Choristoderes are a group of extinct reptiles I’ve talked about before. Now that several years have passed, I’m going to discuss them once more.
What are choristoderes?
Choristoderes are a lineage of extinct, generally fully-aquatic reptiles. Beyond that, however, it’s very hard to tell.
See, it’s been a long standing debate whereas choristoderes are either archosauromorphs (the lineage of reptiles leading to archosaurs, including crocodylomorphs, pterosaurs and dinosaurs), lepidosauromorphs (the lineage leading to squamates and sphenodontians), or neither (more basal than either group). Similar debates have also plagued classical marine reptile groups like plesiosaurs and ichthyosaurs.
Part of the reason why figuring out where choristoderes fit in the phylogenetic tree of reptiles is because they lack definitive features affiliating them to either major reptile group. Their skulls vaguely resemble those of lepidosauromorphs, but lack the same quadrate configuration.
A particularly radical proposition was suggested by Miller 2004, in which choristoderes are linked to classical marine reptiles (ichthyosaurs and sauropterygians) under Euryapsida.
Personally, I lean slightly towards a lepidosaur identity, but further results may prove this hunch wrong.
When did they live?
Hyphalosaurus by Matt Martuniuk.
Pachystropheus and Actiosaurus might represent examples of Rhaetian Triassic choristoderes. There is still some ambiguity; the former has also been regarded as a sauropterygian (Matsumoto 2009) or as a thalattosaur (Renesto 2005), while the latter was initially interpreted as a theropod, then an ichthyosaur before its proposed choristodere identity more recently (Mortimer 2010).
If both or at least one are choristoderes, they’re both fairly specialized marine reptiles and, in Pachystropheus‘ case, fairly derived, implying an even earlier origin. This is consistent with the long temporal gaps in choristodere fossil record history, the Lazarus taxon effect.
Otherwise, choristoderes are first known from the mid/late Jurassic. These Jurassic species are less specialized, being lizard-like freshwater reptiles. This would set a norm for the rest of the group’s existence: highly conservative “living fossils” whose lineages bear a spotty fossil record, attesting to both their resilience as well as avoidance of preservation. These include the complicated “Cteniogenys” assemblage, which first arose in the mid-Jurassic of Europe and North America, including the famous Morrison Formation.
By the Early Cretaceous, choristodere diversity in Asia undergoes an explosive radiation. Besides the more classical, salamander-like monjurosuchids, we also see the arrival of the long-necked hyphalosaurids and the long-jawed neochoristoderes, expanding the ecological niches available to the group.
Its unclear why choristoderes underwent such a sudden diversification. The local extinction of aquatic crocodilians due to colder temperatures might have triggered the evolution of neochoristoderes (Matsumoto 2010), but this doesn’t explain the evolution of hyphalosaurids.
From the mid-Cretaceous onward, there are no Asian choristodere remains, other than the largely understudied Eotomistoma (Carroll 1988). Hyphalosaurids disappear, but North America and Europe see the presence of both salamander-like species (“Cteniogenys”) as well as the diverse neochoristodere Champsosaurus. Both groups co-exist with a variety of aquatic crocodilians, though Champsosaurus apparently prevents long-snouted crocodilians from occurring in freshwater habitats (Matsumoto 2010).
Choristoderes survived the KT event, with Champsosaurus displaying dietary changes (Matsumoto 2015). It undergoes an adaptive radiation, producing several species including C. gigas, the largest known choristodere at around 3.5 meters long. It is joined by another genus, Simoedosaurus, which given its sudden appearance in the fossil record probably evolved from a Cretaceous Asian species.
Cretaceous “Cteniogenys” species are not known to have made it past the KT event. However, the genus Lazarussuchus debuts in the Paleocene of France, a much more basal animal with a ghost lineage quite possibly extending to the Jurassic or Triassic (Hecht 1992), unless they are related to the Cretaceous “Cteniogenys” species (Matsumoto 2013).
Neochoristoderes suddenly disappear in the medial Eocene, for no currently explainable reasons (see below). However, Lazarussuchus would continue to endure until the mid-Miocene, if not possibly until the Pliocene, when Europe grew too cold for these reptiles.
Unlike many clades, in which the most specialized members were the last representatives, choristoderes seem to have ended more or less as they started.
Most choristoderes were fully aquatic animals.
While some basal taxa like Lazarussuchus could be interpreted as amphibious, both monjurosuchids and hyphalosaurids have evidence of vivipary (Wu 2010) while in Champsosaurus only adult females have limbs robust enough to carry them ashore, while males and juveniles could not support their weight on land (Katsura 2007). As such, choristoderes as a whole appear to not have left the water much, if at all.
