Introduction

The crocodylians (order Crocodylia, class Reptilia) are a crown group that comprises the alligators, caimans, crocodiles and gavials (Brochu et al, 2009). It consists of all modern crocodylians and all the extinct relatives that descended from the last common ancestor of all living crocodylian species. Note that the term Crocodylia is restricted to the crown group (Brochu, 2003):
Crocodylia belong to the Archosauria and form a sister group to the Aves (birds). Their total group, known as the Pseudosuchia, contains all crown-group archosaurs that are more closely related to the Crocodylia than to the birds (Benton, 1999).  

The stem group

The phylogeny of the stem-Crocodylia is complex, and several versions have been published. However, most of the trees published in summary form are quite similar to the following tree:​
In order to understand the phylogenetic relationships of these stem fossils, we need to see a more detailed tree that depicts individual stem species. However, such a tree would be too big to show all at once, so we will divide it into the following three parts:
  1. From the Pseudosuchia root to the appearance of the Crocodylomorpha;
  2. From the Crocodylomorpha  root to the appearance of the Crocodyliformes;
  3. From the Crocdyliformes root to the appearance of the Crocodylia. 

The first part of the tree is shown below:
The earliest-known fossil in the crocodylian stem line is Ctenosauriscus koeneni, a member of the Poposauroidea from the Early Triassic (latest Olenekian) Solling Formation at Bremketal, near Göttingen, Lower Saxony, Germany (Butler et al, 2011; Benton et al, 2015). Note that the Poposauridae are by no means the most basal pseudosuchians in the above tree, even though Ctenosauriscus koeneni is the oldest pseudosuchian known. This fossil is illustrated below, together with other members of this basal part of the stem-group (click on image for larger version):
* after name indicates that the image represents a life restoration.
With the exception of Ctenosauriscus koeneni, the above images are numbered in order from most basal to most crownward, but there is no clear trend towards more crocodile-like forms. In fact, only a few (e.g. Riojasuchus, Revueltosaurus, Aetosauroides and Ticinosuchus) bear much resemblance to crocodiles or alligators; the rest look more like lizards or bipedal dinosaurs. One aspect of the evolution of this basal part of the crocodylian stem  group is variation in the nature of the articulation of the vertebrae (Stefanic and Nesbitt, 2018).

The next part of the tree, covering the Crocodylomorpha up to the appearance of the Crocodyliformes, is shown below:
The following images (click on image for larger version) illustrate several of these stem crocodylians :
* after name indicates that the image represents a life restoration.
The above images admittedly represent a small sample, but none of the species show a close resemblance to the modern crown group. For instance, they all have longer and more slender legs relative to their body length, compared to modern crocodiles and alligators.   They all have the same class of jaw structure (Holliday and Nesbitt, 2013).

The third and final part of the stem tree is shown below:
 Most of the stem taxa shown in the above tree are illustrated below (click on image for larger version):
​Names in    red indicate that the fossil is younger  than the oldest known crown-group fossil.
* after name indicates that the image represents a life restoration.
As for the previous image sets, the pictures above are numbered in order from the most basal position in the tree to the position closest to the crown group. In this case there does seem to be a trend towards more crocodile-like forms. However, there is a part of the tree, that side-branch of the Neosuchia that does not lead to the crown group, that contains aquatic forms that resemble whales or dolphins (e.g. Teleidosaurus, Dakosaurus and Metriorhynchus).

Note that some of the stem species (represented by names in red) post-date the appearance of the crown group; these represent branches of the stem line that continued to evolve after the crown-group had appeared.

​​The crocodylian stem group appeared at the beginning of the Triassic and continued to evolve until the crown group appeared in the Late Cretaceous. The following figures show this transition through the three phases considered above (click on each image for a larger version):

The crown group

The crown-Crocodylia appeared when the stem lines of the Gavialoidea and the Brevirostres  separated from one another:
The stem-Gavialoidea and the stem-Brevirostres both appeared almost at the same time, in the Late Cretaceous. The oldest known stem brevirostres belong to the extinct family Borealosuchidae (Jouve et al, 2017). Several species of the genus Borealosuchus have been recognized as stem-Brevirostres (Rio et al, 2019); the oldest known fossils are identified as Borealosuchus sp. in Middle Campanian sediments in South Carolina (Schwimmer et al, 2015). A fossil of a younger member of this genus is shown below:​
The earliest known stem gavialoid is Eothoracosaurus mississippiensis, found in various outcrops of Late Campanian to Early Maastrichtian age in Mississippi, Alabama, Georgia and western Tennessee, USA (Brochu, 2004). No image is available in the public domain.

