User:Armin Reindl/sandbox

Armin Reindl/sandbox Temporal range: Paleocene - Middle Miocene,
Diplocynodon ratelii
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauromorpha
Clade:	Archosauriformes
Order:	Crocodilia
Superfamily:	Alligatoroidea
Subfamily:	†Diplocynodontinae Brochu, 1999
Genus:	†Diplocynodon Pomel, 1847
Species
†D. dalpiazi Del Favero, 1999 †D. darwini (Ludvig, 1877) †D. deponiae (Frey, Laemmert & Riess, 1987) †D. elavericus Martin, 2010 †D. gervaisi †D. hantoniensis (Wood, 1846) †D. levantinicum Huene & Nikoloff, 1963 †D. kochi (Venczel & Codrea, 2022) †D. muelleri (Kälin, 1936) †D. ratelii Pomel, 1847 (type) †D. remensis Martin et al., 2014 †D. tormis †D. ungeri (Prangner, 1845)
Synonyms
Baryphracta Frey, Laemmert & Riess, 1987 Enneodon Pranger, 1845 Hispanochampsa Kälin, 1936 Saurocainus

Diplocynodon is an extinct genus of alligatoroid crocodilian that lived during the Paleocene to Middle Miocene in Europe. Some species may have reached lengths of 3 metres (9.8 ft), while others probably did not exceed 1 metre (3.3 ft). They are almost exclusively found in freshwater environments. The various species are thought to have been opportunistic aquatic predators.

In the nineteenth century, D. steineri was named from Styria, Austria and D. styriacus was named from Austria and France. A third Austrian species, Enneodon ungeri, was placed in its own genus. The Austrian and French species of Diplocynodon were synonymized with E. ungeri in 2011, and because the name Diplocynodon has priority over Enneodon, the species is now called D. ungeri. Other genera have recently been found to be synonymous with Diplocynodon. Hispanochampsa muelleri of Spain was determined to be synonymous with Diplocynodon in 2006, and Baryphracta deponaie of Germany was confirmed to be synonymous with Diplocynodon in 2012.

Well preserved specimens have been found in the Messel Pit and the Geiseltal lignite deposit in Germany. Most articulated Diplocynodon specimens from these localities contain gastroliths. In the Eocene epoch, the German sites were either a swampy freshwater lake (Messel Pit) or a peat bog swamp (Geiseltal).

History

Early finds in the UK

Some of the first remains to now be recognized as those of Diplocynodon were recovered in the early 19th century from Hordle Cliff, an outcrop of the Headon Hill Formation, with the first reports being published as early as 1844. The report, presented by Edward Charlesworth at the behest of Searles Wood, mentions the discovery of a well preserved skull as well as various postcranial remains thought to represent an alligator, which was dubbed Alligator hantoniensis by Wood. This early publication was soon followed up by a more detailed report two years later, figuring the material mentioned previously and once again proposing the name Alligator hantoniensis. Although the name first appeared in Charlesworth's report, the type authority is attributed to Wood's 1846 work due to the fact that only the latter figured the fossil material.

Charlesworth and Wood were not the only ones to take an interest in the Hordle Cliff crocodilian fossils. In 1947 the British Association for the Advancement of Science held its 17th meeting in Oxford, which was attended by both Barbara Rawdon-Hastings, Marchioness of Hastings and Richard Owen, both of whom reported their own findings at the event. Lady Hastings presented two skulls that were noted for their preservation and displayed some anatomical features similar to what is seen in modern crocodiles. These two specimen were noted by Owen in 1848 for that reason, with him placing them in the genus Crocodylus and coining the name Crocodylus hastingsiae after their discoverer. Owen only makes a brief mention of Alligator hantoniensis, as he'd been unaware of the animal's existence until it was brought to his attention by Gideon Mantell during his presentation.

