Simbakubwa: Not a giant carnivore. More like a hippo.

Borths and Stevens 2019 might have been confused by the giant canines
and giant molars of Simbakubwa (Fig. 1). The authors thought they were dealing with a giant carnivore related to hyaenodonts and creodonts (hence the title of their paper).

The large reptile tree (LRT, 1546 taxa) makes no assumptions. The LRT minimizes confusion by testing a wider gamut of taxa, including mesonychids (Fig. 2) and hippos. It turns out the great size of Simbakubwa is actually no big deal because it’s closer to hippos than lions. Most hippos are much bigger than most carnivores.

Figure 1. Simbakubwa from Broths and Stevens 2019, colors added, and compared to a lion mandible. Note the two medial views of the mandible with different shapes. Dorsal view of indented mandible and palate is similar to hippos.

Figure 1. Simbakubwa from Broths and Stevens 2019, colors added, and compared to a lion mandible. Note the two medial views of the mandible with different shapes. Dorsal view of indented mandible and palate is similar to hippos.

Simbakubwa kutokaafrika (Borths and Stevens 2019; Miocene, 23mya; size; Fig. 1) was originally considered a gigantic carnivore, a member of the Hyaeondonta and Creodonta. Here it nests with Ocepeia (Fig. 3) as an offshoot of basal hippos with anteriorly placed eyes, convergent with carnivores, derived from mesonychids (Fig. 2).

Strangely
in dorsal view the mandible (dentary) was originally presented with an unnatural lateral kink/bend, creating a large open space where the teeth do not occlude. The authors report, (dentary is reconstructed with the distal portion medially oriented out of natural position) and “the coronoid should be interpreted cautiously because it is reconstructed.”

Not sure why they published that mandible without fixing it. 
The authors note: “the tooth crowns are unworn”. Relative to the skull size, all the teeth were enormous and they extended far back in the skull. I note the shearing canines of extant hippos are already present here. It is also worthwhile to compare the only dentary premolar of Simbakubwa (Fig. 1) with the identical tooth found in the earlier mesonychid, Harpagolestes (Fig. 4). In any case, the suite to traits preserved nest Simbakubwa with mesonychid hippos, rather than hyaenodont creodonts (which are marsupials, not carnivores).

Hippos are not related to artiodactyls
in the LRT, contra the traditional myth. Hippo ancestors are basal to taxa leading to baleen whales. 

Figure 1. Mesonyx, the first known mesonychid was a sister to Hippopotamus in the large reptile tree. So maybe it was a plant eater.

Figure 2. Mesonyx, the first known mesonychid was a sister to Hippopotamus in the large reptile tree. So maybe it was a plant eater, even though, like Simbakubwa, it looks like a predator with large lower canines.

Wikipedia reports,
Simbakubwa, like other hyainailourids, probably was a specialist hunter and scavenger that preyed on creatures such as rhinoceroses and early proboscideans. It may have been somewhat less specialized in crushing bone than its later relatives such as Hyainailouros. However, like HyainailourosSimbakubwa possessed lingually rotating carnassial blades, ensuring a constant shearing edge throughout its life.” Hippos are also killers, but usually only for defense. They and all their sister taxa prefer plants.

Figure 1. Ocepeia: before and after. The original reconstruction is here compared to a tracing of CT scan, duplicated left to right.

Figure 3. Ocepeia: before and after. The original reconstruction is here compared to a tracing of CT scan, duplicated left to right.

Ocepeia daouiensis (Gheerbrant et al 2001, 2014; Paleocene, 60 mya; 9 cm skull length; Fig. 3) is a Hippopotamus ancestor derived from a sister to Merycoidodon. The original reconstruction was not an accurate representation of the fossil CT scan. The pneumatized skull contains many air spaces. The larger skulls have larger canines and so are considered male. The jugal deepens below the orbit, hiding the posterior molars in lateral view. The premaxilla is transverse. The upper canine rubs against the lower large incsior creating a facet, as in hippos and Harpagolestes (Fig. 4). Ocepeia was found with aquatic taxa and was probably amphibious.

Figure 5. Robust Harpagolestes nests between the hippos and Mesonyx.

Figure 4. Robust Harpagolestes nests between the hippos and Mesonyx. Note the identical lower premolar as in Simbakubwa (Fig. 1).

Several news organizations picked up on the sensational aspects
of this ‘gigantic carnivore’ discovery. Unfortunately, this may become embarrassing for the authors when confirmed.

The good news is:
we have more hippo and mysticete ancestors to study!


References
Borths MR and Stevens NJ 2019. Simbakubwa ￿kutokaafrika, gen. et sp. nov. (Hyainailourinae, Hyaenodonta, ‘Creodonta,’ Mammalia), a gigantic carnivore from the earliest Miocene of Kenya. Journal of Vertebrate Paleontology e1570222 (20 pages) https://doi.org/10.1080/02724634.2019.1570222

wiki/Simbakubwa

https://www.ranker.com/list/killer-hippos-are-dangerous/mariel-loveland

https://www.washingtonpost.com/news/senegals-killer-hippo-problem/

http://blogs.discovermagazine.com/deadthings/2019/04/18/simbakubwa/#.XTX-IRTT63A

https://www.cbsnews.com/news/giant-lion-fossil-found-inside-drawer-at-kenyan-museum-2019-04-19/

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