The last known freshwater coelacanths: New Late Cretaceous mawsoniid remains (Osteichthyes: Actinistia) from Southern France

Coelacanths are iconic fishes represented today by a single marine genus. The group was a little bit more diversified in the Mesozoic, with representatives in marine and continental environments in the Late Cretaceous. Here we describe isolated skull bones of the last know freshwater coelacanths found in several fossil sites from the Early Campanian to the Early Maastrichtian of Southern France (in the Departments of Aude, Bouches-du-Rhône, Hérault, and Var). The sample does not allow distinguishing different species, and all material is referred to Axelrodichthys megadromos Cavin, Valentin, Garcia originally described from the locality of Ventabren in Southern France. A reconstruction of the skull is proposed. Previously unrecognized features are described, including parts of the postparietal portion of the skull, of the suspensorium and of the mandible. The new data confirm the assignation of the species to the mawsoniids, and more specifically to Axelrodichthys. A cladistic analysis scoring new character states provides a similar topology than a previous analysis, i.e. A. megadromos is placed in a polytomy with Axelrodichthys araripensis and Lualabaea lerichei, two species from the Early Cretaceous of Brazil and from the Late Jurassic of the Democratic Republic of the Congo, respectively. A. megadromos appears to have been restricted to freshwater environments, to the contrary of oldest Western Gondwanan representatives of the family that were able to live in brackish and marine waters. A. megadromos is the last representative of the mawsoniids and its occurrence in Europe is probably the result of a dispersal event from Western Gondwana that happened somewhen in the Cretaceous. Based on the available data, the mawsoniids went extinct in the mid-Maastrichthian, i.e. before the end-Cretaceous mass extinction. But it is possible that the fossil record of this family, which has been only recently recognized in Late Cretaceous European deposits, will geographically and stratigraphically widen with further discoveries.


Late Campanian-Early Maastrichtian ("Rognacien" and Maurine Red Marls) localities
Cruzy-Massecaps and Montplo-Nord (Hérault) (CMA). Excavations have been conducted during more than 20 years by the Association ACAP and staff from the CNRS. The composite vertebrate assemblage of these close localities comprises albanerpetontid and anuran amphibians, pleurodiran and cryptodiran turtles, a freshwater mosasaur, an azhdarchid pterosaur [22], crocodilians, dinosaurs (abelisaurids, dromaeosaurids, titanosaurs, the iguanodontian Rhabdodon, the enantiornithine bird Martinavis cruzyensis and the giant ornithurine bird Gargantuavis philoinos) [23] [24 and references herein]. The fish record from the Cruzy localities consists of a lepisosteid indeterminate, and a mawsoniid represented by an angular, the first mawsoniid bone recognized in the Late Cretaceous of Europe [3]. The dinosaur assemblage indicates a Late Campanian-Early Maastrichtian age, an age confirmed by the dinosaur eggshells characteristic of the lower "Rogniacien" [25]. Based on sedimentological studies, the reconstructed environment was a braided river affected by flood episodes under a climate alternating dry and wet seasons [26].
Fox Amphoux-Bastide Neuve (Var) (FBN). The composite faunal assemblage contains hybodont sharks, a lepisosteid and a mawsoniid fish (this study), the bothremydid Foxemys mechinorum [27], the eusuchian Allodaposuchus cf. A. precedens [28], an azhdarchid pterosaur [29] and dinosaurs (titanosaurids, dromaeosaurids [30], [31], abelisaurid [20], the iguanodontian Rhabdodon, ankylosaurs, enantiornithine birds [32] and the giant ornithurine bird Gargantuavis philoinos [33 and references herein] [34]. The vertebrate assemblage and an eggshell assemblage from the nearby locality of Métisson indicate a lower "Rognacien" age, corresponding probably to the Late Campanian [35]. Sedimentological and taphonomical data indicate a fluvial setting of moderate energy with broad floodplain palaeoenvironment. Campagne-sur-Aude-Bellevue (Aude) (C3). This locality has yielded hundreds of bones found during two decades of systematics excavations by the team of the Musée des Dinosaures from Espéraza. The fauna comprises lepisosteid and mawsoniid fishes (this work), turtles, crocodiles and dinosaurs including the titanosaur Ampelosaurus atacis, the iguanodontian Rhabdodon, nodosaurid, and avian and non-avian theropods [36], [37], [38]. The site is located in the upper part of the Maurine Red Marls Formation, regarded as the equivalent to the top of the «Bégudien» and base of the Rognacien facies in the Provence Bas-Languedoc area [39]. Recently, magnetostratigraphical data from the Haute Valley area indicated that the Bellevue bone bed is located in the 'C32n.1n' magnetostratigraphic zone, which corresponds to the Early Maastrichtian (ca 71.5 mya) [40]. Taphonomical and sedimentological features indicate that the environment of deposition was a meandering and braided fluvio-alluvial setting [41].  All necessary permits were obtained for the described study, which complied with all relevant regulations (the Velaux Municipality, the environment department from CD 13, the territorial collectivity of the 'Département des Bouches-du-Rhône', the 'Direction de l'Environnement, des Grands-Projets et de la Recherche', the 'Direction des Forêts et des Espaces Naturels', the 'Service Départemental d'Incendie et de Secours').