By 1950 Owen had managed to access material assigned to Alligator hantoniensis and acknowledged that it and his Crocodylus hastingsiae were very similar to the point of being nearly indistinguishable. Despite this, Owen tentatively maintained both as distinct species due to the specific way the fourth dentary tooth interact with the rest of the jaw. Owen continued to regard C. hastingsiae as a crocodile on account of the fourth dentary tooth sliding neatly into a notch in the upper jaw, whereas in A. hantoniensis he describes it as sliding into a pit that would have obscured it when the jaws were closed. He does concede that this was merely a matter of variation between specimen, but simultaneously hesitated to interpret it as such given that the visibility of the fourth dentary tooth is regarded as a key difference between modern true crocodiles and alligators.

Discoveries in Austria, France and the genus Diplocynodon

Around the same time that the earliest fossils were found in the UK, Engelbert Prangner described Enneodon ungeri from the middle Miocene Schönegg of Styria, Austria, on a the basis of a partial rostrum. However, it wasn't long until Enneodon was lumped into the genus Crocodylus as C. ungeri by multiple authors including Fitzinger, Giebel, Meyer and Hofmann. Adolf Hofmann furthermore described the species Crocodilus (Alligator) styriacus and Crocodilus steineri, with the former known from a complete and articulated skull recovered from the same locality as Prangner's Enneodon.

Pomel also recognized similarities between Diplocynodon and Alligator hantoniensis, creating the new combination Diplocynodon hantoniensis. However, while subsequent researchers also drew comparisson between the two, none acknowledged Pomel's work. Hermann von Meyer compared the two in 1857, but while he too noted similarities he did not make any decisive taxonomic referrals unlike Pomel. Thomas Huxley and Arthur Smith Woodward both concluded that all of the Hordle material belonged to a single species in 1859 and 1885 respectively, dismissing the differences noted by Owne as individual variation of a single species, with both of them opting to use the name Crocodylus hastingsiae rather than the older Alligator hantoniensis. It wasn't until 30 years after Pomel's paper that Richard Lydekker once again committed to lumping the Hordle material into Diplocynodon as D. hantoniensis. Unlike Pomel's previous attempt, this referral saw wider recognition and has become concensus.

Similarily, in 1898 Roger also recognized the Austrian material to represent Diplocynodon, reiterated again by him in 1902. Though this matches later studies like that of Martin and Gross, Roger gave no reasoning for this taxonomic move, nor did he explain why he lumped both of Hofmann's species under the name D. steineri. However little came of this and much as had happened initially with D. hantoniensis the material once more slipped into obscurity with subsequent papers alternating between the use of Diplocynodon and Crocodilus with different studies favouring different species names.

20th Century

1966 saw another attempt at clarifying the state of the Austrian material, as Berg argued that Diplocynodon styriacus should be favoured over Diplocynodon steineri on account of having better material. Berg similarily suggested that Enneodon ungeri should be treated the same, rejecting the previous hypothesis that it may have been a kind of gharial, but ultimately did not finalize this change.

Revisions and new discoveries of the 21st century

2011 saw a revision of the Austrian taxa Enneodon ungeri, Diplocynodon styriacus and Diplocynodon steineri by Jeremy E. Martin and Martin Gross. While the name Enneodon technically predates Diplocynodon, the team concludes that the latter should be regarded as a nomen protectum due to its historical value and the fact that it saw widespread use since its conception, unlike Enneodon which had only been employed sporadically and not at all after 1899. Not only did they thus solidify Diplocynodon ungeri, they further regarded both D. steineri and D. styriacus as junior synonyms of D. ungeri. Martin and Gross also confirm a previous study by Ginsburg and Bulot that assigned material from France to D. styriacus.

In 2014 Martin and colleagues described a new species, Diplocynodon remensis, based on numerous fossils from the Mourras quarry and Lemoine quarry of Mont de Berru, France.

In 2020 Jonathan P. Rio and colleagues published a paper on Diplocynodon hantoniensis with the goal of analyzing it with a modern understanding, as the species had not been revised in over a hundred years. Their study reinforced the validity of the species, established a new diagnosis and commented on previously undescribed features as well as the validity of much of the material that had been referred to the species over the years.

The most recently named species is Diplocynodon kochi, described in 2022 by Márton Venczel and Vlad A. Codrea, although the fossil remains had been known since 1890. The species was described on the basis of an incomplete skull recovered from a Priabonian limestone quarry near Cluj-Mănăștur in Transylvania, Romania. Only a year later Venczel went on to describe multiple additional remains referrable to the species collected from three other localities across Transylvania.

Species

Species
Species	Age	Location	Unit	Notes	Images
D. dalpiazi	Late Rupelian	Italy	Monteviale
D. darwini	Lutetian	Germany	Messel pit	All specimens are from Messel pit of Germany. Synonyms are: D. ebertsi and D. hallense.
D. deponaie	Middle Eocene	Germany	Messel pit	Synonyms are: Baryphracta deponaie.
D. elavericus	Middle Priabonian	France	Domérat	All specimens came from Allier, Massif Central of France.
D. gervaisi	Earliest Rupelian	France	Ronzon	Synonyms are: Saurocainus gervaisi.
D. hantoniensis	Late Eocene (Priabonian)	United Kingdom	Headon Hill Formation	Among the earliest species to be described, D. hantoniensis is exclusively known from Hordwell in the southern UK.
D. levantinicum	Oligocene (Chattian)	Bulgaria	Maritsa Formation
D. kochi	Eocene (Early to Late Priabonian)	Romania	Cluj Limestone Formation	D. kochi is known from four localities within the Gilău and Meses sedimentary areas of the Transylvania Basin. Not only does it mark one of the easternmost occurences of the genus, but it has also been noted for being the only species with remains found in marine sediments, though its appearance in inland environments has lead to the idea that it might have only occasionally entered coastal waters. It was named after Antal Koch.
D. muelleri	Middle Rupelian	Spain	El Talladell	More than 100 are known, all from Lleida Province, Catalonia. Synonyms are: Hispanochampsa muelleri, D. guerini and D. marini.
D. ratelii	Middle Aquitanian Late Eocene to Early Miocene	France	Saint-Gérand-le-Puy	D. ratelii is the type species of Diplocynodon. Most of the specimens came from Allier, Massif Central of France. Synonyms are: D. gracile.
D. remensis	Paleocene (Upper Thanetian)	France	Mont de Berru	Although the oldest named species of Diplocynodon, phylogenetic analysis do not support the idea that it was the basalmost species as might be inferred based on stratigraphy.
D. tormis	Late Bartonian	Spain	Salamanca
D. ungeri	Middle Miocene (Badenian)	Austria France	Pannonian Basin* Paris Basin	Originally described under the genus name Enneodon, most material of D. ungeri stems from the Austrian state of Styria. However, some remains described in 1997 indicate that the species was also found in South and Central France.

Though many more species were at points placed in the genus Diplocynodon, many of them were ultimately found to either be based on insufficient material rendering them nomina dubia or were eventually found to be synonyms of already existing species in the genus or even entirely different crocodilians alltogether.

A lot of material has also been referred to Diplocynodon hantoniensis, either wholy or tentatively. Much of this has been discussed by Rio and colleagues in their 2020 revision of the species. They find that of the European material, that from the Swiss canton of Vaud represented an indetermined alligatoroid, that from the early Oligocene of Hesse, Germany, could belong to either of three described species (D. hantoniensis, D. muelleri or D. ratelii) while fossils from Razac-d’Eymet in France is identified as Diplocynodon sp..

Outside of Europe, Edward Drinker Cope named the species Diplocynodus (Diplocynodon) sphenops from the San Juan Basin in 1875 while Charles C. Mook described Diplocynodon stuckeri based on remains from the middle Eocene of Wyoming. Based on Diplocynodon stuckeri, a singular tooth and osteoderm from Virginia were also assigned to Diplocynodon, specifically D. hantonensis. However, D. sphenops has since then be regarded as possibly being a species of Allognathosuchus and a nomen vanum while Christopher Brochu regarded both D. stuckeri and possibly D. sphenops as junior synonyms of Borealosuchus wilsoni. The isolated virginian material meanwhile was found to simply not be enough to be assigned to anything more specific than an indetermined crocodilian.

