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. darwini (Ludvig, 1877) †D. deponiae (Frey, Laemmert & Riess, 1987) †D. elavericus Martin, 2010 †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

History

Early discoveries

Remains from Saint-Gerand-la-Puy were first identified under the name Orthosuchus by Geoffroy in 1833. However, Geoffroy did little to distinguish Orthosuchus from other crocodilians.

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.

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 (the latter of which further regarded it as a gavialiform). 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.

Around two years after the description of Enneodon, Pomel named the genus Diplocynodon.

Pomel also recognized similarities between Diplocynodon and Alligator hantoniensis in 1853, 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.

In 1956 Aymard coined the name Saurocainus gervaisii for fossil remains from Ronzon, France, tho the publication provided no proper description. This was not improved by Gervais himself, who referred to and illustrated the remains of Saurocainus in 1959. This original material, following Gervais' work, appears to represent a fragment of a mandible, though Berg would later go on to say that Gervais illustration was "hardly analyzable".

Another species was described from Saint-Gerand-Le-Puy by Valiant in 1972, named Diplocynodon gracile.

In 1985 Zittel described more material from Ronzon possibly belonging to D. gervaisi.

1877 saw the description of two crocodilians from the Messel Pit in Germany, penned by paleontologist Rudolph Ludwig. These animals were Alligator darwini and Crocodilus ebertsi, both of which ended up being sunk into the genus Diplocynodon by Richard Lydekker ten years later.

Similarily, in 1898 Roger also recognized the Austrian material to represent Diplocynodon, reiterated again by him in 1902. This created the combinations Diplocynodon steineri. 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 the few subsequent papers alternating between the use of Diplocynodon and Crocodilus and lumping/splitting the three species in different manners.

Early 20th Century

1936 saw the description of Hispanochampsa by Swiss paleontologist Otto Kälin based on two skulls found in the northeast of Spain and held at the Natural History Museum of Basel. Even before this study, crocodilian remains had been recovered from the locality of El Talladell, with Charles Depéret assigning one particular skull to Diplocynodon sp. as early as 1906. However, unable to recognize the classic double caniniform that the genus was named for, Kälin came to the conclusion that the Basel material did not belong to Diplocynodon, with Hispanochampsa being regarded as being more closely related to the modern black caiman (Melanosuchus). Shorty after the description of Hispanochampsa, two more species came to be named from material from El Talladell: Diplocynodon marini and Diplocynodon guerini, both named by Joseph Ramon Bataller in 1941. Tho Bataller did not mention Hispanochampsa at all in his initial publication, by 1956 he did come to regard it and D. guerini to by synonyms of each other (though without providing a anatomical reason). Two years after that, in 1958, Frédéric-Marie Bergonioux came to the same conclusion for D. marini, sinking it too into Hispanochampsa. Both D. guerini and D. marini were likewise considered junior synonyms by Angela D. Buscalioni in 1986.

In 1963 Friedrich von Huene and Ivan Nikoloff described Diplocynodon levantinicum from various bone fragments discovered two years prior in the Nadeshda underground mines in Central Bulgaria. Though collecting material of at least four individuals, the remains were fragmentary and the team did not compare the material to other known species of the genus nor did they justify their decision to erect a new species. They further did not establish a holotype, although later studies would regard a partial dentary as such. Von Huene and Nikoloff coined the name Diplocynodon levantinicum and believed it to have lived during the Levantinian (an antiquated term for the Late Pliocene), though later analysis of the regional stratigraphy would disprove this idea.

1966 revision of the genus

In his 1966 publication on the crocodilians of the Messel Pit in Germany, Dietrich E. Berg also included a section on the genus Diplocynodon in which he recognized 8 species. Given the locality, a big focus of his work was on the two then established species from Messel, Diplocynodon darwini and Diplocynodon ebertsi, both of which he at the time recognized as valid. In addition to adult specimens, Berg also mentioned juvenile material, now regarded as a distinct species.

