Isurus hastalis

A 2.5-inch fossil tooth of an Isurus hastalis (right), with a smaller fossil great white shark tooth for size.

### