Gracilisuchus 2020: Two new specimens added to the genus

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.

Figure 4. The PVL 4597 specimen nests at the base of the Archosauria, not with Gracilisuchus.

Figure 1. The PVL 4597 specimen (above)  nests at the base of the Archosauria, not with Gracilisuchus (below). Gracilisuchus based on holotype  PULSR8 with the skull diagram (MCZ4117)  based on Romer 1971. See figure 3 for an update on that skull.

Gracilisuchus stipanicicorum
(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.

Taxon exclusion
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.

Figure 2. Subset of the LRT focusing on Crocodylomorpha. Matching Nesbitt et al. 2005, the LRT nests Redondavenator near the base of the Crocodylomorpha.

Figure 2. Subset of the LRT focusing on Crocodylomorpha. Matching Nesbitt et al. 2005, the LRT nests Redondavenator near the base of the Crocodylomorpha.

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.

Figure 5. Gracilisuchus skull updated with new colors.

Figure 3. Gracilisuchus skull (MCZ 4117) updated. Note the slender fenestra between the premaxilla and maxilla, as in Dibothrosuchus.

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.

Figure 1. The former Gracilisuchus specimens MCZ4116 and MCZ4118 with colors added.

Figure 4. The former Gracilisuchus specimens MCZ4116 and MCZ4118 with colors added.

Lecuona et al. consider Gracilisuchus to be a member
of the invalid clade, ‘Pseudosuchia’.

Bone histology studies
were performed on the two new specimens:

  1. 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”
  2. 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.

Figure 2. Images from Wu et al. 1993, colors and hind limbs added. Compare to skull in figure 1.

Figure 5. Images from Wu et al. 1993, colors and hind limbs added. Compare to skull in figure 1.

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.


References
Benton MJ and Clark JM 1988. Archosaur phylogeny and the relationships of the Crocodilia in MJ Benton (ed.), The Phylogeny and Classification of the Tetrapods 1: 295-338. Oxford, The Systematics Association.
Brinkman D 1981. The origin of the crocodiloid tarsi and the interrelationships of thecodontian archosaurs. Breviora 464: 1–23.
Butler RJ, Sullivan C, Ezcurra MD, Liu J, Lecuona A and Sookias RB 2014. New clade of enigmatic early archosaurs yields insights into early pseudosuchian phylogeny and
the biogeography of the archosaur radiation. BMC Evolutionary Biology 14:1-16.
Juul L 1994. The phylogeny of basal archosaurs. Palaeontographica africana 1994: 1-38.
Lecuona A and Desojo, JB 2011. Hind limb osteology of Gracilisuchus stipanicicorum(Archosauria: Pseudosuchia). Earth and Environmental Science Transactions of the Royal Society of Edinburgh 102 (2): 105–128.
Lecuona A, Desojo JB and Pol D 2017. New information on the postcranial skeleton of Gracilisuchus stipanicicorum (Archosauria: Suchia) and reappraisal of its phylogenetic position. Zoological Journal of the Linnean Society XX:1–40.
Lecuona A, Desojo JB and Cerda IA 2020. New information on the anatomy and histology of Gracilisuchus stipanicicorum (Archosauria: Pseudosuchia) from the Chañares Formation (early Carnian), Argentina. Comptes Rendus Palevol 19 (3): 40-62. https://doi.org/10.5852/cr-palevol2020v19a3
Parrish JM 1993. Phylogeny of the Crocodylotarsi, with reference to archosaurian and crurotarsan monophyly. Journal of Vertebrate Paleontology 13(3):287-308.
Romer AS 1971. The Chañares(Argentina) Triassic reptile fauna. Two new bu incompletely known long-limbed pseudosuchians. Breviora 378:1–10.
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna. An early ornithosuchid pseudosuchian, Gracilisuchus stipanicicorum, gen. et sp. nov. Breviora 389:1-24.

wiki/Gracilisuchus

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