His paper also discusses the state of other species that had been referred to the genus Diplocynodon. This included another attempt at clarifying the taxonomy of the Austrian material, which had previously been lumped into a single species of Crocodilus by Kühn in 1936 and then split into three distinct species again by Thenius in 1955. Ultimately, Berg continued to use the name Diplocynodon as had been suggested by Roger and Thenius, but alligned more with Kühn in regards to species diversity. Berg argued that at least Diplocynodon styriacus and Diplocynodon steineri represented a single species, with D. styriacus being the name chosen for the animal. The reason D. styriacus was given priority over Diplocynodon steineri is explained by the differences in preservation, with Berg reasoning that the mandible of D. styriacus provided a better diagnosis than the crushed skull remains of D. steineri. Berg similarily suggested that Enneodon ungeri may also belong to this species, rejecting the previous hypothesis that it may have been a kind of gharial, but ultimately did not finalize this idea, retaining Enneodon as distinct.

In regards to D. gracilis, Berg writes that he was unable to distinguish the material in any significant way from that of D. ratelii, with the only differences noted also being explainable by ontogeny or simple variation within a species. Accordingly, Berg argues that D. gracilis was likely synonymous with D. ratelii. Additional similarities are noted between D. gracilis and "Crocodilus" aeduicus, which Berg interpreted as a potential subspecies of D. gracilis. Berg also touches on Orthosuchus, concluding that Geoffroy's description is insufficient and that the name Diplocynodon was too widespread to be so easily replaced.

All in all Berg mentions several species as valid, consisting of D. ratelii from France, D. darwini and D. ebertsi from Messel, D. hallensis from the Geiseltal (though Berg took note of an intended revision of the species), D. stuckeri from the North American Bridger Beds (though other North American forms were not recognized as valid),D. styriacus from Austria, D. levantinicum from Bulgaria, D. marini and D. guerini from Spain, D. hantoniensis from the UK as well as D. gervaisi and a further unnamed form from France

Berg also weighed in on material from Italy described in the late 1800s and early 1900s, with the taxa Crocodilus bolcensis and Crocodilus vicetinus from the Eocene locality of Monte Bolca and Crocodilus dalpiazi and Crocodilus monsvialensis from the Oligocene locality of Monteviale. Berg concludes that Diplocynodon was present at both localities, though he does not definitely assign any of the then named species to the genus. Subsequent authors would sink Crocodilus monsvialensis into Diplocynodon dalpiazi.

Late 20th century

The material thought to belong to juvenile individuals of the Messel Diplocynodon would eventually come to be regarded as a distinct taxon by Frey, Laemmert, and Riess in 1987. Though rather than just naming the material as a new species of Diplocynodon, they coined an entirely new genus, Baryphracta. While phylogeny was not tackled by the team, later studies would establish a close relationship with Diplocynodon, sink it into the genus outright or suggest ties with the small-bodied Allognathosuchus.

In 1998 Brinkmann and Rauhe suggested that the Austrian material simply represented Diplocynodon ratelii on the basis of an Oligocene skull from Céreste, France. However, the Céreste skull was not described in full and Martin and Gross note that the anatomy and proportions do not support this line of thinking.

In 1999 Christopher Brochu recovered Baryphracta as the sister taxon to Diplocynodon

Revisions and new discoveries of the 21st century

The 2000s saw multiple papers with the aim of either describing new species of Diplocynodon or trying to clarify the taxonomic status of already established taxa. Among the newly named forms was Diplocynodon elavericus, described by Jeremy E. Martin in 2010 based on fossils recovered from the Eocene Massif Central in France, specifically the locality of Domérat. Another French specimen was named by Martin and colleagues just four years later, Diplocynodon remensis from the Mourras quarry and Lemoine quarry of Mont de Berru.

After having already been redescribed one, 2006 saw yet another study reinterpreting the material of Hispanochampsa from El Talladell, this time with the conclusion that the species actually represents a species of Diplocynodon rather than simply a related genus.

