New Champsosaurus paper perpetuates old myths

Whenever taxon exclusion mistakes are made and reviewed here,
I try to write to the lead author of the paper. Below is a recent email directed to Professor Dudgeon et al. 2020 on their recent review of the well-preserved skull of Champsosaurus (Figs. 1, 3), which they re-examined using computed tomography analysis.

Figure 1. Champsosaurus from Dugeon et al. Here the nasal is the ascending process of the premaxilla. The prefrontal is the nasal fused to the prefrontal. The postorbital (pro) is the postfrontal (pof) and vice versa.

Dear Dr. Dudgeon:

It’s always good to see new studies on old skulls.

Based on phylogenetic bracketing the bone traditionally identified as the ‘nasal’ is the ascending process of the premaxilla. That makes the purported ‘prefrontal’ a fused nasal + prefrontal. The postorbital and postfrontal are mislabeled with the other bone identity based on Tchoria (Fig. 2), a taxon not mentioned in your text. See attached.

Choristoderes are not ‘neodiapsid reptiles.’ Phylogenetically they are archosauriformes arising from Proterosuchus, Elachistosuchus and Tchoria. Phylogenetic miniaturization in that lineage lost the antorbital fenestra. See links below.

https://pterosaurheresies.wordpress.com/2013/08/13/champsosaurus-and-its-snorkel-nose/
http://reptileevolution.com/reptile-tree.htm
http://reptileevolution.com/champsosaurus.htm
http://reptileevolution.com/youngina-bpi2871.htm
http://reptileevolution.com/hyphalosaurus.htm
http://reptileevolution.com/lazarussuchus.htm

Best regards,

Figure 2. Tchoria and phylogenetic bracketing help identify bones in the skull of Champsosaurus (Fig. 2).

So, the Dudgeon et al. paper
is yet another great example of a situation in which phylogenetic analysis and bracketing (= comparing related taxa) sheds more light on a specimen than high-resolution micro-computed tomography scanning and/or adding characters (= looking more deeply into one taxon to the exclusion of others).

Figure 3. Champsosaurus skull with premaxilla in yellow, nasal + prefrontal in pink. Bone identities determined by phylogenetic bracketing with Tchoria. See figure 2.

The greatest benefit 
available from the large reptile tree (LRT, 1631 taxa) is this sort of phylogenetic bracketing based on the validated nesting of sisters that have never been tested together in prior studies. You can look more deeply into one skull, as Dudgeon et al. did. Or you can examine many skulls, as ReptileEvolution.com and the LRT enable workers to do (Figs. 2, 4). In this case, using computed tomography on one skull did not put an end to traditional myths regarding the identity of bones in Champsosaurus.

Note to readers who like to harp on these issues:
More characters were not needed to resolve these problems. More taxa were needed.

Firsthand access + computed tomography did not help Dudgeon et al. Rather, a century-old drawing (Brown 1905, Fig. 3), access to several sister taxa for comparison (Figs. 2, 4) and Adobe Photoshop were the tools needed to resolve this issue.

It helps to know what you are dealing with.
Only a wide-gamut phylogenetic analysis that minimizes taxon exclusion can tell you where a specimen nests in the cladogram. Too often workers like Dudgeon et al. rely on vague citations, rather than running tests themselves or citing ongoing and self-repairing studies like the LRT. Publishing a mistake is to be avoided no matter how trivial.

Figure 4. Dorsal, lateral and palatal views of Late Triassic BPI 2871 with bones colorized above. Below, reconstructed images of BPI 2871 tracings. It is more complete than illustrated by Gow 1975. Note the tiny remnant of the antorbital fenestra and the long ascending process of the premaxilla.  The squamosal has been broken into several parts. This is a tiny phylogenetically miniaturized sister to the ancestor of Champsosaurus.

Champsosaurus annectens (Cope 1876, Brown 1905) ~1.5 m in length, Late Cretaceous to Eocene. Champsosaurus was derived from a sister to Tchoiria, and was a sister to other choristoderes, such as Cteniogenys and Lazarussuchus. This clade must have originated in the Late Permian or Early Triassic, but fossils are chiefly from late survivors, hence the wide variety in their morphology.