Distribution

Origins and Paleocene record

Diplocynodon was a long-lived and widespread genus, first appearing during the Paleocene and lasting until the middle Miocene. This results in an impressive spatial distribution that covers over 40 million years, though their range is entirely confined to the European continent. Due to the relatively poor fossil record of Paleocene freshwater eusuchians, little is known about the exact origins of Diplocynodon, though phylogenetic analysis suggest a link to North American taxa.

The oldest named species of Diplocynodon, D. remensis, has been recovered from Thanetian deposits at Monte de Berru in the Paris Basin. Though the age of the species would lend itself well to the hypothesis that it was the basalmost member of the genus, something that finds support by its unique anatomy, phylogenetic analysis do not support this line of thinking and instead recover it as a more intermediate taxon. The overall presence of D. remensis indicates that Diplocynodon or its ancestors must have arrived in Europe prior to the Paleocene-Eocene Thermal Maximum (PETM). Though Europe's terrestrial Paleocene sites are poorly studied, enough information is present to show that most mammals that immigrated from North America only did so around 56 million years ago during the PETM. Two hypothesis might explain the origins of Diplocynodon. One proposes that a North American origin is possible, as Mont de Berru already features plesiadapids, arctocyonids and multituberculids. The other hypothesis meanwhile suggests an Asian origin, which would parallel the origins of the other crocodilian (Asiatosuchus) and turtles at the locality. Yet another hypothesis suggests that the presence of Diplocynodon could reach back even beyond the Late Paleocene, especially if a connection to the middle to late Paleocene Menatalligator can be recovered. However, the remains of this taxon have been lost.

Eocene

During the Late Eocene Diplocynodon hantoniensis inhabited the southern UK.

One of the last Diplocynodon species of the Eocene was D. kochi from Romania. Fossils of this taxon have been recovered from four localities across two sediment areas in Transylvania, Romania. These are the localities of Cluj-Mănăştur (Cluj Limestone Formation), Rădaia (Valea Nadăşului Formation) and Leghia-Tabăra (Viştea Limestone Formation) of the Gilău sedimentary area and the Treznea locality (Turbuta Formation) of the Meseş sedimetnary area. All of these correspond to the Priabonian, with Cluj-Mănăştur being Late and all the other localities being Early Priabonian in age. It is hypothesized that Diplocynodon arrived in Romania from the west in what was likely a comparably slow process, as the genus had to contend with the sea levels being much higher than today, effectively splitting the continent into a series of archipelagos separated by epicontinental seas. Still, D. kochi does show that the genus ranged as far east as Romania by the Priabonian.

Oligocene

Romania also helps understand the survival of Diplocynodon across the Eocene-Oligocene boundry. Remains assigned to Diplocynodon are known from the Early Oligocene (Rupelian) localities of Suceag and Cetățuia Hill, showing that the genus survived in its range despite deteriorating environmental conditions. Venczel and Codrea argue that this was possible due to the fact that the Eocene-Oligocene boundry did not see major climate pressures that affected Diplocynodon, which casts doubt over Jeremy E. Martin's hypothesis that Diplocynodon migrated southward in this timeframe to avoid extinction.

Early Oligocene occurences of unnamed or indetermined species are known from Razac-d’Eymet in France as well as the Melanian clay of Germany.

Miocene and extinction

The last Diplocynodon survived until the middle of the Miocene, specifically MN Zone 4 to 5 which corresponds to the early Badenian age of the Paratethys. These last holdouts of Diplocynodon are represented by the species D. ungeri, which is known from France and Austria. The Austrian remains are known exclusively from the Eibiswald Formation in Styria. The Styrian basin that was home to D. ungeri was a subbasin of the Pannonian basin and by extension part of the central Paratethys.

