Mastodontosauroidea indet.

(Figs 5; 6)

REFERRED MATERIAL. — Zambia. Large postcranial elements studied in 2015 by one of us (BRP) in the historical collections of the NHMUK, and labeled as ‘cf. Parotosuchus ’ (Fig. 5): NHMUK R10271, a huge partial interclavicle of nearly ½ meter breadth (Fig. 5A); NHMUK R10272-R10273, two very large intercentra of 130 × 80 × 40 mm (Fig. 5B, C); NHMUK R10274, a partial right scapulocoracoid (Fig. 5D); and NHMUK R10275, a right complete ilium (Fig. 5E).

Tanzania. NMT RB537 (Fig. 6), a very large and robust intercentrum (132 × 88 × 43 mm) found isolated by one of us (SJN) in 2007 at the locality Z30, near the village of Gingama, in the grey siltstones of the Lifua Member, Manda Beds, Middle-?Late Triassic (e.g., Nelsen et al. 2001; Smith et al. 2018) of the Ruhuhu Basin.

LOCALITY AND HORIZON. — The Zambian specimens were collected in 1963 by a NHMUK and London University expedition which comprised Barry Cox, Alan Charig and colleagues (Attridge et al. 1964) at Locality 15 of Drysdall & Kitching (1963), about 6 km west of the village of Sitwe, upper horizon of the Ntawere Formation, Middle-?Late Triassic (Peecook et al. 2018) of the Luangwa Basin (sensu Barbolini et al. 2016).

DESCRIPTION

Zambian material

All the elements are fully ossified, robust and very large: they therefore belong to adult individual(s). The partial interclavicle NHMUK R10271 (Fig. 5A) bears radiating ridges and deep grooves on its ventral side. Most of the ridges are eroded, which suggests that this bone has been strongly weathered. Unfortunately, the margins of this interclavicle are not preserved, preventing a more precise assignment. The intercentra NHMUK R10272-R10273 (Fig. 5B, C) are crescentic, but with relatively low and short lateral flanks. The limited lateral flanks suggest a relatively reduced notochordal canal. The bone surfaces are relatively rugose, probably for contact with the cartilaginous intervertebral disks. The parapophyses are not well developed. The partial right scapulocoracoid NHMUK R10274 (Fig. 5D) is also massive and robust. Although only its central part is preserved, the typical fan shape is recognizable, with the supraglenoid fossa visible in lateral view. The supraglenoid fossa is oval in shape and very large compared with the width of the mid-shaft. It turns inside into the supraglenoid foramen. Lateral to the supraglenoid fossa, a small expansion of the bone could correspond to the scapular tubercle but its preservation is not sufficient to confirm this identification. The complete right ilium NHMUK R10275 (Fig. 5E) is well preserved. In lateral view, its dorsal iliac process is high and slightly widens dorsally. The supracetabular buttress is well pronounced ventrolaterally, even though its surface is relatively weathered.

Tanzanian material

NMT RB537 (Fig. 6A, B) is crescentic, with a reduced anteroposterior length (43 mm) compared to its height (88 mm). Its parapophyses are located posteriorly suggesting an attribution to a stereospondyl (e.g., Warren & Snell 1991; Witzmann & Schoch 2006). Compared with other stereospondyl taxa, this intercentrum is very robust, with large elevated flanks drawing dorsally the base of the central tube for the notochordal canal. This character is typical of the neorachitomous pattern (e.g., Watson

1919). The parapophyses are well separated from the rest of the vertebral element. Together with the strong degree of ossification of the intercentrum, this suggests that this element belongs to an adult individual (e.g., Steyer 2000). Its ventral, anterior, and posterior surfaces are very rugose. The latter two surfaces show a subcircular rugose zone for the contact with the cartilaginous intervertebral disks. The subcircular ventral side of the intercentrum, as well as the dorsal position and subtriangular shape of its parapophyses, suggests that this element was originally positioned between the middle and the posterior end of the presacral column. More precisely, the height of the parapophyses relative to the height of the intercentrum in lateral view suggests a rather posterior presacral element, by comparison with the vertebral columns of Paracyclotosaurus davidi Watson, 1958 and Mastodonsaurus giganteus (Jäeger, 1828) (respectively Watson 1958; Schoch 1999). The lateral expansions or “winglets” of the intercentrum suggest a relatively reduced notochordal canal. The ventral side of the intercentrum is wider than long. The parapophyses are entirely dorsally located on the corpus of the bone (not laterally and in between two intercentra).

One of us (TA) produced a µCT-scan of NMT RB537 at AST-RX (MNHN) (Fig. 6C). The radiographic image shows two inner zones: a large, dense and dark central zone transitioning relatively abruptly into a lighter peripheral zone. The abrupt transition between the two zones may correspond to a volume of decreasing or arrested growth (VAG, i.e., the 3D equivalent of an annulus or a line of arrested growth in 2D). The strong density of the central zone, if of biological origin, suggests osteosclerosis, a common phenomenon observed in giant temnospondyls (Schoch 2012). In this case, the central zone limited by the VAG corresponds to the juvenile shape of the intercentrum. If this strong density is of taphonomical origin, the central zone is therefore the medullar zone whose spongiose structure was infilled by dense minerals. In this interpretation, the peripheral zone is the cortical zone composed of endosteal bone. Reduced but localized black patches are also visible in the lateral regions of the intercentrum: they are interpreted as hypermineralized zones or inclusions.

IDENTIFICATION

Zambian material

These elements were labeled as ‘cf. Parotosuchus ’ in the NHMUK historical collections, probably because they are very large and robust, and only Parotosuchus (Damiani, 2001) was known as a unique giant Triassic temnospondyl in the 1960s, when they were discovered. However, no diagnostic Parotosuchus characters have been observed on this postcranial material: we therefore prefer to broaden its assignment to Mastodonsauroidea.

Tanzanian material

The proportions of the bone and comparisons with other taxa in which intercentra are known (e.g., Howie 1970; Schoch 1999; Pawley 2006) suggest an individual of about 4 m in total body length. However, because of the lack of diagnostic characters visible on this intercentrum and the high variability of the shape of the intercentra along a single vertebral column (e.g., Schoch 1999), we prefer to broaden its assignment to Mastodonsauroidea.