Tyrannosauridae, Osborn, 1905

Peecook, Brandon R., Wilson, Jeffrey A., Hernández-Rivera, René, Montellano-Ballesteros, Marisol & Wilson, Gregory P., 2014, First tyrannosaurid remains from the Upper Cretaceous “ El Gallo ” Formation of Baja California, México, Acta Palaeontologica Polonica 59 (1), pp. 71-80 : 73-75

Tyrannosauridae gen. et sp. indet.

Material. — IGM 6130 , a left metatarsal IV ( Fig. 3 ) from locality Arroyo del Rosario , outside the town of El Rosario, Baja California, México ( Fig. 1 ). An overbank deposit in the sandstone/siltstone El Disecado Member of the “El Gallo” Formation, which is bracketed by age determinations of 75.21 ± 0.07 Ma and 74.55 ± 0.09 Ma (late Campanian; Renne et al. 1991) .

Remarks.—Two features support this referral: the proximal articular surface of metatarsal IV bears a deep, pointed medial notch that articulates with the crescent-shaped proximal end of metatarsal III; and the posterior surface of the shaft is slightly concave between the facet for metatarsal III on the medial surface of the shaft and the M. gastrocnemius lateralis insertion site on the posterior surface of the shaft (see Discussion below).

Description. — IGM 6130 is a complete left fourth metatarsal ( Fig. 3 ). Its total length is 391 mm, and its circumference is 150 mm at midshaft. The minimum circumference (140 mm) is present in two regions of the shaft: above the proximal point of the teardrop articulation for metatarsal III and just above the distal condyle ( Table 1). The bone has excellent preservation of surface detail, but there is moderate damage to the perimeter of the proximal end and posterior portion of the distal condyle. The shaft is relatively straight, with the exception of its distal one-fifth, which sharply angles laterally approximately 10º as in Tyrannosaurus rex (FMNH PR2081) . The lateral divergence of the distalmost portion of the shaft is not noticeable in lateral or medial view, in which the shaft appears straight.

The proximal surface of metatarsal IV is shaped like a U ( Fig. 3E ). The rounded portion of the U faces laterally, and

Length of metatarsal IV 391.0

Proximal surface, circumference 240.0

Proximal surface, anteroposterior width 76.8

Proximal surface, mediolateral width 69.2

Midshaft, circumference 150.0

Midshaft, anteroposterior width 43.2

Midshaft, mediolateral width 42.7

Distal surface, circumference 193.0

Distal surface, anteroposterior width 47.4

Distal surface, mediolateral width 60.5

there is a deep notch on its medial side. In tyrannosauroids such as Tyrannosaurus , the medial notch receives the crescentic proximal end of metatarsal III ( Brochu 2003; Holtz 2004), and the anteromedially-directed arm of the U contacts metatarsal II anterior to metatarsal III, which is a feature of the arctometatarsalian complex ( Holtz 1994). Weathering has removed evidence for this articulation from the Baja metatarsal ( IGM 6130). The posteromedial corner of the proximal surface is also weathered, though less severely. The posteromedial portion of the proximal surface of metatarsal IV bears a well-marked articulation for distal tarsal 4, which was not recovered. The anterolateral rim of the proximal surface is pitted with many small openings that are a few millimeters in diameter.

The medial side of the shaft of metatarsal IV has an elongated, teardrop-shaped articular facet for metatarsal III ( Fig. 3A ). This facet, which is oriented with the point directed proximally, starts roughly one-third of the way down the shaft and extends to where the axis of the shaft diverges laterally near its distal end. The edges of the facet are raised on the anterior margin around the midpoint of the metatarsal, but on the posterior margin more distally, and the interior of the facet bears a roughened texture. Between the proximal notch for metatarsal III and the teardrop articular facet is an ovoid rugosity that is roughly twice as long as it is wide. This rugosity marks the insertion site for M. tibialis anterior ( Carrano and Hutchinson 2002).

The posterior surface of the shaft of metatarsal IV is flat transversely and is set off from the medial and lateral margins of the shaft by sharp corners ( Fig. 3B ). The most noticeable feature of the posterior surface of the shaft is a sharply pointed, elliptical rugosity, about 120 mm long, lying along its lateral border. The rugosity is centered on the midpoint of the shaft and is well textured, most heavily along the lateral border. This rugosity represents the insertion site for M. gastrocnemius lateralis ( Carrano and Hutchinson 2002). The area between this rugosity and the teardrop articular facet for metatarsal III on the medial side of the shaft is slightly concave. A low ridge rises from the medial margin of the midshaft and reaches a maximum thickness of ca. 10 mm before merging with the distal condyle. A similar distolaterally-trending ridge is present on the anterior surface but disappears at the lateral flaring of the distal fifth of the bone.

The shaft is pinched anteroposteriorly above the distal condyle to 140 mm, 10 mm less than the midshaft circumference. This feature is most prominent in lateral view and least prominent in posterior view. The articular surface itself, when viewed distally, is asymmetrical with the medial condyle broader transversely and anteroposteriorly longer than the lateral condyle ( Fig. 3F ). Between the two condyles open a dozen or so pits similar to those on the proximal surface of the metatarsal. The lateral collateral ligament pit, which is weakly developed, and the anterior hyperextensor pit are connected distally by a depression, together creating a larger roughly U-shaped depression just above the distal condyle ( Fig. 3C, D ). This U-shaped depression lies just dorsal to the distal articular surface on the shared anterolateral face of the shaft. This depression can be seen in Tyrannosaurus rex (FMNH PR2081) , although it is much more compressed proximodistally. The collateral ligament pit on the medial side of the distal shaft is well marked and ovoid in shape. It is deep, with vertical walls on three sides and a gentle slope proximally that extends towards the teardrop articulation for metatarsal III.

Figures

Fig. 3. Left metatarsal IV of the Campanian tyrannosaurid (IGM 6130) from Baja California, México, in medial (A), posterior (B), lateral (C), anterior D), proximal (E), and distal (F) views.Anterior is toward the top of the page in E and F. Diagonal line pattern indicates broken bone surface; hatching pattern indicates reconstructed areas; dashed lines indicate inferred extent of the bone. Inferred attachment sites for M. gastrocnemius lateralis (M. gl) and M. tibialis anterior (M. ta) based on reconstruction of Tyrannosaurus hindlimb musculature by Carrano and Hutchinson (2002).

Fig. 1. Generalized geography and stratigraphy of the study area. A. Geographic map of southwestern North America with fossil locality marked. B. Stratigraphic map of the area around El Rosario. Modified from Renne et al. (1991).