The relationship between the ages of appearance mentioned above and that of the earliest stem crocodylian is shown in the figure below. Also shown are representative images of species from the respective stem groups.
​
The figure below depicts the total uncertainty in the age of the crocodylian crown node. This uncertainty represents the maximum period of time for the stem-to-crown transition; the time between the origin of the crocodylian stem group and the initiation of the crown group could have been as long as 173 million years:
Image credits - Crocodylians
  • Header (American Alligator, Alligator mississippiensis)    By Tim Vickers [Public domain], from Wikimedia Commons​
  • ​Vertebrae of   Ctenosauriscus koeneni    By Richard J. Butler, Stephen L. Brusatte, Mike Reich, Sterling J. Nesbitt, Rainer R. Schoch, and Jahn J. Hornung [CC BY 2.5  (https://creativecommons.org/licenses/by/2.5) or CC BY 2.5  (https://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons
  • Ctenosauriscus koeneni     By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)], from Wikimedia Commons
  • Ornithosuchus longidens     By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5  (https://creativecommons.org/licenses/by/2.5)], from Wikimedia Commons
  • ​Skull of Riojasuchus tenuisceps     By Maria Belen von Baczko, Julia Brenda Desojo [CC BY 2.5  (https://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons
  • Riojasuchus tenuisceps      By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], from Wikimedia Commons
  • Skull of   Revueltosaurus    sp.     By National Park Service [Public domain], via Wikimedia Commons
  • Revueltosaurus      sp.     By National Park Service/Jeff Martz [Public domain], via Wikimedia Commons
  • Assemblage of Aetosaurus ferratus     By Ghedoghedo [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons
  • Aetosaurus ferratus     By O.C. Marsh. (http://sauroposeidon.net/marsh/) [Public domain], via Wikimedia Commons
  • Gracilisuchus stipanicicorum     By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5  (https://creativecommons.org/licenses/by/2.5)], from Wikimedia Commons
  • Turfanosuchus dabanensis  (6a)   By User:Captmondo (Own work (photo)) [Copyrighted free use], via Wikimedia Commons  
  • Turfanosuchus dabanensis  (6b)     By Nobu Tamura http://paleoexhibit.blogspot.com/ http://spinops.blogspot.com/ [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
  • Nundasuchus songeaensis     By Aquakeeper14 [CC BY-SA 4.0  (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
  • Ticinosuchus ferox  (8a)     By Ghedoghedo [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons
  • Ticinosuchus ferox   (8b)      By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5  (https://creativecommons.org/licenses/by/2.5)], from Wikimedia Commons
  •   Arizonasaurus babbitti     By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], from Wikimedia Commons
  • Effigia okeeffeae     By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5  (https://creativecommons.org/licenses/by/2.5)], from Wikimedia Commons
  • Shuvosaurus inexpectatus     By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], from Wikimedia Commons
  • Sillosuchus longicervix  (12a)     By Kentaro Ohno (Flickr: 地球最古の恐竜展) [CC BY 2.0  (https://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons
  • ​Sillosuchus longicervix   (12b)     By Nobu Tamura (http://spinops.blogspot.ca/) [CC BY-SA 4.0  (https://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons
  • Poposaurus gracilis     By Petrified Forest from Petrified Forest, USA (Poposaurus gracilisUploaded by FunkMonk) [CC BY 2.0  (https://creativecommons.org/licenses/by/2.0) or Public domain], via Wikimedia Commons
  • Prestosuchus chiniquensis   (14a)     By Vince Smith, Attribution 2.0 Generic (CC BY 2.0)
  • Prestosuchus chiniquensis    (14b)     By Creator: Dmitry Bogdanov (dmitrchel@mail.ru) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
  • Batrachotomus kupferzellensis   (15a)     By Ghedoghedo [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons
  • Batrachotomus kupferzellensis   (15b)     Dmitry Bogdanov [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
  • Skull of    Fasolasuchus tenax     By Ghedoghedo [CC BY-SA 4.0  (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
  • Fasolasuchus tenax     By Ornitholestes [Public domain], from Wikimedia Commons
  • Postosuchus kirkpatricki   (17a)     By Richie Diesterheft from Chicago, IL, USA [CC BY 2.0  (https://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons
  • Postosuchus kirkpatricki    (17b)     By Petrified Forest from Petrified Forest, USA (Postosuchus kirkpatrickiUploaded by FunkMonk) [CC BY 2.0  (https://creativecommons.org/licenses/by/2.0) or Public domain], via Wikimedia Commons
  • ​Hesperosuchus agilis    By Dr. Jeff Martz/NPS [Public domain], via Wikimedia Commons 
  • Skull of      Sphenosuchus sp.   By Ghedoghedo [CC BY-SA 4.0  (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
  • ​Vertebrae and limb bones of  Dibothrosuchus elaphros   By Smokeybjb [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons   
  • Dibothrosuchus elaphros      By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], from Wikimedia Commons
  • Terrestrisuchus gracilis  (21a)     By Jaime A. Headden (User:Qilong) [CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
  • Terrestrisuchus gracilis   (21b)    By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5  (https://creativecommons.org/licenses/by/2.5)], from Wikimedia Commons