All known choristoderes possess laterally flatted tails, which were probably their main propulsion mechanism, with neochoristoderes also having paddle-like limbs. At least hyphalosaurids, monjurosuchids and neochoristoderes all have smooth skins with small, non-overlapping scales, though monjurosuchids do bear rows of scutes similar to those of modern alligator-lizards (Gao 2000). In neochoristoderes, the torso is dorsoventrally flattened, the gastralia are large and the ribs are short and massive, further adaptations for diving. There was a source mentioning pachyostic bones, but I can’t seem to find it.
Probably the most remarkable adaptation for life in the water are the nostrils. Aside from basal choristoderes like Lazarussuchus, which simply have receded nostrils, in more derived taxa the nostrils are fused and oriented towards the tip of the snout, allowing the animals to effectively use them as a snorkel, surfacing only the very tip of the snout while the rest of the body remained underwater (Acorn 2007).
Perhaps tellingly, the eyes are forwardly oriented. This is in contrast to amphibious animals like crocodilians and hippos, which have dorsally-oriented nostrils and eyes.
Palaeogeographical distribution of Choristodera in the Jurassic and Cretaceous periods.
Choristoderes as a whole have a pan-Laurasian distribution, ocurring in fossil sites in North America, Europe and Asia. There are two possible exceptions: basal choristoderes from the Jurassic/Early Cretaceous of North Africa (Haddoumi 2014) and possible neochoristodere teeth from Timor (Umbgrove 1949).
Other than these examples, however, nearly all choristodere remains occur in high latitudes. Although some do occur in sites that had a paratropical climate, most occur in temperate regions. Some Champsosaurus fossils even ocur in the high Arctic, in the Cretaceous of Greenland and the Eocene and Cretaceous of Axel Heiberg (Matsumoto 2010).
Thus, it can be seen that choristoderes tolerated colder environments, if not outright preferred them (Matsumoto 2014). Their ability to survive in colder climates in fact probably allowed them to diversify in the absence of crocodilians during the Early Cretaceous, when colder temperatures in Asia caused the local extinction of aquatic crocodilians (Matsumoto 2014). Both groups ultimately co-existed in warmer areas, however.
I previously speculated that neochoristoderes might have been endothermic, which fits well with their extreme speciation to an aquatic lifestyle. However, given that cold-tolerance was also present in more basal taxa, monjurosuchids and hyphalosaurids, I wonder if it endothermy was more widespread across the clade.
Ironically, cooling temperatures in Europe probably lead to the extinction of the group, as Lazarusussuchus probably survived until the glaciations.
Palatine teeth in Simoedosaurus.
Basal choristoderes were probably generalistic feeders, ambushing small fish, crustaceans and other prey. Monjurosuchus has evidence of arthropod cuticle in its intestines, suggesting that it fed on aquatic arthropods (Gao 2000).
Hyphalosaurids were certainly specialized animals, bearing long necks. It’s likely that they were doing whatever the convergently similar plesiosaurs were doing, whatever that was. In particular, they appear to have preferred softer prey, which they probably hunted actively on the deeper areas of their lake environments (Gao 2008).
By contrast, the long-snouted neochoristoderes are much easier to figure out. These predators were doing what crocodilians, whales, gars and countless other large, predatory aquatic vertebrates do best: snatch up larger prey. Long, thinner jaws allowed them to accomplish this task with less drag.
While they are often compared to gharials, neochoristoderes may not be exactly analogous. Unlike other crocodilians, gharials have weak bite forces, rarely exceeding 497 N; they can tackle animals as large as goats, but they prefer small fish. By contrast, Champsosaurus had a bite force of 1194 to 1910 N, more comparable to that of crocodiles (James 2010). This may not have necessarily translated to a preference for larger prey, but it does convey a much larger degree of strength and speed when catching small fish.
Simoedosaurus dakotensis and Champsosaurus gigas, by contrast, have semi-brevirostrine snouts, and might have a diet more similar to that of crocodiles, feeding on not just fish but aquatic tetrapods like turtles and waterfowl and maybe ambushing terrestrial animals near the water. Notably, they co-existed with crocodile-like crocodilians like Borealosuchus (Matsumoto 2013 and 2014).
A neat feature choristoderes have is the retention of palatine teeth. Lost in various tetrapod groups like mammals, crocodylomorphs and dinosaurs, these teeth basically serve the role palatal grooves have in these groups, helping the animal to hold and manipulate food items in the mouth. In basal choristoderes these teeth are fairly generic, while in neochoristoderes they are more specialized, indicating a higher degree of food manipulation in the mouth. Such speciation might have evolved multiple times, as the simoedosaurid Ikechosaurus has palatine teeth more similar to those of non-neochoristodere choristoderes (Matsumoto 2015).
Through subtle differences in palatine teeth, we know that Champsosaurus changed its diet across the KT event, and that Simoedosaurus lindoei prefered softer prey than S. dakotensis (Matsumoto 2015).
Lazarrusuchus, the last choristodere, by Nobu Tamura.
As mentioned previously, the extinction of choristoderes isn’t clear.
Competition with crocodilians is sometimes attributed to the demise of neochoristoderes, but both groups co-existed across the Late Cretaceous, Paleocene and Eocene of Laurasia and possibly Timor, and if anything neochoristoderes seem to have been dominant over long-snouted crocodilians in freshwater habitats (Matsumoto 2010 & 2014).
This also doesn’t explain the demise of hyphalosaurids, or of basal choristoderes in North America, North Africa and Asia.
Ultimately, European choristoderes such as Lazarussuchus appear to have died out due to glaciations in Europe.
There is still much I haven’t covered. Hopefully this will introduce you to these amazing extinct reptiles.
Ryoko Matsumoto; Shigeru Suzuki; Khisigjav Tsogtbaatar; Susan E. Evans (2009). “New material of the enigmatic reptile Khurendukhosaurus (Diapsida: Choristodera) from Mongolia”. Naturwissenschaften. 96 (2): 233–242. doi:10.1007/s00114-008-0469-6. PMID 19034405.
Silvio Renesto (2005). “A possible find of Endennasaurus (Reptilia, Thalattosauria) with a comparison between Endennasaurus and Pachystropheus“. Neues Jahrbuch für Geologie und Paläontologie – Monatshefte. Jg. 2005 (2): 118–128.
R. Matsumoto and S. E. Evans. 2010. Choristoderes and the freshwater assemblages of Laurasia. Journal of Iberian Geology 36(2):253-274
R. L. Carroll. 1988. Vertebrate Paleontology and Evolution. W. H. Freeman and Company, New York 1-698
R. Matsumoto and S. E. Evans. 2015. Morphology and function of the palatal dentition in Choristodera Article in Journal of Anatomy 228(3):n/a-n/a · November 2015 DOI: 10.1111/joa.12414
Hecht, M.K. (1992). “A new choristodere (Reptilia, Diapsida) from the Oligocene of France: an example of the Lazarus effect”. Geobios. 25: 115–131. doi:10.1016/S0016-6995(09)90041-9.
Matsumoto, R.; Buffetaut, E.; Escuillie, F.; Hervet, S.; Evans, S. E. (2013). “New material of the choristodere Lazarussuchus(Diapsida, Choristodera) from the Paleocene of France”. Journal of Vertebrate Paleontology. 33 (2): 319. doi:10.1080/02724634.2012.716274.
Ji Q., Wu, X.-C. and Cheng, Y.-N. (2010). “Cretaceous choristoderan reptiles gave birth to live young.” Naturwissenschaften, 97(4): 423-428. doi:10.1007/s00114-010-0654-2
Yoshihiro Katsura. 2007. Fusion of sacrals and anatomy in Champsosaurus (Diapsida, Choristodera), doi:10.1080/08912960701374659
Gao, K.; Evans, S.; Ji, Q.; Norell, M.; Ji, S. (2000). “Exceptional fossil material of a semi-aquatic reptile from China: the resolution of an enigma”. Journal of Vertebrate Paleontology. 20 (3): 417–421. doi:10.1671/0272-4634(2000)020[0417:efmoas]2.0.co;2.
John Acorn, Deep Alberta: Fossil Facts and Dinosaur Digs, University of Alberta, 07/02/2007
Hamid Haddoumi, Ronan Allain, Said Meslouh, Grégoire Metais, Michel Monbaron, Denise Pons, Jean-Claude Rage, Romain Vullo, Samir Zouhri, Emmanuel Gheerbrant, Guelb el Ahmar (Bathonian, Anoual Syncline, eastern Morocco): First continental flora and fauna including mammals from the Middle Jurassic of Africa, doi:10.1016/j.gr.2014.12.004
J. H. F. Umbgrove, Structural History Of The East Indies
Gao, K.-Q. and Ksepka, D.T. (2008). “Osteology and taxonomic revision of Hyphalosaurus (Diapsida: Choristodera) from the Lower Cretaceous of Liaoning, China.” Journal of Anatomy, 212(6): 747–768. doi:10.1111/j.1469-7580.2008.00907.x
James, Michael, The jaw adductor muscles in Champsosaurus and their implications for feeding mechanics, 2010-08-30T19:01:39Z