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 as a valid combination, 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.

Study of three-dimensionally prepared specimens of Baryphracta eventually allowed for a reappraisal of the taxon to be authored by Massimo Delfino and Thierry Smith in 2012, resulting in the authors placing the animal once more in the genus Diplocynodon under the name Diplocynodon deponiae.

Various fossils were recovered from the Vallès-Penedès Basin of Spain and were attributed to Diplocynodon ratelii in 2017.

In 2019 Macaluso and colleagues provided a reinterpretation of material recovered from Vicenza in Italy, which had previously been referred to as Diplocynodon dalpiazi and was eventually assigned to Diplocynodon cf. ratelii from 1999 onward. The team did find significant differences that showed that the material was not referrable to D. ratelii and instead more closely resembled D. muelleri, but also acknowledged that said species required revisions of its own. Subsequently, they referred the material to D. cf. muelleri. Furthermore, their study showed that material previously thought to have originated at the older locality of Monte Bolca actually stems from Monteviale, restricting Diplocynodon at Vicenza to the Oligocene sediments.

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.

In 2022 Massonne and Böhme published a revision of Diplocynodon levantinicum, recognizing additional remains from a second locality and establishing a new age for the type locality. Originally, it was believed that this taxon came from Pliocene deposits, which would have made it the youngest species of the genus by far. Several subsequent authors have come to cast doubt over this however and Massonne and Böhme presented a range of arguments showing that this early estimate was indeed inaccurate, instead proposing a much more reasonable Late Oligocene age. While having to revise the age of the animal, the team did nonetheless conclude that Diplocynodon levantinicum was a valid taxon distinct from all other species of the genus, with a cast of a dentary serving as the lectotype (the original fossil had been lost since its discovery).

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. darwini	Middle Eocene, Lutetian	Germany	Messel pit	All specimens are from Messel pit of Germany. Synonyms are: D. ebertsi and D. hallense.
D. deponaie	Middle Eocene, Lutetian	Germany	Messel pit	Once thought to represent juvenile remains, D. deponiae was later regarded as a distinct genus (Baryphracta) before being placed in Diplocynodon. It can easily be distinguished from the contemporary D. darwini by its much smaller size (less than 1 m (3 ft 3 in)) and the fact that its tail was entirely covered in osteoderms.
D. elavericus	Late Eocene, Middle Priabonian (MP18-20)	France	Domérat	All specimens came from Allier, Massif Central of France. The name "elavericus" stems from the Latin name of the Allier River. The name is similar but unrelated to that of Crocodilus elaveris, a nomen dubium coined in 1843 for various unfigured and undescribed crocodilian remains from Allier.
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	Though originally thought to be Pliocene in age, recent revisions have shown that it was much younger, living some 26 million years ago during the Late Oligocene.
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 Italy	El Talladell Monteviale	More than 100 are known, all from Lleida Province, Catalonia. Synonyms are: Hispanochampsa muelleri, D. guerini and D. marini.
D. ratelii	Early Miocene (MN2 to MN4)	France Spain	Saint-Gérand-le-Puy Vallès-Penedès Basin	D. ratelii is the type species of Diplocynodon and from Allier department of France, with later finds adding material from the site of Saint-Gérand-le-Puy (initially described as D. gracile). Material has also been recovered from the Vallès-Penedès Basin of Spain.
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. It was named for the Remes, the gallic natives of the Reimes region.
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 France. The species Diplocynodon styriacus and Diplocynodon steineri are both synonyms of D. ungeri.

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.

Some species that have been described but require reassesment as to their validity include Diplocynodon gervaisi and Diplocynodon buitekonensis.

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..

Diplocynodon gracile was described from material found at Saint-Gérand-le-Puy, but is now regarded as being a junior synonym of Diplocynodon ratelii.

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.

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).