Remark
In the present state, we are not able to distinguish distinct species within the bone sample collected in the various Campanian-Early Maastrichtian localities of Southern France. Consequently, we regard all this material as belonging to a single species, Axelrodichthys megadromos, keeping in mind that few bones are represented by several similar items. Further discoveries might show that several taxa of mawsoniids are actually present. In order avoid confusion in future studies, we provide an anatomical description that distinguished each locality.

Ventabren-Aire de Repos
No new material from this locality is described herein. The specimens from the other localities are compared, as far as is it possible with the holotype from Ventabren. Pieces of this specimen are used for the reconstruction of the skull.

Bouc-Bel-Air-Sousquières
This locality has yielded the most abundant material referred to A. megadromos.
Basisphenoid: A single, incomplete, basisphenoid is present (CD13-Pal.2019.6.1, Fig 2A). It is slightly smaller, but very similar in shape to the basisphenoid from the holotype of A. megadromos (Fig 2B), i.e. the antotic process protrudes laterally, the sphenoid condyles are close and moderately developed and they are squarish in dorsal view. In lateral view, the processus connectens is elongated and arched as in A. megadromos, but the sphenoid condyle is a little less protruding. This difference is possibly due to the imperfect state of preservation of the specimen from Bouc-Bel-Air.
Supratemporal: Two complete left supratemporals were found (CD13-Pal.2019.6.2 and -3, Fig 2C and 2D). One is about 1.5 times larger than the other. The ossifications are oval in shape with a regularly curved lateral margin. A well-marked angle separates the anterior margin from the anteromedial margin indicating that the median portion of the postparietal was longer than the lateral portion. The anterior margin is proportionally shorter on the small specimen. The suture between postparietal and supratemporal is interdigitate. The external (dorsal) surface of the bone is strongly ornamented with a reticulated pattern in the center of the ossification and irregular radiating rides at the periphery, which are more developed in the large specimen and especially marked along the lateral border. The internal (ventral) side is smooth, except a poorly developed descending process. The otic sensory canal enters the supratemporal close to the medial corner of the anterior margin of the bone and exits posteriorly at the posterior most tip of the bone, just above a little process. The otic sensory canal gives off lateral diverticula, which open and extend in deep grooves on the surface of the bone (arrows in Fig 2C). The supratemporal commissure is marked by an opening on the medial margin of the bone, close to its posterior corner. There are apparently no anterior extensions of the supratemporal commissure. The supratemporals from Bouc-Bel-Air are reminiscent of the supratemporal of all Cretaceous mawsoniid because of the strong ornamentation and the shallow descending process [1], [42], [43]. They are notably reminiscent of the supratemporal of a specimen from the Kem Kem beds, Morocco, referred to an indeterminate genus of mawsoniid sharing characters with Axelrodichthys [5]. Similarities between both specimens are the regularly curved lateral margin and the angle made by the suture with the posterior parietal. The well-preserved supratemporals from Bouc-Bel-Air allows re-identifying a bone belonging to the Ventabren type specimen originally identified as a left postparietal (Fig 2E). Because the margins of the bone from Ventabren are damaged, its original shape was supposedly more angled on the living fish, which led to conclude that it was a postparietal. Also, a small articular facet was regarded as a facet for intracranial joint less developed than in other mawsoniids [8]. This facet corresponds actually to the posterior small process present above the posterior opening of the otic sensory canal in the Bouc-Bel-Air supratemporals.
lachrymojugal: A comma-shaped bone is identified as a lachrymojugal (CD13-Pal.2019.6.4, Fig 2F). The external face is ornamented with very strong ridges and grooves. The dorsal margin, which marked the ventral border of the orbit, is regularly curved. One extremity is broader than the other. In most coelacanths, including the mawsoniids, the anterior extremity of the lachrymojugal is more expanded than the posterior one, which indicates that the bone described here is a right one. The infraorbital sensory canal is marked by two pores located at each extremity of the bone, better visible in internal view, and by at least three pores located within the strong ornamentation of the posterior half of the lateral face (arrows in Fig 2F). The lachrymojugal of Mawsonia brasiliensis is highly derived, with a very elongated straight posterior half and a curved anterior third, which marks the position of the orbit ([42] [43]). In Axelrodichthys araripensis, the dorsal margin of the lachrymojugal is curved along its whole length ( [42] [1]) as in the bone from Bouc-Bel-Air, thus confirming the attribution of the French Late Cretaceous form to the genus Axelrodichthys.
Squamosal: An approximately trapezoidal bone, with a strong ornamentation, a series of pores aligned parallel to one of its margins and crossed by a sensory canal as indicate large openings visible in internal view, is tentatively identified as a squamosal (CD13-Pal.2019.6.5, Fig 2G).
Preopercle: An ovoid bone with a strong ornamentation and crossed along its long axis by a sensory canal is tentatively identified as a preopercle (CD13-Pal.2019.6.10, Fig 2N), because of is general shape reminiscent of the corresponding bone in A. araripensis ([1], [42]).
Other tick and ornamented dermal bone are present, but too badly preserved to be identified with accuracy.
Quadrate: A left articular head of a quadrate, showing the typical actinistian double and asymmetrical condyles for articulation with the lower jaw is present (CD13-Pal.2019.6.6, Fig  2H). It is similar in shape to the corresponding bone in the holotype of A. maegadromos ( Fig  2I).
Pterygoid: Two plate-like fragments of bones covered with densely packed small bulbous teeth with radiating ridges are identified as pieces of pterygoids (CD13-Pal.2019.6.7 and -8, Fig  2J and 2K). This teeth morphology is typical of the mawsoniids, although it can also be found in other coelacanth genera. Another fragment, still attached to an indeterminate piece of bone, is covered with the same kind of teeth. It is tentatively identified as a pterygoid, although it could also be a fragment of prearticular or parasphenoid (CD13-Pal.2019.6.9, Fig 2L).

