Garzapelta
| Garzapelta Temporal range:
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Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Archosauria |
| Clade: | Pseudosuchia |
| Order: | †Aetosauria |
| Family: | † Stagonolepididae
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| Genus: | †Garzapelta Reyes, Martz & Small, 2024 |
| Species: | †G. muelleri
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| Binomial name | |
| †Garzapelta muelleri Reyes, Martz & Small, 2024
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Garzapelta is an extinct
History and naming
The remains of Garzaspelta were discovered in 1989 in the UU Sand Creek locality (MOTT 3882) in the middle units of the
The first part of the name Garzapelta reflects the geographic origin of the animal, as the type locality lies in
Description
The armour of Garzapelta, like that of other aetosaurs, can generally be split into several regions depending on their position relative to the underlying skeleton. The cervical osteoderms just above the neck, the trunk vertebrae that cover the torso (themselves separated into anterior, mid and posterior trunk vertebrae), the sacral osteoderms above the hip and the caudal osteoderms that surround the tail (which can be further separated much like the trunk vertebrae. Additionally, the armour is composed of paramedian and lateral osteoderms depending on their position relative to the spine, with paramedian osteoderms being those located directly above the vertebral column and meeting in the middle, with the lateral osteoderms being those that attach to either side of this central double row.
Paramedian osteoderms
Towards the middle of the trunk the osteoderms are notably wider than they are long, with the width to length ration being somewhere around 3.0 to 3.5. This is similar to the proportions of Rioarribasuchus, but not as extreme as the values seen in other typothoracines. The dorsal eminence of the paramedian osteoderms, a prominent ridge on their surface, is offset from the midline and located further towards the back, yet does not touch the edge of the element. The anterior margin of the dorsal paramedians before the tail is described as a thin and smooth strip of bone that lacks the ornamentation seen on the rest of the surface. The anterior paramedian osteoderms above the tail have a distinct sinuous back edge that also features a "tongue-like" process just behind the dorsal eminence. It is noted that the paramedian and lateral osteoderms of Garzapelta, unlike those of desmatosuchines, do not articulate in the same rigid groove and ridge articulation and instead overlap the anteromedial corner of the lateral osteoderms. The surface of the osteoderms is ornamented by various grooves and ridges that radiate outward from the dorsal eminence.
Lateral osteoderms
The lateral osteoderms consist of three primary regions, the flat dorsal flange that articulates to the paramedian osteoderms along their medial edge (which is distinctly sigmoid in the anterior trunk vertebrae), the downward-facing lateral flange that forms the outermost edge of the element and a large spike-like dorsal eminence similar to those seen in Longosuchus, Lucasuchus and Sierritasuchus, which is formed in the region where the dorsal and lateral flanges meet. In top view the dorsal eminences are swept back, whereas in anterior or posterior view (from the front or from behind) they protrude outward and up. Among the defining features of the lateral osteoderms of Garzapelta is that the osteoderms of the cervical and anterior trunk region bear protrusions on the anterior edge of these spikes. Garzasuchus lacks an embayment at the back of the base of the dorsal eminence, which sets it apart from Lucasuchus and Desmatosuchus. The lateral flanges of Garzapelta are reduced and in the mid-trunk region they connect to the dorsal flange at an obtuse angle. This is similar to what is seen in most other aetosaurs, but clearly distinct from the derived desmatosuchines and typothoracines, in which the two elements connect at an angle of 90° or less. Further back, on the early osteoderms of the caudal armour, the lateral flange takes on a distinct semi-circular shape that is shared by paratypothoracines.
Classification
Reyes and colleagues conducted both
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The results recovered for lateral and paramedian osteoderms together agrees with the results yielded by just looking at the lateral osteoderms, again placing it as a stagonolepidoid just outside of Desmatosuchini. Here too the strict consensus is notably less well resolved than the Bayesian consensus, featuring a massive polytomy that includes almost all non-desmatosuchin aetosaurs. The resulting Bayesian consensus tree is shown below.[1]
| Aetosauria |
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The conflicting positions of Garzapelta had been known to science since long before the taxon was named. The fact that the lateral osteoderms resemble those of desmatosuchins while the paramedian osteoderms resemble those of paratypothoracines poses the question whether or not Garzapelta was a desmatosuchine with paramedian osteoderms that evolved convergently to those of paratypothoracines or the other way around. The results of the overall phylogeny would support the former interpretation, which in turn would suggest that the paratypothoracine paramedian osteoderms are the ancestral condition, which was simply lost by all other aetosaurs and reversed in Garzapelta. However, this might not be the case. The interpretation that it was the lateral osteoderms that are the convergent element finds support in the way the lateral and paramedian osteoderms articulate with one another. Desmatosuchini are in part defined by the fact that the inner frontal corner of the lateral osteoderms overlaps the outer frontal corner of the paramedian osteoderms, forming a rigid point of articulation. Furthermore, the fact that the lateral osteoderms of the mid trunk region flex at an obtuse angle sets them apart from the 90° angle seen in Desmatosuchini and the strongly acute angle of paratypothoracines. Reyes and colleagues thus conclude that it is far more likely that Garzapelta was a typothoracine that exhibits convergent evolution with desmatosuchines, rather than the other way around. They further note that the fact that it was nonetheless recovered as a desmatosuchine in the third phylogenetic analysis including all material suggests that the dataset that aetosaur phylogenetics are built upon do not fully account for convergent evolution in the dorsal armour, meaning future analysis would have to be done with caution.[1]
References
- ^ PMID 38206046.