---

References
Brown B 1905. The osteology of Champsosaurus Cope. Memoirs of the AMNH 9 (1):1-26. http://digitallibrary.amnh.org/dspace/handle/2246/63
Cope ED 1876.On some extinct reptiles and Batrachia from the Judith River and Fox Hills beds of Montana: Proceedings of the Academy of Natural Sciences, Philadelphia. 28, p. 340-359.
Dudgeon TW, Maddin HC, Evans DC & Mallon JC 2020. Computed tomography analysis of the cranium of Champsosaurus lindoei and implications for choristoderan neomorphic ossification. Journal of Anatomy (advance online publication)
doi: https://doi.org/10.1111/joa.13134
https://onlinelibrary.wiley.com/doi/10.1111/joa.13134

http://reptileevolution.com/champsosaurus.htm

Champsosaurus and its snorkel nose

Choristoderes are a varied clade of mostly aquatic, often croc-like reptiles descending from certain long-snouted younginids, like the BPI-2871 specimen (Fig. 2). Doswellia (Fig. 2) is closely related to choristoderes, splitting off at the base to form its own clade. Diandongosuchus likewise split off early, giving rise to later parasuchids (phytosaurs), proterochampsids and chanaresuchids.

Many from this varied clade of pararchosauriformes had nostrils shifted back from the snout tip, reaching an acme with parasuchians. Champsosaurus (Figs. 1, 2) was different. The naris was located at the very tip, probably to act as a snorkel, in order to breathe without surfacing. Given the unusual morphology of the snout tip, it’s very possible that the reversion to the tip was a secondary adaptation.

Figure 1. Neochoristoderes including Champsosaurus, Simoedosaurus and Ikechosaurus. Premaxilla in yellow. Nasals in pink. Lacrimals in orange. The prefrontals were fused to the nasals. Note: the largest genus here is the most primitive with lateral temporal fenestra oriented laterally and the nares still dorsal on the rostrum.

The identification of the rostral bones in Champsosaurus is controversial. Here we’ll look at some heretical labels for traditional paradigms.

Figure 2. Various choristoderes and their kin with a focus on the bones surrounding the naris and comprising the snout. The nostrils migrate posteriorly by convergence in Lazarussuchus and Diandongosuchus.

Traditionally
In Champsosaurus (Fig. 1) the dorsal medial bone is traditionally considered the nasal and the paired bones following it are considered the prefrontals. However if you look at all the closest kin to Champsosaurus it becomes clear that the paired bones remain traditional nasals. The prefrontals are simply missing, likely due to fusion with the nasals. That means the tooth-bearing portions of the premaxilla wrapped completely around the rostrum and nares until they came into contact with the ascending process of the premaxilla, which extends beyond the naris in many related taxa.

Did Champsosaurus once have an antorbital fenestra?
Related taxa, including Diandogosuchus and Doswellia had an antorbital fenestra and there are signs of a nascent or vestigial antorbital fenestra in certain Champsosaurus (Fig. 3). If it’s there, it’s tiny, but worth searching for.

Figure 3. The rostrum of a champsosaur color coded to identify the premaxilla (yellow), nasals (pink), prefrontals (purple) and lacrimals (orange). The vestigial or nascent antorbital fenestrae are in black, along with the snout-tipped nares (at far right). If anyone has better data, please send it along.

Choristoderes are underrepresented in the fossil record.
So are doswellids and basal parasuchians. What this means, with present data, is basal taxa appear to be large forms, which goes against the grain of evolutionary patterns. These named taxa are all quite derived at their first appearance. I’m guessing we’re likely to find smaller basal and transitional forms, more like the BPI-2781 specimen (Fig. 2) as they become known.

It’s also interesting that the largest choristodere, Simoedosaurus (Fig. 1) has the more primitive skull, with a dorsal set of nares and more laterally-oriented lateral temporal fenestra. The smaller Champsosaurus has the more derived snout tip and more dorsally open lateral temporal fenestrae.

Popping paradigms is what we do here.
If you have data that supports other positions, please send them forward.

Updated were made to today to the post on Varanosaurus and archosauromorph diapsid origins.

New material of the choristodere Lazarussuchus – Matsumoto et al. 2013

What a wonderful new fossil!
A complete and articulated Lazarussuchus (Hecht 1992) has just been described by Matsumoto, et al. (2013). The new specimen (Fig. 1, below) comes from the Paleocene of France.

Figure 1. Click to enlarge. Lazarussuchus, the older Hecht 1992 specimen (above) and the newer specimen (below) from Matsumoto et al. 2013, to the same scale. The differences are subtle, including a longer neck and more robust limbs. These two were definitely congeneric but not conspecific.

Distinct from an earlier specimen
the new specimen has a taller skull, a distinct pelvis and more robust limbs. At the same scale the skulls were nearly identical in length, but in the new specimen the neck was more robust and nearly twice as long.

Unfortunately, by way of taxon exclusion,
the writers (Matsumoto, et al. 2013) got their phylogeny backwards. They reported, “Despite its age, most phylogenetic analyses place Lazarussuchus at or close to the base of the choristoderan tree, implying a very long unrecorded history.”  In contrast, the large reptile tree (also see fig. 2) nested Lazarussuchus as a derived choristodere. After all it had lost its lateral temporal fenestra, a primitive trait present in other choristoderes. Moreover, the Matsumoto (2013) tree nested choristoderes with sauropterygians and their tree improperly rooted with  Youngina.

One problem:
Matsumoto et al. included two basal sauropterygians and two lepidosaurs in their tree. These are not closely related to choristoderes in the large reptile tree. So, sadly, this group of scientists had no scientific basis or guidance for choosing their inclusion set as a subset from a much larger tested set. Instead they chose what they felt like choosing.

Another problem:
Matsumoto et al. did not include the real closest known taxa to choristoderes.

Figure 2. Matsumoto et al. (2013, left) nested sauropterygians with choristoderes. Here with the addition of Ichthyostega and Cephalerpeton for rooting, much the same tree topology is present. Same tree with added taxa from the large reptile tree properly separates the sauropterygians from the choristoderes.

Testing Matsumoto (2013) with the Large Reptile Tree
Starting with the 235+ taxa in the large reptile tree and deleting all but those taxa used by Matusmoto et al. (2013) and, like them, setting Youngina as the outgroup, I recovered much the same tree, except Gephyrosaurus nested at the base of Petrolacosaurus + Araeoscelis and Prolacerta nested closer to the Choristodera. Within the Choristodera Champsosaurus nested as a basal taxon.

Non-controversial outgroup taxa
Instead of guessing which taxon should be the outgroup, why not go all out?  Ichthyostega, a basal tetrapod, and Cephalerpeton, a basal reptile serve as non-controversial outgroups for all reptiles. Adding them to the matrix (Fig. 2, left) moves Youngina closer to Prolacerta and little else.

Adding several more pertinent taxa
Let’s add more taxa from and based on the large reptile tree (Fig. 2, right) as another test. That recovers a tree topology much more like the large reptile tree in which a longer list of various younginoids, Doswellia and Diandongosuchus nest close to the Choristodera. The sauropterygians now correctly nest further away with other Enaliosauria. The new Lepidosauromorpha, including Mesosuchus, nest together (in blue) and so do the new Archosauromorpha (in yellow), but they did not split from each other following Cephalerpeton, as in the large reptile tree. The reason: Too few pertinent taxa at the critical point were missing to make that division appear where it should.

The Nasal Question
Conventional thinking and Matsumoto et al. (2013) identify the two medial bones anterior to the frontals as prefrontals (Fig. 3, lavender). This is a common and traditional mistake when dealing with choristoderes. Following this pattern Matsumoto et al. identify the nasals (pink) as those tiny supranarial bones. This is false paradigm that probably goes back to Champsosaurus, in which the premaxilla is literally split into two separate bones, the ascending process and the tooth-bearing portion, due to the migration of the once dorsal nares back to the tip of the snout from its plesiomorphic position on top of the snout. All choristodere workers have since assumed the ascending process was the nasal since the real nasal and prefrontals are fused in Champsosaurus. So, given the examples of the closest known taxa (Figs. 3,4), in Matsumoto et al. (2013) the ascending process of the premaxilla was falsely labeled a nasal and the nasals were falsely labeled prefrontals.

 

Figure 3. The misidentification of the rostral bones in Lazarussuchus according to Matsumoto (2013, above) and relabeled below. Sister taxa demonstrate the continuing presence of large nasals that meet medially and the extent of the premaxilla ascending process. This false paradigm has to stop. Also see figure 4 for Youngina BPI 2871, which has a longer snout.