Ollières-Autoroute Nord/Sud sites
Postparietal: A right postparietal is relatively poorly preserved but shows details of its general outline and of its anterior morphology (MHN.AIX.PV.2019.13.1, Fig 3A). It is the first known postparietal of Axelrodichthys maegadromos because the supposedly postparietal of the holotype specimen [8] is actually a supratemporal (see above). The external surface of the bone is worn and it is likely that the ornamentation has been partly erased after death. The anterior portion of the bone is smooth and grooves radiate on the posterior and lateral portions. The ossification was flattened during fossilization. As a consequence, the angle between the median horizontal portion and the lateral portion visible in anterior view, was more acute on the living fish than it is on the fossil (i.e. the lateral side was more inclined.) As a result, the postparietal as figured in Fig 3A is broader than it actually was. The descending lamina of the postparietal is damaged, but it was obviously weakly developed. The facet for the intracranial joint is well developed.
Extrascapular: a trapezoid bone with a strong ornamentation, found near the postparietal described above, is tentatively identified as an extrascapular (MHN.AIX.PV.2019.13.2, Fig 3B). The main feature supporting this identification is the presence of a groove along the posterior (?) margin, which correspond to the canal of the supratemporal commissure with a broken wall. Based on its general shape and proportions, it probably corresponds to the right lateral extrascapular. The reconstruction of the skull roof indicates that a small medial extrascapular was likely present, a character present in Axelrodichthys and absent in Mawsonia [42].

