The origin of marine crocs re-revised

Revised August 09, 2019
with the addition of the Dyoplax skull (Fig. 7b) recently downloaded from Maisch et al. 2013

Also revised March 31, 2019, 
with a repaired nesting for Fruitachampsa (Figs. 4,5) a sister to Protosuchus in the LRT (subset Fig. 3), not a sister to marine crocs.

Figure 2. Several Jurassic sea crocs, apparently derived from Late Triassic Dyoplax.

Figure 1. Several Jurassic sea crocs, apparently derived from Late Triassic Dyoplax.

Dr. Andrea Cau 2019
recently revised the affinities of the extinct marine crocs (Figs. 1,2). Here (Fig. 3), with more outgroup taxa, the affinities of the outgroups are more refined by adding taxa omitted by Cau. The in-group marine croc clade of Cau 2019 continue as is untested.

Figure 1. Reduced from Cau 2019 showing Fruitachampsa as the proximal outgroup for marine and river crocs.

Figure 2. Reduced from Cau 2019 showing Fruitachampsa as the proximal outgroup for marine and river crocs. The outgroup Postosuchus is not related to Crocodylomorpha in the LRT.

As we learned earlier
choosing outgroup taxa is not as scientific as letting a wide gamut phylogenetic analysis, like the large reptile tree (LRT, 1549 taxa, subset Fig. 3), choose outgroup taxa for you. 

Figure 3. Subset of the LRT with the addition of Lagosuchus next to Saltopus among the basal bipedal Crocodylomorpha. The nesting of skull-only Yonghesuchus near the skull-less taxa provides clues to the morphology of the skulls in the headless taxa.

Figure 3. Subset of the LRT with the addition of Lagosuchus next to Saltopus among the basal bipedal Crocodylomorpha. The nesting of skull-only Yonghesuchus near the skull-less taxa provides clues to the morphology of the skulls in the headless taxa.

Strangely,
the proximal outgroup taxon for marine crocs recovered by Dr. Cau (Fig. 2) was tiny Fruitachampsa (Figs. 3, 4), a small, gracile Late Jurassic biped sprinter that nests with Protosuchus in LRT and Cau’s cladogram. Fruitachampsa would seem to have few traits in common with river and marine crocs. Dyoplax has more.

Figure 1. Fruitachampsa reconstructed. Note the homologies with Scleromochlus.

Figure 4. Fruitachampsa reconstructed. Note the many homologies with Scleromochlus and the few with marine crocs.

Cau did not include Dyoplax
in his cladogram.

Figure 5. Fruitachampsa skull. The vomers are missing. The chonae are conjoined medially, contra Clark 2011.

Figure 5. Fruitachampsa skull. The vomers are missing. The chonae are conjoined medially, contra Clark 2011.

While we’re on the subject of Fruitachampsa,
it had an enormous notch for a mandibular fang, much larger than necessary. The medial choana is similar to sister taxa.

Despite appearances
Dyoplax (Fig. 7) is not considered a crocodylomorph, let alone an outgroup to marine crocs, as we learned earlier here.

Figure 7. Dyoplax arenaceus Fraas 1867 is a mold fossil recently considered to be a sphenosuchian crocodylomorph. Here it nests as a basal metriorhynchid (sea crocodile) in the Late Triassic.

Figure 7. Dyoplax arenaceus Fraas 1867 is a mold fossil recently considered to be a sphenosuchian crocodylomorph. Here it nests as a basal metriorhynchid (sea crocodile) in the Late Triassic.

Figure 7b. Added 08/09/19 from Maisch et al. 2013. DGS sutures do not match sutures found by Maisch et al. Hypothetical missing parts based on phylogenetic bracketing ghosted on.

Figure 7b. Added 08/09/19 from Maisch et al. 2013. DGS sutures do not match sutures found by Maisch et al. Hypothetical missing parts based on phylogenetic bracketing ghosted on.

In the LRT
(subset Fig. 3) Dyopolax (Figs. 7, 7b) is the outgroup taxon to marine crocs while Dibrothosuchus (Fig. 8) is basal to this clade + river crocs.  

Figure 8. Dibothrosuchus nests basal to all later quadrupedal crocs, including marine crocs, in the LRT.

Figure 8. Dibothrosuchus nests basal to all later quadrupedal crocs, including marine crocs, in the LRT. The hind limbs are not known. Phylogenetic bracketing suggests shorter legs are more likely.

Once again,
a wide gamut phylogenetic analysis is key to recovering interrelationships.


References
Cau A 2019. A revision of the diagnosis and affinities of the metriorhynchoids (Crocodylomorpha, Thalattosuchia) from the Rosso Ammonitico Veronese Formation (Jurassic of Italy) using specimen-level analyses. PeerJ, DOI 10.7717/peerj.7364
Clark JM 2011. A new shartegosuchid crocodyliform from the Upper Jurassic Morrison Formation of western Colorado. Zoological Journal of the Linnean Society, 2011, 163, S152–S172. doi: 10.1111/j.1096-3642.2011.00719.x

 

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