Saved by the sister taxa
Related taxa (Figs. 3, 4), like Younginia, Younoides, Cteniogenys and Diandongosuchus demonstrate the conservative arrangement of the rostral bones. The prefrontals of Lazarussuchus did not meet medially, but were located at their conventional places, separated by large nasals. Not sure why (other than the Champsosaurus issue mentioned above) experts are missing this. Lack of more parsimonious related taxa indicated Matsumoto et al. (2013) have no idea where choristoderes nest in the reptile family tree. Here, when given 235+ other opportunities, choristoderes nest with younginiforms and within the pararchosauriforms: Doswellia, parasuchia and proterochampsia in the large reptile tree.

Fenestrae Coming and Going
Lazarussuchus is a diapsid lacking the lateral temporal fenestra present in sister taxa like Diandongosuchus and Younginoides (Fig. 3) and another choristodere, Cteniogenys (Fig. 4). Diandongosuchus is among the first taxa in its lineage to develop an antorbital fenestra, convergent with several other taxa with antorbital fenestra.

Figure 4. Youngina BPI 2871 and its descendants, according to the large reptile tree, the choristodere Cteniogenys and the chanaresuchid, Gualosuchus.

It’s discouraging when paleontologists do no apply due diligence to their work and leave it to  amateurs to correct their mistakes. Such mistakes can be eliminated and tested with widespread use of the large reptile tree, which has been available world wide for nearly two years now.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Reference
Evans SE and Hecht MK 1993. A history of an extinct reptilian clade, the Choristodera: longevity, Lazarus-Taxa, and the fossil record. Evolutionary Biology 27:323–338.
Hecht MK 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 and Evans S 2013. New material of the choristodere Lazarussuchus (Diapsida, Choristodera) from the Paleocene of France.

wiki/Lazarussuchus

A new Doswellia snout!~!

Traditional paleontologists are still a little off about Doswellia (Fig. 1). It is a strange one with transverse and square ribs, a horizontal ilium, and a low wide skull that fills in a former lateral temporal fenestra. Unfortunately the rostrum has not been known for the last 30 years. Neither have the feet.

Even so, the large reptile tree firmly nested Doswellia at the base of the Choristodera, derived from Youngoides (RC91) and more distantly related to taxa at the base of the Parasuchia and Proterochampsia, all members of the Pararchosauriformes.

Figure 1. Doswellia in several views from Weems (1980). Missing pieces from 1980 are in black.

News about the rostrum!
Thankfully Heckert et al. (2012) discovered some of the last missing pieces, the premaxilla and maxilla of Doswellia (Fig. 2). Unfortunately they could not bring more focus to relationships, but repeated Dilkes and Sues (2009) assessment that Doswellia was close to proterochampsids, again ignoring the Choristodera and younginoids.

Figure 2. The newfound elements of Doswellia found by Heckert et al. (2012). The naris is dorsal. A tiny antorbital fenestra is present. The ventral maxilla is wavy. The premaxilla is deeper anteriorly and tips downward.

So what’s new?
The maxilla has teeth of several sizes and the ventral margin is wavy, not straight as in sister taxa.

There is an antorbital fenestra, small, and without much of a fossa. This follows the pattern seen in some (but certainly not all) Youngina and proterochampsids, and not seen  in the Choristodera.

The naris is dorsal in position, but still at the jaw tips. This is totally in line with the entire clade, which, other than Champsosaurus, all have dorsal nares. The premaxilla is also deeper anteriorly, downturned at the tip, as in several sisters.

The teeth are stout cones ideal for capturing prey.

This is a welcome discovery by Heckert et al. (2012) and fills a minor gap with real data. Glad to see it. Thanks to Dr. Heckert for sending the pdf.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Dilkes D and Sues H-D 2009. Redescription and phylogenetic relationships of Doswellia kaltenbachi (Diapsida: Archosauriformes) from the Upper Triassic of Virginia. Journal of Vertebrate Paleontology 29(1):58-79.
Heckert AB, Lucas SG and Spielmann JA 2012. A new species of the enigmatic archosauromorph Doswellia from the Upper Triassic Bluewater Creek Formation, New Mexico, USA”. Palaeontology (Blackwell Publishing Ltd) 55 (6): 1333––1348. 
Weems RE 1980. An unusual newly discovered archosaur from the Upper Triassic of Virginia, U.S.A. Transactions of the American Philosophical Society, New Series 70(7):1-53

wiki/Doswellia

More About Doswellia and Archeopelta

The phylogenetic position of Doswellia (Weems 1980) has been troublesome because it is so different from other Triassic reptiles. Recent cladistic analyses (Desojo, Ezcurra  and Schultz 2011) have shed light on the nesting of Doswellia and the large study has nailed it down.