Velaux-La Bastide Neuve
This locality has yielded a subcomplete parasphenoid (MMS/VBN.09.001 G, Fig 4). The posterior extremity of the bone and most of its anterior half are preserved, but no contact exists between both pieces. In ventral view, the posterior extremity broadens. Although poorly preserved, the section at this level of the bone forms a half circle. The posterior broken margin of  the anterior portion is located at the level of the posterior extremity of the tooth patch. The tooth patch is tapering posteriorly. It broadens anteriorly and forms two lateral wings, which extend laterally the width of the ossification. At this level, two lateral longitudinal grooves separate a median ridge. At the anterior extremity of the preserved portion, the median ridge itself is dug by a medial depressed longitudinal area. In dorsal view, the anterior portion is only slightly curved in section but the curvature increases backwards. There is no shallow median ridge on the dorsal side as described by Wenz [47] in 'Mawsonia' lavocati, but this difference may be caused by the different size of the specimens. The structure of the bone, in particular the contour of the posterior extremity of it toothed area, is similar in the corresponding bone in the holotype of A. maegadromos form Ventabren, in particular the heart-shaped posterior extremity of the patch, which is regarded as a diagnostic character of this species ( [8]; Fig 5). We can add to the description of this bone in the holotype that the anterior portion of the median toothed ridge is dug by a central groove. The teeth are small and hemispherical, with a minute spiny apex on some of them. The radiating grooves on the teeth typical of mawsoniids are hardly visible, probably for preservational reason.

Cruzy-Massecaps
The first recognised mawsoniid element from the Late Cretaceous of France, an isolated angular, was discovered in the locality of Massecaps in Cruzy [3]. The dentary described here comes from the same site (M2253, Fig 5). The bone is typical of dentaries of mawsoniids, i.e. it bears a hook-shaped process, a depression for the pseudomaxillary fold and a lateral swelling. The latter character is an apomorphy of the more derived mawsoniids (Mawsonia, Axelrodichthys, Lualabaea) [1]. Cupello et al. (2016) [45] showed that in Mawsonia the ventral extension of the dentary is broader and longer than the dorsal one, while in Axelrodichthys both extensions are almost equal in size, an arrangement also observed in M2253. The ventral body of the bone, which sutured with the splenial, is marked by deep grooves. An opening, corresponding to the enlarged sensory pore present in the dentary of some coelacanths, opens just posteriorly to the lateral swelling.

Fox Amphoux-Bastide Neuve
A right prearticular (collection Méchin 745, Fig 6A) was found during recent excavations in the site of Bastide-Neuve. It is very similar to the less complete right prearticular of A. megadromos from Ventabren ( Fig 6B). Both are thin bones with a subrectangular edentulous posterior lamina, which expands dorsally frontward, mirroring the general outline of the lateral angular (preserved in the holotype of A. megadromos). The deeper portion of the bone bears numerous tiny hemispherical teeth with the typical radiating ridges present in all mawsoniids.

Campagne-sur-Aude-Bellevue
A small fragment, slightly less than 1 square centimetre, corresponds to a pavement of closely packed teeth (MDE C3-02-57, Fig 7A). The teeth are very similar in size and shape to the parasphenoid teeth from Velaux (compare Fig 4 with Fig 7A), or to teeth borne by an unidentified bone from Bouc-Bel-Air (Fig 7B is a detail of the specimen illustrated in Fig 2L), i.e. they are small, hemispherical and marked by fine striations that radiate from the apex. These features indicate that this piece likely belongs to a mawsoniid, likely a fragment of parasphenoid.