Evolutionary history and range

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. Its range covered most of the continent, with fossils recovered from over 300 localities, from Spain and the UK in the West to Ukraine in the east. The southern most records come from Spain and Italy, while it ranged north as far as Germany. 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.

Still, the origins of the genus remain ellusive. The most common hypothesis is that Diplocynodon has American ancestry, as indicated by phylogenetic analysis. Jeremy E. Martin highlights that the oldest species of Diplocynodon appears to predate the large-scale immigration of North American mammals into Europe during the Paleocene-Eocene Thermal Maximum. At the same time however, he goes on to acknowledge that the same locality that yielded these fossils did feature some mammals with ties to America, namely plesiadapids, arctocyonids and multituberculids. Counter to this runs the hypothesis that Diplocynodon had its origins in Asia. Martin has noted that this could parallel the dispersal of the crocodyloid Asiatosuchus as well as various turtles that coexisted with Diplocynodon. This hypothesis might be supported by the findings of Kuzmin and Zvonok, who identify a plethora of Diplocynodon-like remains from the Cenomanian to Santonian of Central Asia (Kazakhstan, Tadzhikistan and Uzbekistan). Though these remains are significantly older than any known remains of Diplocynodon, the dispersal into Europe might have still occured during the Paleocene when landbridges were present between Europe and Asia. Another animal that might give clues to the origins of Diplocynodon is Menatalligator, however, the remains of this taxon have been lost, rendering it inaccessible for study.

The first true records of Diplocynodon come from Thanetian deposits at Monte de Berru in the Paris Basin and were described as Diplocynodon remensis. Though its age would lend itself well to the hypothesis that it was the basalmost member of the genus, with the surrounding fauna showcasing both Asian and North American influences, phylogenetic analysis does not support this line of thinking. Instead, Diplocynodon remensis has been recovered as quite derived member of the genus, meaning that it is not useful in solving the question of where the genus originated.

Eocene

Diplocynodon occurs next in the fossil record in early Eocene (MP7) strata of Silveirinha, Dormaal and Le Quesnoy.

Fossils tentatively referred to Diplocynodon have been recovered from Lutetian strata of Ikovo, Ukraine.

By the late Eocene Diplocynodon had spread into the UK, with the species Diplocynodon hantoniensis being found in Priabonian strata of southern England.

Diplocynodon elavericus was found in Middle Priabonian sediments at Domérat, France, in the d´ epartement de l’Allier.

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.

Survival into the Oligocene

The Eocene-Oligocene boundry marks a pivotal time for crocodilian diversity in Europe, with many of the previously established faunas falling victim to the global drop in temperatures that occured at the Eocene-Oligocene boundry, though certain lineages appear to have began to disappear even before that. For example, while many Early and Middle Eocene faunas had up to four or even seven contemporary crocodilians (notably localities like the Messel Pit, Monte Bolca and the Duero Basin), later strata would often be limited to a single taxon per formation. Jeremy E. Martin notes that ziphodont crocodilians like planocraniids last appear during the Bartonian, with the same being applicable to the large crocodyloid Asiatosuchus. Linking the change in temperature with the disappearance of ectothermic animals like crocodilians, Jeremy E. Martin notes a certain trend in Diplocynodon distribution towards the Oligocene. Martin notes that the early Priabonian D. hantoniensis occured in the southern UK, while the slightly younger D. elavericus occured further south in France.

During the early Rupelian Diplocynodon gervaisi is found in the then subtropical to tropical region of Ronzon and Diplocynodon ratelii at Céreste. Early Oligocene occurences of unnamed or indetermined species are known from Razac-d’Eymet in France as well as the Melanian clay of Germany. Diplocynodon ratelii has been described from the Rupelian of France. Around the same time Diplocynodon muelleri was found in Spain and at the locality of Monteviale in northern Italy.