Figure 1. Doswellia in several views from Weems (1980).

Doswellia Basics
As a diapsid, Doswellia lost its lateral temporal fenestrae. The orbits were on top of the flattened skull. The missing rostrum was narrow and elongated if it fit the narrow and elongated mandible. The posterior mandible was deeper than the skull. Different than its sisters, the Doswellia ilia have rotated laterally such that the lateral surface now faces ventrally. The posterior dorsal ribs extended only laterally matching the elongated transverse processes of the anterior caudals. The anterior dorsal ribs extended laterally then abruptly turned ventrally at mid-length. The femur was relatively short, indicating a low-slung configuration. The seven cervicals were elongated such that the elongated, but relatively small skull was shorter than the cervicals.

That Very Strange Ilium Compared
(Desojo, Ezcurra  and Schultz 2011) compared the ilium of Doswellia (updated from Weems 1980) to several other reptiles. Unfortunately they included several unrelated taxa (Mesosuchus, Vancleavea and Erythrosuchus) and excluded Champsosaurus and Youngoides RC91, two Doswellia sisters in the large study. Fortunately Proterochampsa was tested against Doswellia and it nests as a close sister, but unfortunately no ilium has been published for this taxon. In this case, there were no closely related taxa with a similar ilium. Thus Doswellia is alone with regards to its ilium.

Fortunately the large study relies on a large suite of characters. Until a closer sister taxon comes along, Doswellia will continue to nest between Youngoides and Champsosaurus, with Archeopelta closer (but not yet included in the large study).

Figure 2. Comparing various ilia to that of Doswellia (in pink). Desojo (2011) included an unrelated lepidosaur, Mesosuchus, an unrelated thalattosaur, Vancleavea, and an unrelated euarchosauriform, Erythrosuchus due to poor prior poorly assembled inclusion sets. Chanaresuchus is the closest sister in the top row and there was no resemblance. On the bottom row are Youngina and Champsosaurus, two closer sisters to Doswellia and even here there was no distinct synapomorphy. The ilium of Doswellia was oriented laterally, not vertically, like the others and that difference sets Doswellia apart from all known sister taxa. The ventral pelvis was medially oriented in Champsosaurus and Doswellia and the acetebulum was partly open ventrally.

Archeopelta
Archeopelta (Desojo, Ezcurra  and Schultz 2011) is known from less complete material (dorsals and a few other nearby parts), but appears to be the closest known sister with regard to a suite of characters not listed in the large study.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Desojo JB, Ezcurra MD and Schultz CL 2011. An unusual new archosauriform from the Middle–Late Triassic of southern Brazil and the monophyly of Doswelliidae. Zoological Journal of the Linnean Society, 2011, 161, 839–871. DOI: 10.1111/j.1096-3642.2010.00655.x
Dilkes D and Sues H-D 2009. Redescription and phylogenetic relationships of Doswellia kaltenbachi (Diapsida: Archosauriformes) from the Upper Triassic of Virginia. Journal of Vertebrate Paleontology 29(1):58-79.
Weems RE 1980. An unusual newly discovered archosaur from the Upper Triassic of Virginia, U.S.A. Transactions of the American Philosophical Society, New Series 70(7):1-53

wiki/Doswellia

What are Choristoderes? (you know…Champsosaurus, Cteniogenys, Doswellia, etc.)

The Choristordera constitute a clade of elongated aquatic to semi-aquatic, lizard-like to croc-like diapsid reptiles. Traditional taxa include: Champsosaurus, Cteniogenys, Lazarrusuchus and Hyphalosaurus. The first two-headed fossil reptile came from this clade.

What Wiki Sez:
“Cladists have placed [choristoderes] between basal diapsids and basal  archosauromorphs but the phylogenetic position of Choristodera is still uncertain. It has also been proposed that they represent basal lepidosauromorphs.”

So we have an enigma taxa, an ideal opportunity to use the large study to narrow down choristodere outgroup relations.

Figure 1. Several choristoderes (in white), their predecessor and sisters (in yellow).

Choristoderes are Pararchosauriformes
The large study nested choristoderes within the Archosauriformes and within the Pararchosauriform branch between Youngoides (the RC91 specimen) and Proterochampsa.

Figure 2. A section of the large study focusing on choristodere relations.

Doswellia was also a Choristodere
Doswellia (Weems 1980) has been considered an enigma taxon, different enough from all other known taxa to create more questions than answers. Dilkes and Sues (2009) proposed a nesting with Proterochampsa, which is confirmed here.

Parsimonly Rules
Side by side, the resemblance of several choristoderes to Youngina, Doswellia and parasuchians is clear and reasonable. In the present taxon list, there is no more parsimonious nesting to be found. Think of choristoderes as successors to Youngoides (RC91 specimen), a taxon that has never been tested with choristoderes before.

The Dorsal Naris
Most choristoderes have a dorsal naris, similar to Cerritosaurus, parasuchians and Proterochampsa. Champsosaurus has a naris at the tip of it snorkel like snout. This appears to be a reversal because the premaxilla has no ascending process.

Another Appearance of the Antorbital Fenestra
This nesting highlights an important taxonomic fact: the antorbital fenestra appeared in reptiles at least four times. Parasuchians and Cerritosaurus had an antorbital fenestra. Precursors, including choristoderes, did not. This means the antorbital fenestra in parasuchians and their kin developed independently of the antorbital fenestra in Euarchosauriformes, such as Proterosuchus and its successors.

The Longevity and Variety Within the Choristodera
Choristoderes appeared in the Late Triassic, but probably originated in the Late Permian, along with their sister taxa. Some survived into the Early Miocene. Despite the longevity of this clade, relatively few modifications to the basic body plan appeared. Oh, sure, the lateral temporal fenestra disappeared in Doswellia and Lazarussuchus. The rostrum elongated in Champsosaurus. The neck elongated in Hyphalosaurus. The unguals were enlarged in Lazarussuchus, which means it was probably more terrestrial than its aquatic sisters and may have climbed trees. Doswellia was the giant of the clade, reaching 1.6 m in length, or slightly larger than Champsosaurus at 1.5 m. No choristoderes developed an herbivorous diet, a mammal-like dentition, a bipedal stance or wings.

Summary
Traditional enigmas, choristoderes were a monophyletic clade that nested between Youngoides and Parasuchia + Proterochampsa, close to the base of the Archosauriformes. Relatively conservative in morphology, choristoderes were a relatively minor presence throughout the Mesozoic and into the Cenozoic.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Brown B 1905. The osteology of Champsosaurus Cope. Memoirs of the AMNH 9 (1):1-26. http://digitallibrary.amnh.org/dspace/handle/2246/63
Cope ED 1876. On some extinct reptiles and Batrachia from the Judith River and Fox Hills beds of Montana: Proceedings of the Academy of Natural Sciences, Philadelphia. 28, p. 340-359.
Dilkes D and Sues H-D 2009. Redescription and phylogenetic relationships of Doswellia kaltenbachi (Diapsida: Archosauriformes) from the Upper Triassic of Virginia. Journal of Vertebrate Paleontology 29(1):58-79
Evans SE and Hecht MK 1993.A history of an extinct reptilian clade, the Choristodera: longevity, Lazarus-Taxa, and the fossil record. Evolutionary Biology 27:323–338.
Foster JR and Trujillo KC 2000. New occurrences of Cteniogenys (Reptilia, Choristodera) in the Late Jurassic of Wyoming and South Dakota. Brigham Young University Geology Studies 45:11-18.
Gao K-Q, Tang Z-L and Wang X-L 1999. A long-necked reptile from the Upper Jurassic/Lower Cretaceous of Liaoning Province, northeastern China. Vertebrata PalAsiatica 37:1–8.
Gilmore CW 1928. Fossil lizards of North America. Memoirs of the National Academy of Sciences 22(3):1-201.
Hecht MK 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 and Evans SE 2010. Choristoderes and the freshwater assemblages of Laurasia. Journal of Iberain Geology 36(2):253-274. online pdf
Weems RE 1980. An unusual newly discovered archosaur from the Upper Triassic of Virginia, U.S.A. Transactions of the American Philosophical Society, New Series 70(7):1-53

wiki/Champsosaurus
wiki/Cteniogenys
wiki/Hyphalosaurus
wiki/Lazarussuchus