Over the last several weeks
the large reptile tree (LRT, 1660+ taxa, subset Fig. 1) was updated once again with a focus on the Crocodylomorpha. Two congeneric taxa known from a few scraps were eliminated. More insightful identification of skull bones (Figs. 1, 5) settled old issues. Over the next several posts some of the newly recovered hypothetical interrelationships will be presented for review.
We’ll start here
with a new nesting in the LRT (subset Fig. 1) for the small specimens (MCZ4116 and MCZ4118, Fig. 2) formerly assigned to Gracilisuchus (Figs. 4, 5). Now they nest either as hatchling Trialestes (Fig. 3), or, just as likely, as phylogenetically miniaturized Middle Triassic predecessors to the much larger and highly derived Late Triassic basal crocodylomorph, Trialestes. In either case, now Trialestes and its tiny doppelgänger nest together in the LRT, closer to each other than either is to any other taxon, despite a magnitude or two difference in size (Fig. 3). Gracilisuchus nests several nodes away in the next clade (Fig. 1).
Hatchling? Trialestes? (MCZ 4116, MCZ 4118, originally Gracilisuchus, Brinkman 1981; Middle Triassic; Fig. 2). These two specimens have a taller, narrow skull than Gracilisuchus (Figs. 4, 5) and a long list of other distinct traits and proportions that nest them with the very much larger Trialestes (Fig. 3) in the LRT (Fig. 1).
Trialestes romeri (Bonaparte 1982) = Triassolestes (Reig, 1963/Tillyard 1918) Carnian, Late Triassic ~235 mya) is known from scattered parts here reconstructed and restored (Fig. 3). Clark, Sues and Berman (2000) redescribed the known parts and admitted the possibility that this taxon combined dinosaurian and crocodylomorph characters.
is indeed different than most basal bipedal crocodylomorphs (see Pseudhesperosuchus), but it has elongate proximal carpals (Fig. 3) and a long list of other croc clade traits. The elongate ilium is typical of bipedal taxa indicating a bipedal ancestry. Additional sacrals that would have filled out the sacral set between the ilia (Fig. 3) are not known, but likely were present.
the vertebral centra had excavated lateral surfaces, for bird-like air sacs. The radius was longer than the humerus, a character otherwise known only in dinosaurs. The long radiale was slightly shorter than the ulnare. The fingers were tiny, another indicator of a bipedal ancestry. The pelvis was semi-perforated with a well-developed supraacetabular crest, as in basal dinosaurs. The femoral head was inturned, indicating an erect posture. The ankle joint had a crocodile normal configuration and a functionally pentadactyl pes.
The MCZ 4116 and MCZ 4118 specimens
are coeval with Gracilisuchus in the Middle Triassic and similar in size, but share more traits in the LRT with highly derived Late Triassic Trialestes. As we’ve seen before, new morphologies often express their genesis in phylogenetically miniaturized taxa. That may be the case with the MCZ specimens, appearing millions of years before the much larger Trialestes. More discoveries, like an adult Trialestes in the Middle Triassic, will someday settle this ontogenetic and phylogenetic issue. This blogpost is where this issue starts. If this is not a novel hypothesis of interrelationships, let me know so I can promote the older citation.
Updates have been a continuing feature
of the LRT since its origin nine years ago, along with the steady addition of taxa to the present total of 1658 taxa, plus several hundred taxa in the pterosaur and therapsid cladograms. Correcting mistakes is standard practice in every science and every correction is another rewarding moment of discovery. Holding on to outdated and invalid hypotheses has been an acknowledged problem in paleontology.
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Clark JM, Sues H-D and Berman DS 2000. A new specimen of Hesperosuchus agilis from the Upper Triassic of New Mexico and the interrelationships of basal crocodylomorph archosaurs. Journal of Vertebrate Paleontology 20(4):683-704.
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