Old African fossils provide new evidence for the origin of the American crocodiles

Abstract

Molecular and morphological phylogenies concur in indicating that the African lineages formerly referred to Crocodylus niloticus are the sister taxon the four Neotropical crocodiles (Crocodylus intermedius, C. moreleti, C. acutus and C. rhombifer), implying a transoceanic dispersal from Africa to America. So far the fossil record did not contribute to identify a possible African forerunner of the Neotropical species but, curiously, the oldest remains referred to the African C. niloticus are Quaternary in age, whereas the oldest American fossils of Crocodylus are older, being dated to the early Pliocene, suggesting that another species could be involved. We re-described, also thanks to CT imaging, the only well-preserved topotipic skull of Crocodylus checchiai Maccagno, 1947 from the late Miocene (Messinian) African site of As Sahabi in Libya. As previously suggested on the basis of late Miocene material from Tanzania, C. checchiai is a valid, diagnosable species. According to our phylogenetic analyses, C. checchiai is related to the Neotropical taxa and could be even located at the base of their radiation, therefore representing the missing link between the African and the American lineages.

Figure 1

Life appearance of Crocodylus checchiai from As Sahabi (Libya). The reconstruction is based on the information from literature^– updated with the complete cranium sn813/lj. Artwork by D. A. Iurino.

Figure 2

Geographic location of the As Sahabi paleontological locality. Artwork by D. A. Iurino.

Figure 3

Crocodylus checchiai from As Sahabi. Specimen sn813/lj in dorsal (a1,a2), ventral (b1,b2), right lateral (c1,c2) and left lateral (d1,d2) views. Anatomical abbreviations: bo, basioccipital; bs, basisphenoid; ect, ectopterygoid; en, external naris; f, frontal; if, incisive foramen; itf, infratemporal fenestra; j, jugal; l, lacrimal; m, maxilla; n, nasal; o, orbit; pa, parietal; pal, palatine; pf, prefrontal; pm, premaxilla; po, postorbital; pt, pterygoid; q, quadrate; qj, quadratojugal; soc, supraoccipital; sof, suborbital fenestra; sq, squamosal; stf, supratemporal fenestra; t, tooth. Scale bar: 10 cm.

Figure 4

Implied weighting tree of a matrix comprising 53 taxa and 189 characters based on Brochu and Storrs. Crocodylus checchiai sn813/lj is retrieved in a polytomy along with all the four extant American Crocodylus.

Figure 5

The addition of the coding of Crocodylus checchiai based on the late Miocene Libyan specimen sn813/lj to an updated version of the matrix by Scheyer et al. provides (“implicit enumeration search” with implied weighting) a topology with C. checchiai in intermediate position between the African Crocodylus niloticus and American crocodiles, with the early Pliocene Crocodylus falconensis nested within the American clade and not at its base as in Scheyer et al..

Figure 6

CT investigation of the relationship between the nasals and the premaxillae in sn813/lj. Top left: anterior tip of the snout in dorsal view showing the planes of the sections in (A), (B) and (C). (A) anterior transversal section do not involve the nasals; (B): posterior transversal section cutting the nasals and showing that the premaxillae are entirely separated by the nasals; (C): sagittal section showing that the nasals are slightly lowered in the area of the posterior process of the premaxillae. Note in (A) and (B) how the nasals are raised to form the medial boss of the snout. Scale bar: 3 cm.