Ichthyosaurus cf. communis, IST-MDT

Correspondence, OctAEvio Mateus, 2021, The southernmost occurrence of Ichthyosaurus from the Sinemurian of Portugal, Fossil Record 24 (2), pp. 287-294 : 288-292

Ichthyosaurus cf. communis, IST-MDT 85

1952, Ichthyosaurus intermedius (cited in Zbyszewski and Moitinho de Almeida, 1952)

Material

Forelimbs (IST-MDT 85; Figs. 2 and 3 ): almost complete left forelimb, in dorsal view, plus disarticulated phalanges from the right limb.

Locality

Praia de Nossa Senhora da Victória. Cross-referencing the Sinemurian rocks with the original indications by Zbyszewski and Moitinho de Almeida (1952), the probable coordinates are around 39 ◦ 42 0 25 00 N, 9 ◦ 3 0 4 00 W, in Municipality of Alcobaça, Portugal .

Age and horizon

In the area, the Sinemurian is observable at Praia de Nossa Senhora da Vitória, in dolomite layers with Unicardium costae (Sharpe) (Zbyszewski and Assunçªo, 1965), which is the lower section of Coimbra Formation, considered lower Sinemurian (199.3–195.8 Ma) .

3.1 Description

The more complete left fin is exposed dorsally, with the humerus slightly proximodistally longer than anteroposteriorly wide, and in dorsal view is equally as wide proximally and distally, lacking a differentiated proximal head. The anterior margin is markedly concave, while a proximally reduced anterior flange is present. The dorsal trochanter, a ridge-like structure across the length of the humerus, is observable,

290 J. Sousa and O. Mateus: The southernmost occurrence of Ichthyosaurus but a protruding deltopectoral crest and a plate-like dorsal ridge are absent. The distal facets are terminally placed, approximately equal in size, apparently continuous, and the ulnar facet does not deflect posterodistally. The anterodistal extremity shows no tuberosity and no anterior or posteriorly distal facets for the sesamoid. There is no contact between the humerus and the intermedium. The epipodial and metapodial elements are flattened and plate-like, with no notching on the anterior and posterior edges of the fin, and one postaxial accessory digit is observable. The radius and the ulna are equal in size, lacking an interosseous space between them, and the radius is wider than long. The ulna is severely damaged and difficult to discern, but it is apparently equally wide anteriorly and posteriorly, the posterior margin is convex, and as thick as the rest of the ulna. The manual pisiform is absent. The intermedium, damaged like the ulna, is smaller than the ulnare, located between the radius and the ulna, and apparently as wide as it is long, with a pointed proximal shape, an angular distal edge, and two directly supported digits. The ulnare sports one distal facet, supporting a single digit. There are six observable carpals, all of them are of comparable size and a postaxial row of phalanges identified as part of an accessory digit. There are four observable ossified elements in the metacarpal row, and a preaxial accessory digit is absent, with the smaller elements observable on the anterior edge of the fin identified as belonging to the more incomplete right fin, while a postaxial accessory digit is observable. The manual metacarpals ii–iv are polygonal with rounded edges. The 55 observable manual phalanges are tightly packed polygons proximally and growing rounded distally, and a digital bifurcation is observable on the anterior side of the second phalangeal row, apparently stemming from digit iii.

3.2 Phylogenetic analysis results

Specimen IST-MDT 85 had the following characters scored based on character list by Moon (2019): 198–252: 121 1010?10110 000000101? 2121010001 0101002?1?0 001?1001211. The analysis yielded 50 000 MPTs (mostparsimonious trees), each with a length of 1694 steps.

The strict consensus is not well resolved, placing the specimen within the Ichthyosaurus genus ( Fig. 4 ), as determined by the ulnare being larger than the intermedium (Char. 229: 1 → 0); the angular distal shape of the intermedium (Char. 233: 1 → 0); the subequal size of the distal facets of the intermedium (Char. 235: 1 → 0); and the four elements on the metacarpal row (Char. 241: 1 → 0). Within the genus, the specimen is placed into a polytomy, alongside I. communis and I. anningae and a subgroup of I. breviceps , I. somersetensis and I. larkini as determined by the humerus being of approximately equal width anteroposteriorly (Char. 206: 0 → 1) and the presence of digital bifurcation (Char. 252: 0 → 1).

292 J. Sousa and O. Mateus: The southernmost occurrence of Ichthyosaurus

Figures

Figure 2. Ichthyosaurus cf. communis (IST-MDT 85), anterior left limb in dorsal (a) and ventral (b) view, from Praia de Nossa Senhora da Victória, Sªo Pedro de Moel. H – humerus; R – radius; U – ulna; r – radial; u – ulnar; i – intermedium; a.d – accessory digit; bi – digital bifurcation; 2–4 – distal carpals; ii–v – metacarpals. Bar marks 2 cm.

Figure 3. Ichthyosaurus cf. communis (IST-MDT 85), humerus in dorsal (a) and ventral (b) view. from Praia de Nossa Senhora da Victória, Sªo Pedro de Moel. ad – depression in the articular face; dt – dorsal trochanter. Bar marks 2 cm.

Figure 4. Reduced consensus tree of IST-MDT 85, I. cf. communis, after the matrix by Moon (2019).