Megistotherium: Not a gigantic hyaenodont creodont. More like a very basal seal.

Traditionally described as a gigantic hyaenodont creodont,
Megistotherium (Savage 1973; Miocene; Fig. 1) nests in the large reptile tree (LRT, 1544 taxa) at the base of the clade within Carnivora that ultimately produced extant seals and extinct Palaeosinopa (Figs. 1,2). In the LRT creodonts are large marsupial predators, convergent with members of the clade Carnivora.

Therefore 
Megistotherium is also a sister to the Machaeroides clade (which gave rise to Stylinodon) both derived from the Kerbos and Gulo (wolverine) clades (which gave rise to terrifying short face bears, like Arctodus). So, several gigantic, fearless bear-like taxa arise from this branch within Carnivora.

Figure 1. Megistotherium skull in several views. It is 2/3 of a meter in length. Don't overlook the skull of tiny relative, Palaeosinopa with a 10cm skull length.

Figure 1. Megistotherium skull in several views. It is 2/3 of a meter in length. Don’t overlook the skull of tiny relative, Palaeosinopa with a 10cm skull length.

Megistotherium osteothlastes (Savage 1973; Miocene, 23mya; 66cm skull length) was originally considered a giant hyaenodontid creodont based on tooth data. Here, because it nests basal to Palaeosinopa and seals it was probably semi-aquatic. Premaxillary teeth were weak and disappearing. The jaw muscles were enormous judging by the widespread cheek arches and tall cranial crest. The large diameter canines were housed in large, laterally expanded maxillae. The braincase was narrow. Note the prefrontal and lacrimal are no longer fused to one another.

Earlier mistakes in nesting Megistotherium
may be assigned to an over reliance on dental traits, which tend to converge more often than traditionally realized, and to taxon exclusion, something the LRT tends to minimize due to its wide gamut, getting bigger every week.

Figure y. Palaeosinopa in situ with tail reconstructed from disturbed elements.

Figure 2. Palaeosinopa in situ with tail reconstructed from disturbed elements. This taxon provides clues to the post-crania of Megistotherium by way of phylogenetic bracketing.

Side note:
In overall size and general features, the skull of Megistotherium was similar, by convergence, to the giant Eocene elephant shrew, the mighty Andrewsarchus.

Figure 1. Harpagolestes macrocephalus compared to sisters Sinonyx and Andrewsarchus to scale.

Figure 1. Harpagolestes macrocephalus compared to sisters Sinonyx and Andrewsarchus to scale. Compare these elephant shrews to Megistotherium (Fig. 1)/

In the past,
several mammal taxa achieved gigantic proportions not found in today’s relatives.


References
Savage RJ 1973. Megistotherium, gigantic hyaeonodont from Miocene of Gebel Zelten, Libya. Bulletin of the British Museum (Natural History) Geology 22(7):483–511.

wiiki/Megistotherium

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