  •  Litargosuchus leptorhynchus     By Smokeybjb [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons
  • Protosuchus richardsoni     By Colbert, Edwin Harris, 1905-2001Mook, Charles Craig, 1887-1966. [Public domain], via Wikimedia Commons
  • Protosuchus  sp.     By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5  (https://creativecommons.org/licenses/by/2.5)], from Wikimedia Commons
  • Skull of   Gobiosuchus kielanae     By Halszka Osmólska, Stephane Hua, and Eric Buffetaut (http://app.pan.pl/article/item/app42-257.html) [CC BY 2.0  (https://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons
  • Hsisosuchus twojiangensis    By Einar Fredriksen (Kina 2009 1513 Uploaded by FunkMonk) [CC BY-SA 2.0  (https://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons 
  • Anatosuchus minor   (26a)     By Sereno PC, Larsson HCE [CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
  • Anatosuchus minor   (26b)    By Todd Marshall [CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
  •  Several   Araripesuchus wegeneri   By Sereno PC, Larsson HCE [CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons  
  • Araripesuchus wegeneri      By Todd Marshall [CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
  • Uruguaysuchus aznaresi    By Nobu Tamura (http://spinops.blogspot.com)] [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], from Wikimedia Commons 
  • Simosuchus clarki   (29a)    By D. Gordon E. Robertson [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons 
  • Simosuchus clarki   (29b)     By Smokeybjb [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons
  • Notosuchus terrestris    By Nobu Tamura (http://spinops.blogspot.com), head modified by User:FunkMonk [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5   
  • ​Jawbone of   Sphagesaurus huenei    By Diego Pol, Paulo M. Nascimento, Alberto B. Carvalho, Claudio Riccomini, Ricardo A. Pires-Domingues, Hussam Zaher [CC BY 2.5  (https://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons 
  • Sebecus icaeorhinus   (32a)     By Ghedoghedo [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
  • Sebecus icaeorhinus   (32b)    By FunkMonk (Michael B. H.) [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons
  • ​ Uberabasuchus terrificus     By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5  (https://creativecommons.org/licenses/by/2.5)], from Wikimedia Commons
  • Skull of  Pholidosaurus meyeri      By FunkMonk [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons
  • Dyrosaurus sp.     By incidencematrix (DSC_0056) [CC BY 2.0  (https://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons
  • Dyrosaurus   phosphaticus     By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], from Wikimedia Commons
  • Teleosaurus   sp.     By JaviDex [CC BY-SA 4.0  (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons
  • Teleidosaurus sp.     By User:ArthurWeasley [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5  (https://creativecommons.org/licenses/by/2.5)], from Wikimedia Commons
  • Skull of     Dakosaurus maximus     By Ghedoghedo [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons
  •  Dakosaurus maximus      By Dmitry Bogdanov, Mark T. Young1, Stephen L. Brusatte, Marco Brandalise de Andrade4, Julia B. Desojo, Brian L. Beatty6, Lorna Steel, Marta S. Fernández, Manabu Sakamoto, Jose Ignacio Ruiz-Omeñaca10, Rainer R. Schoch. [CC BY 2.5  (https://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons
  • Metriorhynchus superciliosus   (39a)     By FunkMonk [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons
  • Metriorhynchus superciliosus    (39b)     By Creator:Dmitry Bogdanov (dmitrchel@mail.ru) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
  • Atoposaurus oberndorfi     By Ghedoghedo [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
  • Goniopholis simus   (41a)     By Ghedoghedo [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
  • Goniopholis simus    (41b)    By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], from Wikimedia Commons
  • Lower jaw of    Laganosuchus thaumastos  By Carol Abraczinskas [CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons   
  • Laganosuchus thaumastos      By Nobu Tamura (http://spinops.blogspot.com) [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons
  • Bernissartia fagesii   (43a)   By Ghedoghedo [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
  • Bernissartia fagesii    (43b)     By Nobu Tamura aka Arthur Weasly [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
  • Skull of   Shamosuchus djadochtaensis     By Alan H. Turner [CC BY 2.5  (https://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons
  • Isisfordia duncani     By Smokeybjb [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons
  • Skull of   Acynodon sp.     By Ghedoghedo [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
  • Skull of   Iharkutosuchus makadii     By Zoltan Csiki-Sava, Eric Buffetaut, Attila Ősi, Xabier Pereda-Suberbiola, Stephen L. Brusatte (https://zookeys.pensoft.net/articles.php?id=4474) [CC BY 3.0  (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
  • Borealosuchus wilsoni    By   James St. John,   Attribution 2.0 Generic (CC BY 2.0)
  • Gryposuchus croizati    By   Joerim [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)]
  • Brachychampsa montana    By Nobu Tamura email:nobu.tamura@yahoo.com  http://spinops.blogspot.com/ http://paleoexhibit.blogspot.com/ [CC BY-SA 4.0  (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons