant Protopterus, which suggests a similar mode of life. The Late Triassic Dipnoan freshwater Fish faunas of the northern Pangea are characterized by a frequent occurrence of Ptychoceratodus.

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