Machimosaurus, SPP. VON MEYER, 1837

Scavezzoni, Isaure, Fischer, Valentin, Johnson, Michela M. & Jouve, Stéphane, 2024, Form and function of the pelvic girdle of Thalattosuchia and Dyrosauridae (Crocodyliformes), Geodiversitas 46 (6), pp. 135-326 : 250-254

MACHIMOSAURUS SPP. VON MEYER, 1837

For measurements, see Tables 7-9 .

The number of species constituting the genus Machimosaurus Von Meyer 1837 is still debated (e.g. compare Young et al. (2014 b); Fanti et al. (2015); Martin et al. (2015)). In parallel, the pelvic girdle elements associated with either species of Machimosaurus do not match with one another (i.e. an isolated pubis in one case, and a partial ilium and ischium in the other), preventing further comparisons. For these reasons, we chose to describe the pelvic girdle elements of Machimosaurus at the generic level.

The bone referred to the pubis of Machimosaurus in Hua et al. (1993); Hua (1999); Young et al. (2014 b) is not included in our description. Indeed, the bone displays all the characteristics of a derived teleosauroid humerus: shortening of humeral shaft; posterior deflection of the articular proximal head; distal torsion of the condyles in relation to the coronal plane; continuous surface between the deltopectoral crest and the proximal articular head.

Ilium

The ilium of Machimosaurus ( Fig. 61 ) corresponds to the posterior half of a left ilium. The posterior extension of the postacetabular process only reaches about half of the total dorsoventral height of the bone resulting in a relatively short posteacetabular process, which is similar to chat is observed in Sericodon jugleri . Neosteneosaurus edwardsi (i.e. NHMUK PV R 3701, NHMUK PV R 2076, NHMUK PV R 3898) also displays a relatively short postacetabular process compared to Lemmysuchus obtusidens , but it is not as short as Machimosaurus . Similar to Neosteneosaurus edwardsi and Lemmysuchus obtusidens , the postacetabular process of Machimosaurus takes the overall shape of a Lancet arch whose apex points dorsoposteriorly. Indeed, the dorsal and ventral margins of the postacetabular process are both convex, with the dorsal one – the iliac crest – displaying a stronger curvature (which differs from Sericodon jugleri ). The entirety of the lateral surface bordering the margins of the postacetabular process up to the base of the preacetabular process is strongly rugged: it shows a series of densely packed shallow ridges and sulci indicating the presence of a cartilage cap in vivo. Anteriorly, the transition from the postacetabular to the preacetabular process is marked by a dorsoventrally shallow but anteroposteriorly elongated notch as in Charitomenosuchus leedsi , but contracts with Lemmysuchus obtusidens and also Neosteneosaurus edwardsi (i.e. NHMUK PV R 3701, NHMUK PV R 2076, NHMUK PV R 3898). Around its mid-length, the ventral margin of the postacetabular process inverts its concavity and becomes concave. This point corresponds to the transition between the postacetabular process and the posterior margin of the ilium, which culminates ventrally to form the posterior corner of the ischial peduncle. Unfortunately, it is not certain if a small crest or tubercle was present around the concavity inversion as in Neosteneosaurus edwardsi (preserved in NHMUK PV R 2076).

The ischial peduncle of Machimosaurus forms a shallow crescent as in Lemmysuchus obtusidens , Neosteneosaurus edwardsi (i.e. NHMUK PV R 3701, NHMUK PV R 2076, NHMUK PV R 3898), and also Charitomenosuchus leedsi . Indeed, its dorsal and ventral margins are both strongly arched without forming an angled peak. It is possible that the pubic peduncle towered the ischial peduncle as in Neosteneosaurus edwardsi , but it is not preserved. The bony acetabulum was presumably anteroposteriorly wide as seen in other teleosauroids (e.g. Charitomenosuchus leedsi ; Neosteneosaurus edwardsi PETMG R 178, NHMUK PV R 3701, NHMUK PV R 2076, and NHMUK PV R 3898; Lemmysuchus obtusidens ; etc.).

The posterior half of the supraacetabular crest stops around the apex of the bony acetabulum as in other teleosauroids, and its surface appears to have be similarly pitted.

Ischium

The posterior half of the ischium of Machimosaurus ( Fig. 61 ) is preserved, which shows the presence of a dorsoventrally thick posterior process. Indeed, the posterior margin of the ischium forms a strong sinusoidal shape: it is markedly concave dorsally and convex ventrally, with the inflection point located around the mid-length of the margin. The surface of the posterior margin surrounding the inflection point is almost parallel to the ventral margin of the ischium (corresponding to the ischial blade). This is mainly due to the strong concavity of the dorsal portion of the posterior margin which forms a marked bent. Similarly, the strong convexity of the ventral portion of the posterior margin almost forms an obtuse corner. As a result, the posterior process of the ischium appears relatively tubular whereas being dorsoventrally thick throughout its length as in Sericodon jugleri . Lemmysuchus obtusidens also possesses a dorsoventrally thick posterior process although not tubular, and the overall shape of its posterior margin is smoother without any marked bent like Machimosaurus . Proexochokefalos cf. bouchardi also shows a relatively tubular posterior process, but relatively more slender than Machimosaurus . The extremity of the posterior process of Machimosaurus appears to form a ventroposteriorly oriented peak, rather than a flat to slightly convex surface as in Charitomenosuchus leedsi, Proexochokefalos cf. bouchardi, Neosteneosaurus edwardsi , and Lemmysuchus obtusidens .