Identification and affinities
Because all the mawsoniid material described in this study is incomplete or fragmentary, it is difficult to demonstrate that they all belong to a single species, i.e. A. megadromos. The fragmentary parasphenoid from Velaux, however shows the typical shape of the tooth patch of the parasphenoid of the type material of A. megadromos, a feature regarded as a diagnostic character [8]. The locality of Ventabren is situated at the base of the "Fuvélien" deposit (Early Campanian) while the locality of Velaux is situated in the "Bégudien" (late Campanian), indicating that approximatively 8 million years (Fig 1) separate both parasphenoids. Other bones know in duplicate, such as the basisphenoid and the supratemporal from the holotype of Ventabren  and equivalent bones from Bouc-Bel-Air are also very similar, even though they do not show diagnostic characters of the species. The locality of Bouc-Bel-Air is situated in the same "Fuvélien" unit as Ventabren, making likely the occurrence in both sites of the same species. The issue of the presence of another species arises with the material from Cruzy, Fox-Amphoux and Campagne-sur-Aude because the former two sites are located in the "Rognacien" facies (probably late Campanian) and the latter site is located in the Maurine Red Marls (probably early Maastrichtian), i.e. these fishes are about 10 million years younger than the type specimen from Ventabren. However, duplicate bones from these younger localities, i.e. an angular from Cruzy, a prearticular from Fox-Amphoux and parasphenoid teeth from Campagne-sur-Aude show no characters differing enough from the type of A. megadromos for characterizing another species.  mawsoniid features, some of them present only in the Mawsonia-Axelrodichthys complex: the strongly ornamented bones of the skull roof and of the cheek, the shape of the postparietal and of the supratemporal and notably their shallow descending processes, the peculiar arrangement of the bones of the postparietal portion, with the extrascapulars included in the skull roof, the typical shape of the angular, the shape of the prearticular and pterygoid teeth, the lateral swelling of the dentary, and the medial expansion of the cleithrum. Other features, i.e. the probable presence of a medial extrascapular, the curved dorsal margin of the lachrymojugal, the ovoid shape of the preopercle, the posterior extensions of the dentary almost equal in size, and the general organization and proportions of the ethmosphenoid portion of the skull roof confirm the inclusion of this species in the genus Axelrodichthys.
In order to test the phylogenetic affinities of this species, we include A. megadromos in the recent phylogenetic cladistic analysis of Cavin et al. [7]. In the latter analysis, A. megadromos was resolved in a trichotomy with A. araripensis and Lualabaea lerichei, from the Early Cretaceous of Brazil and the Late Jurassic of the Democratic Republic of the Congo, respectively. The scoring of the character states of A. maegadromos was based on the holotype specimen only [8]. If [1] (for details of characters and character states definitions, see [7]). The new data matrix (S1 Data) was analysed using PAUP � 4.0a167 (SWOFFORD, 2000) with a heuristic search using random addition sequence, replicated 10000 times, 10 trees held at each iteration, and tree bisection and reconnection branch swapping was carried out with Latimeria and Macropoma as outgroup. The exact topology and the amount of number of parsimonious trees are similar to the results obtained by [7] (Fig 9), and the indices are very close (18 MPT, length 100, CI = 0.660, RI = 0.663, RC = 0.438; previous metrics: 18 MPT, length 100, CI = 0.660, RI = 0.653, RC = 0.431). In consequence, the new material described herein brings new information for reconstructing the anatomy of this species, but it does not allow improving the understanding of its phylogenetic relationships. For a discussion of the distribution of the characters within the mawsoniid phylogeny, see [7]. Eventually, it should be mention that the stratigraphical ranges of A. megadromos is relatively long, i.e. circa 10 million years, but it is still short compared to the stratigraphical range of Mawsonia gigas, which is circa 30 million years in South America [7]. These long ranges point to a slow morphological evolution and illustrates the status of 'living fossils' of the coelacanths taken as a whole, although with notable exceptions [60].

Conclusion
Late Cretaceous European continental mawsoniids have been recognised only recently, about fifteen years ago. At that time, remains of these fishes appeared to be very rare, but examination of palaeontological collections from several localities reveals that they actually formed a non-negligible portion of the vertebrate assemblages. Fossils of mawsoniid coelacanths are more abundant and better preserved in the early Campanian "Fuvélien" facies than in the Late Campanian "Bégudien" and early Maastrichtian lower "Rognacien" facies. At the moment, we cannot decide if this trend: 1) corresponds to a real rarefaction of these fishes during this time interval; 2) is due to taphonomical or environmental biases linked to differences in the sedimentological and preservational features between facies or; 3) is caused by a sampling artefact.
The current data indicate the occurrence of mawsoniids only during a short time interval and in a restricted area of the Ibero-Armorican Island. But we hypothesize that new field discoveries and re-examination of palaeontological collections, especially from southern Pyreneans and Eastern Europe (Hungary and Romania) may enlarge the geographical range of the family. Similarly, it is likely that the stratigraphical range of mawsoniids will be extended towards the past, as indicate preliminary observation, but potentially towards the present as well. Mawsoniid fishes do not share the typical profile of the fish victims of the Cretaceous-Palaeogene mass extinction event [61], and it is not unlikely that representatives of this lineages crossed this boundary. So far, the fossil record of coelacanth is inexistent in the Cenozoic.
Supporting information S1 Data. Character list and character-taxon matrix in nexus format. (NEX)