Description

Diplocynodon kochi possessed a prominent ridge before its eyes and a similar ridge is recognized from D. remensis, although in the former the crest is wide and overhanging while in the latter it is only weakly developed.

The way the teeth occlude with another varies among species. Most have premaxillary teeth that form an overbite, however, in D. tormis and D. ratelli these teeth interlock.

Size

Diplocynodon was comparably small for a crocodilian, with its size

The smallest species did ot exceed 1 m (3 ft 3 in) in length, with the German Diplocynodon deponiae generally reaching lengths of 0.73–0.83 m (2 ft 5 in – 2 ft 9 in). Several species can be regarded as medium sized by the genus' standards, with both Diplocynodon tormis and Diplocynodon darwini ranging between 1.4–1.8 m (4 ft 7 in – 5 ft 11 in). Similiar estimates were proposed for the Romanian Diplocynodon kochi, which likely grew to around 1.76 m (5 ft 9 in) in length, tho some estimates suggest they might have been as small as 1.61 m (5 ft 3 in). The largest species of Diplocynodon is D. hantonensis, with large specimens suggesting a length of up to 2.9 m (9 ft 6 in).

Paleobiology

In the case of Diplocynodon kochi, the slender, flattened snout, enlarged attachment site for the external jaw adductors and the slightly compressed teeth in the back of the jaw are well suited for an animal that was actively hunting. Due to the well developed jaw adductors it would have been able to perform rapid jaw movements, which in turn would have allowed the compressed teeth to shear through prey.

Given the small size of Diplocynodon, it likely preyed on a variety of invertebrates and small vertebrates.

Paleoenvironment

Diplocynodon kochi and salt tolerance

One exception to this general preferance for freshwater is seen in the Romanian species Diplocynodon kochi. While the localities of Rădaia and Treznea both matching the inland freshwater biomes inhabited by other species, with the former interpreted as a flood plain and the latter as a marshy lacustrine environment, the other two localities are known to have been shallow marine deposits. Cluj-Mănăştur, where the holotype originates from, preserves a shallow marine environment that in addition to Diplocynodon was also home to softshell turtles (which are known to venture into saltwater), the sulid Eostega and the sirenian Protosiren. A similar environment has also been inferred for the Leghia-Tabăra locality, which preserves marine fauna such as sea urchins, marine algae, various molluscs and shark teeth. In both instances, the fossil material is well preserved, suggesting that the bones did not undergo extensive transport before being burried by sediments in the shallow water (though short distance transport is still possible).

This has lead to some speculation on the ability of Diplocynodon to tolerate saltwater. Of the three crocodilian families still alive today, both crocodylids and gavialids are known to possess salt glands that allow them to manage salinity levels and spend extended periods of time in saltwater, something that factored into their dispersal across the globe during the Cenozoic. In contrast to that, alligatorids do not possess salt glands and by extension tend to avoid saltwater. However, it is not clear if the loss of salt glands in alligatorids necessarily affects Diplocynodon, given that these glands are likely ancestral to Eusuchia as a whole and the fact that Diplocynodon is regarded as a basal alligatoroid. Furthermore, even in modern alligators the lack of salt glands does not entirely prevent them from venturing into saltwater, as can be observed in adult American alligators which will at times enter brackish and even coastal waters, with records finding that they might travel up to 60 km (37 mi) out to sea in some cases.

Much like modern alligators, Diplocynodon kochi may have been primarily native to inland bodies of freshwater with only limited osmoregulation while still being able to at least occasionally travel into more saline environments such as the shallow marine deposits the holotype was recovered from. One argument to support this conclusion is that Diplocynodon seems to have dispersed relatively slowly, likely limited by the fluxuating sea levels present throughout most of the Eocene, with Europe broken up into various archipelagos that were only temporarily linked by land bridges.