Pubis

The pubis of Machimosaurus ( Figs 62 ; 63 ) displays an overall slender shape due to a long shaft and narrow pubic apron similar to that of Suchodus durobrivensis . Other crocodyliforms like Hyposaurus natator or Thalattosuchus superciliosus NHMUK PV R 2054 display relatively slender pubis, but those are slightly less similar to that of Machimosaurus ( Fig. 62 ). Hence, the pubis of Machimosaurus contrasts with those of rhacheosaurines (i.e. Cricosaurus suevicus , Cricosaurus albersdoerferi , Cricosaurus bambergensis ) and geosaurines (i.e. Geosaurus giganteus ) which possess reduced pubic symphysis and wide pubic apron.

In Machimosaurus , the medial margin forms a marked bent around the level of the pubic symphysis (like in Teleosaurus sp. ) which creates an external obtuse angle of approximately 135°. As a result, the area surrounding the pubic symphysis strongly protrudes from the main body of the pubic apron. Such an abrupt transition is seen in few other thalattosuchians, namely Cricosaurus albersdoerferi and Lemmysuchus obtusidens . The pubic symphysis of Machimosaurus corresponds to about 35% of the total length of the bone (reconstructed based on both pubes; Fig. 62 ), which is slightly longer than in other teleosauroids (i.e. Lemmysuchus obtusidens , Charitomenosuchus leedsi , Neosteneosaurus edwardsi ) except Macrospondylus bollensis for which the pubic symphysis of Machimosaurus is greatly larger. The length of the shaft of Machimosaurus is slightly shorter than that of the pubic symphysis as it reaches about 30% of the the total reconstructed length of the bone. Comparatively, in Suchodus durobrivensis , the pubic shaft and pubic symphysis display similar lengths, and those reach around 36% of the total length of the bone. In Thalattosuchus superciliosus NHMUK PV R 2054 and Hyposaurus natator , however, the pubic symphysis is shorter than the shaft, and reaches between 18 – 24% of the total length of the pubis. Another similarity between Suchodus durobrivensis and Machimosaurus ( Fig. 62 ), which is also shared with Lemmysuchus obtusidens , is the acute angle formed between the pubic symphysis and the median of the shaft: about 22° for Machimosaurus , and 28°for Suchodus durobrivensis and Charitomenosuchus leedsi , and 30° for Lemmysuchus obtusidens and Neosteneosaurus edwardsi . Nevertheless, the other teleosauroids appear dissimilar to Machimosaurus due to a larger pubic plate laterally to the median of the shaft. Indeed, in Machimosaurus , the distal margin of the bone connecting the pubic symphysis with the lateral margin of the pubis is relatively short (less than the length of the pubic symphysis unlike in Lemmysuchus obtusidens ). Also, the junction between the pubic symphysis and the distal blade of Machimosaurus forms a relatively small angle of approximately 127° which contributes to the shortness of the distal blade, as in Neosteneosaurus edwardsi compared with Lemmysuchus obtusidens or Charitomenosuchus leedsi . The exact shape of the distal margin of Machimosaurus ( Figs 62 ; 63 ) is uncertain, but was presumably slightly arched as in other thalattosuchians (i.e. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Cricosaurus albersdoerferi , Geosaurus giganteus , Lemmysuchus obtusidens ).

In Machimosaurus , the junction between the distal and the lateral margins is achieved through a blunt right-angle corner which does not appear to protrude laterally, as in Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 2054 , and Lemmysuchus obtusidens , contra ( Mycterosuchus nasutus or ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 to a lesser extent. The lateral margin of the pubis of Machimosaurus forms a tenuous sinusoid, with the portion bordering the pubic apron being convex and gradually switching to concave as it climbs back up towards the peduncle. The undulation of the lateral margin of the pubis is even less perceptible in Suchodus durobrivensis , but is more clear in Lemmysuchus obtusidens . Comparatively, the medial margin of the pubis of Machimosaurus appears almost straight with a slight undulation around the junction between the shaft and the distal blade. This shape is not present in other thalattosuchians (i.e. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Cricosaurus albersdoerferi , Geosaurus giganteus , Lemmysuchus obtusidens ). The pubic peduncle of Machimosaurus flares out from the shaft but is only slightly larger mediolaterally than the thinnest portion of the shaft similar to Dakosaurus maximus , but unlike in Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Cricosaurus bambergensis , Geosaurus giganteus .

Figures

FIG. 61. — Left ilium and ischium of the lost neotype of Machimosaurus mosae Sauvage & Liénard, 1879: A, left ilium in lateral view; B, hypothetical reconstruction of left ischium in lateral view; C, pelvic girdle in lateral view as it appears in Hua et al. (1993). Arrow point anteriorly. Picture in A modified from Young et al. (2014). Pictures in C modified from Hua et al. (1993). Scale bars: 5 cm.

FIG. 62. — Right and left pubes of Machimosaurus sp., SMNS 81608: A, right pubis in anterior view; B, left pubis in anterior view. Target indicates anterior. Scale bar: 1 cm.

FIG. 63. — Reconstruction of the pelvic girdle of Machimosaurus sp. (SMNS 81608, A-F) and Lemmysuchus obtusidens (Andrews, 1909) (NHMUK PV R 3168, D-F): A, right lateral view; B, anterior view; C, ventral view; D, right lateral views; E, anterior views; F, ventral views. Target indicates anterior. Arrow points anteriorly. The right pubis of SMNS 81608 is mirrored, the left pubis is rapaired. 3D models of SMNS 81608 courtesy of Roland Sookias. Reconstructed bones only serve as a qualitative representation of the pelvic girdle of Machimosaurus sp. Scale bars: 5 cm.