Lucuona, Desojo and Cerda 2020 bring us new information
on Gracilisuchus (Figs. 1, 3), a basal bipedal crocodylomorph in the large reptile tree (LRT, 1734+ taxa; subset Fig. 2). Gracilisuchus was originally considered an ornithosuchid by Romer (1972). Others considered it to nest between Parasuchus and Stagonolepis (Benton and Clark 1988), as the sister to Postosuchus (Juul 1994) or as a sister to Postosuchus and Erpetosuchus (Benton and Walker 2002). All suffered from taxon exclusion.
(Romer 1972; Butler et al. 2014; Ladinian, Middle Triassic, ~230 mya, 30 cm long; holotype PULSR8) is a basal crocodilomorph. In the LRT Gracilisuchus was derived from a sister to Dibothrosuchus (Fig. 5), and preceded both Saltopus and Scleromochlus in the LRT. These three taxa are not mentioned in the Lucuona et al. text.
is the major and continuing problem in vertebrate paleontology. The LRT is trying to repair that problem simply by adding taxa.
From the Lucuona et al. 2020 abstract:
“Gracilisuchus stipanicicorum Romer, 1972 is a basal suchian from the Late Triassic Chañares Formation (Argentina), nested in the recently erected Gracilisuchidae, along with Turfanosuchus dabanensis Young, 1973 and Yonghesuchus sangbiensis Wu et al., 2001 from China.”
Adding taxa separates these three genera in the LRT (subset Fig. 2) and invalidates any clade with only these three polyphyletic members. These three taxa and all their descendants do form an unnamed clade: Poposauria + Archosauria in the LRT, which was not their intention, nor that of Butler et al. 2014, who erected the clade with these three members with the same criticism about 5 years ago.
Continuing from the abstract:
“The six known specimens of Gracilisuchus Romer, 1972 preserve most of the skeleton, lacking only most of the shoulder girdle and forelimb. Our latest fieldwork has recovered two specimens that preserve previously unknown elements, including the humerus, radius, and ulna, as well as the femur, presacral vertebrae, and paramedian osteoderms.”
By contrast the LRT (Subset Fig. 2) separates the PVL 4597 specimen from the PULSR8 holotype. The former nests at the base of the Archosauria (Crocodylomorpha + Dinosauria) at least two nodes away at present.
By combining two or more specimens
Lecuona et al. 2020 created an unwanted and confusing chimaera. A better practice is to score each specimen individually and let those that nest together do so. THEN create a chimaera if warranted.
The traditional short-faced Gracilisuchus specimen
MCZ 4116 (Fig. 4), was not mentioned in the Lecuona et al. 2020 text. Perhaps that’s a good thing since earlier the LRT nested the short-faced specimen with Trialestes.
Lecuona et al. consider Gracilisuchus to be a member
of the invalid clade, ‘Pseudosuchia’.
Bone histology studies
were performed on the two new specimens:
- CRILAR PV 480, “one and a half incomplete cervical centra articulated with each other and with two incomplete ribs, a series of three incomplete vertebrae articulated with one rib, dorsally in contact with the left row of the paramedian osteoderms, and posteriorly four ribs with no articulating vertebrae but in anatomical position (Fig. 2A, B), a series of six incomplete cervicodorsal vertebrae with some of their ribs preserved and half of a centrum attached posteriorly (Fig. 2C, D), and moulds of two short fragments of paramedian osteoderms”
- CRILAR PV 490, “two articulated cervical vertebra in contact with a short paramedian osteoderm series, one isolated dorsal vertebrae, left humerus, right ulna, right radius, left femur, and six histological sections of the femoral diaphysis and osteoderms”
Until phylogenetic analysis is performed on each specimen,
we can’t be sure that these are indeed Gracilisuchus specimens.
Until Lucuona et al. 2020 add pertinent taxa
they will not understand the phylogenetic context of the holotype specimen, the referred specimens and all pertinent, but unrelated taxa. That means whatever they have to say about the new fossils has to be considered with some reservation. They think all this material belongs to Gracilisuchus. The LRT demonstrates at least one specimen nests apart from the holotype. Better to know with validation, than to guess.
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