Martin hypothesizes that the seeming southwards trend during the later part of the Eocene might represent Diplocynodon surviving the Grand Coupure by inhabiting low-latitude climate refugia in which temperatures never dropped beyond what was survivable for crocodilians. Once temperatures warmed again, the genus might have been capable of recolonizing Europe, accounting for animals like Diplocynodon gervaisi. This hypothesis is however far from uncontested, as Martin acknowledges that the spotty fossil record of the late Eocene does prove to be a hinderance. The absence of crocodilians from more northern localities during the latest Eocene is not entirely certain and a mandible from the Melanienton formation at Borken could indicate the presence of Diplocynodon across the Eocene-Oligocene boundry. However, the specific stratigraphy of the material is uncertain, rendering it ambiguous whether the remains belong to the Late Eocene or Early Oligocene strata.

Another point against the southern climate refugia hypothesis was raised by Venczel and Codrea with the description of Diplocynodon kochi from Romania. Remains assigned to this species are known from the Rupelian localities of Suceag and Cetățuia Hill, showing that the genus survived in its Romanian range despite deteriorating environmental conditions. Venczel and Codrea argue that this was possible due to the fact that unlike what is assumed by Martin, the Eocene-Oligocene boundry did not see major climate pressures that affected Diplocynodon.

Alternatively, it could be hypothesized that Diplocynodon could have simply been more cold-tolerant than other crocodilians of its time. Martin speculates that if not surviving in southern refugia, Diplocynodon could have been capable of surviving several months in conditions below 0°C like today's Chinese alligator. This adaptation may have given it an edge over the other crocodilians of its time, explaining how it survived into the Oligocene when others like Asiatosuchus did not. This might further prove to be something that would later be optimized in other alligatoroids, leading up to the more cold-tolerant modern alligators, however, this hypothesis has yet to be tested using isotopic analysis.

Late Oligocene finds are rare however, with most consisting of non-diagnostic bone fragments collected in western Europe. Though referrable to Diplocynodon, they give no precise information about the species identity. An exception to this is presented through Diplocynodon levantinicum, which lived approximately 26 million years ago in the Upper Thracian Basin of what is now Bulgaria. It was initially believed that this species lived much later, but this notion was eventually debunked through a better understanding of the stratigraphy of the Thracian Basin.

Miocene and extinction

Remains of Diplocynodon ratelii have also been recovered from the Early Miocene site of Els Casots in the Vallès-Penedès Basin of Iberia, which dates to MN Zone 4 (specifically 16.5-16.3 Ma). This makes it the third species of Diplocynodon from Spain in addition to D. tormis and D. muelleri. The Els Casots material has been noted to be smaller than that from France, though this might be as simple as stemming from younger animals.

Diplocynodon may have undergone a southward shift similar to the one that may have occured in the Eocene-Oligocene.

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, located in the Styrian basin (a subbasin of the Pannonian basin, itself a part of the Central Paratethys) while the French remains were discovered in the Paris basin. This time period has been noted for being particularily warm, though it was followed by drastic cooling.

Perhaps even younger remains have been reported from Spain and possibly Greece.

It is generally believed that Diplocynodon disappeared towards sometime before the genus Crocodylus dispersed across Europe, though some evidence may suggest that the two genera could have met for a brief period of time. Böhme and Ilg take note of several Late Miocene Diplocynodon remains (not identified on a species level), while remains tentatively referred to Crocodylus appear in the Mediterranean as early as 9 million years ago.

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. Venczel argues that this enabled D. kochi to capture fast moving prey.

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

Paleoenvironment

Mont de Berru, where the oldest remains of Diplocynodon are found, is located in the Paris Basin of France. Aside form Diplocynodon, the locality preserves the fossils of a crocodyloid similar to Asiatosuchus.

At Domérat Diplocynodon likewise inhabited freshwater biomes. Initially, it was believed that the deposits preserve a mass death assemblage by flash flood and subsequent transportation based on the incomplete nature of the bones and the pebbles recovered from the locality. The same does not appear to have been the case for the remains of Diplocynodon elavericus, which are much more complete and thus unlikely to have been transported. It is therefore possible that Domérat represents a river channel and that the mammal bones were transported from a more distant point of origin into the same area where the Diplocynodon remains were burried. Regardless of their exact point of origin, the mammal bones of Domérat belong to animals like Palaeotherium, Plagiolophus, Anoplotherium, Xiphodon and Amphimeryx, all in all animals typical for the middle Priabonian.

Messel

Another major lake that has yielded remains of Diplocynodon was likely present at the locality of Els Casots in Catalonia, which is described as lacustrine-palustrine, tho much of the surrounding basin was fluviate in nature. This presence of a permanent body of water is well established thanks to the discovery of a plethora of fish, amphibians and of course crocodilians at the site. The lake is thought to have been surrounded by dense, humid forests, which may have been partially flooded. These woods were inhabited by a variety of animals including the small tragulid Dorcatherium, which is often found in association with water, the suid Eurolistriodon, the bovid Eotragus and even large proboscideans in the form of deinotheres.

Macroflora indicates that during the Oligocene, the Ebro basin was covered by tropical-deciduous forest, with D. muelleri inhabiting freshwater and low salinity ponds and lakes. The climate was subtropical to warm with mean temperatures between 22° C and 26° C.

Diplocynodon levantinicum inhabited the freshwater biomes of Bulgaria after the reteat of the Paratethys from the region it inhabited. The Maritsa Formation preserves swamps and freshwater lakes and swamps. It coexisted with the stem-tragulid “Dorcatherium” bulgaricum and a medium-sized anthracothere. The formation has yielded abundant fish remains and the turtle Promalacoclemmys cf. laharpi.

The Austrian species likewise inhabited freshwater biomes, with Wies and Eibiswald both being described as lacustrine in nature.

Coexistence with other crocodilians

Especially during the Paleogene, Diplocynodon was just one of many crocodilians to inhabit Europe. Oftentimes up to four different crocodilians inhabited the same localities, which depending on the locality could range from four to up to seven different taxa. A key example is the Messel Pit in Germany, which was not only home to two different species of Diplocynodon, D. darwini and D. deponiae, but also the terrestrial Boverisuchus, the notosuchian Bergisuchus, the large generalist Asiatosuchus germanicus, the small Hassiacosuchus and an indetermined tomistomine gavialoid.

Today sympatry between multiple crocodilians tends to be more limited compared to what appears to be the case at Messel. The co-occurance of two species of Diplocynodon at this fossil site could however be explained by several different lines of thinking. For one, it is unclear if the animals truly inhabited the same locality at the same time or whether their bones were simply preserved together due to taphonomy. As an example, Delfino and Smith note that in the modern day Amazon multiple crocodilians inhabit the same wider geographic area, but are in actuality separated by their individual habitat preferences. Another factor raised by the team concerns size-based differentiation. Similar to what occurs between saltwater and freshwater crocodiles in modern Australia, the two species may have been able to coexist due to their different sizes causing them to fill different ecological niches. The anatomy of the posterior maxillary and dentary teeth would further support niche partitioning, with the two species likely differing in their prey items and how they dispatch of them.

Things began to change with the collapse of the Eocene greenhouse biomes and the transition to the much cooler Oligocene, when Europe experienced a noticable drop in crocodilian diversity. This is especially notable at the locality of Monteviale, with its close proximity to the Eocene deposits of Monte Bolca. While Monte Bolca in many regards represents a typical Eocene crocodilian fauna, featuring Asiatosuchus, Allognathosuchus and a possible example of Boverisuchus, the Oligocene deposits of Monteviole show a drastic decline in diversity, with Diplocynodon cf. muelleri being the only crocodilian present.

Other types of crocodilians did continue to exist in Europe elsewhere, tho they tended to inhabit different biomes from Diplocynodon. Gavialoids for example are still found in central Europe around the same time as Diplocynodon, but were typically more coastal animals.

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 features 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.