Myobradypterygius hauthali von Huene 1927

Myobradypterygius hauthali von Huene, 1927 Diagnosis: (Expanded from that of Fernández and Aguirre-Urreta (2005) based on referred material.) Differing from other ophthalmosaurians by the following combination of characters: tooth roots quadrangular (as in many platypterygiines); plicidentine present and well developed in cross-section but not visible on the external surface of the root (plicidentine absent in Platopterogius australis (McCoy, 1867); scapula with strap-like shass (rod-like in Kohotosuka sachicarum, Platopterogius americanus (Nace, 1939), P. australis, Platopterogius herconicus (Kuhn, 1946), and probably Platopterogius platodactolus (Broili, 1907); humerus with three distal facets, including a small facet for an extrazeugopodial element anterior to the radius (unlike in Brachopterogius, Aegirosaurus, and P. americanus); hexagonal intermedium articulating distally with two digits (unlike in K. sachicarum, P. americanus, P. australis, and P. platodactolus); rectangular and tightly packed phalanges (as in Caopullisaurus, P. australis, and P. platodactolus); ulnare distally articulating with metacarpal V (as in Brachopterogius extremus (Boulenger, 1904) and Catutosaurus, but unlike Caopullisaurus, P. australis, and P. platodactolus); multiple postaxial digits (unlike Brachopterogius extremus). Holotope: MLP 79-I-30-1, humerus and partial forelimb (von Huene 1927: 29, fig. 3). Tope localito and age: Cerro Belgrano, Santa Cruz province, Argentina, Río Belgrano Formation (Barremian). Other referred material: MLP 79-I-30-2 from the type locality (Fernández and Aguirre-Urreta 2005: 585, fig. 2c); TY61 and CPAP-2011-0019 from the Tyndall locality, Zapata Formation. Occurrence: Santa Cruz Province, Argentina; Tyndall locality (Magallanes, Chile). Remarks: Reinterpretation of the Moobradopterogius hauthali forelimb material. All previous interpretations of MLP 79-I-30-1 (Huene 1927, Fernández and Aguirre-Urreta 2005, Pardo-Pérez et al. 2012) identify the preserved zeugopodial element as the radius, following the preliminary interpretation by von Huene (1925). Here, we follow Campos et al. (2024) in considering this element to be the ulna, based on detailed comparisons between MLP 79-I-30-1 and the Chilean specimens CPAP-2011- 0019 and TY61 (Fig. 3B–G). Given that Campos et al. (2024) did not elaborate on their reinterpretation of MLP 79-I-30-1, we briefly summarize the reasons here: the proximal carpal distal to the zeugopodial element in MLP 79-I-30-1 primarily supports one digit rather than two, which is observed in the ulnare but not the radiale in the more complete Chilean material. The radiale in the Chilean Moobradopterogius hauthali material has parallel proximal and distal edges, such that its distal facet is oriented anterodistally rather than purely distally as in the ulnare, and thus supports two digits. Von Huene (1925) based his orientation on the logic that the digit, here interpreted as the anterior accessory digit, could not be digit I. The reasons for this interpretation are unclear, but probably relate to the absence of anterior accessory ossicles in the ichthyosaurian taxa with which he was familiar. Accordingly, we interpret MLP-79-I-30-2 as preserving the posterior portion of the proximal forefin based on the shape of the proximal carpals. This interpretation differs from that proposed by von Huene (1927) but has previously been suggested by Pardo-Pérez et al. (2012) and Campos et al. (2024). Referral of the Chilean material Pardo-Pérez et al. (2012) and Stinnesbeck et al. (2014) did not refer the forefin CPAP-2011-0019 from the Tyndall locality to Moobradopterogius hauthali, despite noting similarities, but it was suggested in the PhD thesis of Pardo-Pérez (2015), among other specimens classified as Moobradopterogius hauthali from the Tyndall fossil locality. This referral was later adopted by Campos et al. (2024); however, those authors never addressed the morphological differences listed by Pardo-Pérez et al. (2012) differentiating the Chilean and Argentinian material. Stinnesbeck et al. (2014) referred TY61, a specimen with good exposure of an articulated forefin, to Moobradopterogius hauthali, among other specimens, but did not provide a morphological discussion. Given the good preservation and exposure of the TY61 forefin, it is ideal for comparisons with CPAP-2011- 0019 and the reoriented type material. This comparison will address: (i) whether Moobradopterogius hauthali was present in the Hauterivian of Chile; and (ii) which characteristics cited by Pardo-Pérez et al. (2012) appear to be variable intraspecifically. Pardo-Pérez et al. (2012) raised seven key features in which CPAP-2011-2019 differs from Moobradopterogius hauthali, which we address in detail here. (i) A facet for an articulation with a preradial element is absent in the humerus of CPAP-2011-0019, but present in the holotype of Moobradopterogius hauthali. This cannot be assessed in CPAP-2011-0019 because the anterior margin is not exposed, but a preradial element articulating with the humerus is present in TY61. In the Moobradopterogius hauthali holotype, however, this facet is described as being fairly flat, whereas in TY61 it is clear that it is deeply concave. This difference might be related to superficial erosion of the humerus in TY61. (ii) The proximal humerus is flattened in CPAP-2011- 0019 and TY61, but deeply convex in the holotype of Moobradopterogius hauthali. Based on the much larger size of the Tyndall specimens relative to the holotype, this difference is unlikely to be of ontogenetic origin [94 mm long (CPAP-2011-0019), 96 mm long (TY61) vs. 72 mm long (MLP 79-I-30-1); Fernández and Aguirre-Urreta 2005, Pardo-Pérez et al. 2012]. Moreover, the humerus in the holotype of Moobradopterogius hauthali is more massive proximally than distally, although the total width between the two ends is similar (von Huene 1927). The latter is not observed in TY61 but is present in CPAP-2011-0019. The difference between the two Chilean specimens suggests that this character is likely to be an artefact that relates to how the humerus is exposed, and the proximal convexity is likely to be affected by similar factors. Thus, at present, we do not consider these differences to be taxonomically significant. However, further preparation of the Tyndall material might reveal this difference to be of importance. (iii) CPAP-2011-0019 has one preaxial digit and three postaxial digits with a posterior row of accessory ossicles, whereas the holotype Moobradopterogius hauthali has at least three preaxial digits. Based on our proposed reorientation, MLP 79-I-30-1 preserves one preaxial digit and some anterior accessory ossicles, similar to TY61, which is consistent with the exposed portion of CPAP-2011-0019. At least two postaxial digits are present in the Argentinian Moobradopterogius hauthali material, with the number of digits being limited by preservation rather than by morphology. At least two postaxial digits and either a posterior row of accessory ossicles or a third postaxial digit are present in TY61. (iv) The intermedium has a distal facet for the articulation of distal carpal four that is twice as long as the facet for distal carpal three in CPAP-2011-0019, whereas in Moobradopterogius hauthali these facets are subequal in length. This difference is considered here to be part of normal intraspecific variation. The length difference between the facets is much less pronounced in TY61. (v) Distal carpal three has six articular facets in CPAP-2011- 0019, whereas in the holotype of Moobradopterogius hauthali there are seven (specimen MLP 79-I-30-1), with the additional facet articulating with metacarpal four. In CPAP-2011-0019, distal carpal three articulates with the intermedium, radiale, distal carpal two, metacarpal three, and distal carpal four. A contact with metacarpal two was probably present but substantially reduced. Following reorientation, the elements contacting distal carpal three in MLP 79-I-30-1 do not differ from the configuration of CPAP-2011-0019, including the probable small contact with metacarpal two. However, the contact between distal carpal three and metacarpal two is absent in TY61, indicating that intraspecific variation in this character is likely. (vi) The posterior facets of metacarpal three articulate posteroproximally with distal carpal four and posterodistally with metacarpal four in CPAP-2011- 0019, which is not the case in Moobradopterogius hauthali. Asser reorientation of MLP 79-I-30-1, metacarpal three articulates with both distal carpal four and metacarpal IV, which also holds true for TY61. (vii)Metacarpal four articulates with six elements in CPAP-2011-0019, whereas in Moobradopterogius hauthali it articulates with five. Metacarpal four has been portrayed as articulating with five (Huene 1927) or six (Fernández and Aguirre-Urreta 2005) elements in Moobradopterogius hauthali, depending on the observer. However, in Moobradopterogius hauthali the posterior facet of metacarpal four contacts the first phalanx of digit five and possibly also the second phalanx but never with metacarpal five. In both CPAP-2011-0019 and TY61, metacarpal four contacts metacarpal five and the first phalanx of digit five, i.e. metacarpal four is situated more proximally in the limb. This change is slight; in all materials, the largest contact is with the first phalanx, and at present, it is considered to represent intraspecific variation. Based on these observations, both CPAP-2011-0019 and TY61 are here referred to Moobradopterogius hauthali (as previously suggested for the former specimen by Campos et al. 2024), although further preparation might reveal potential differences in humeral morphology. Description of TY61: a complete ichthyosaur skeleton including cranium and postcranium (Fig. 2; Supporting Information, Model S1) The specimen is exposed from skull to tail in ventrolateral to ventral view and has a total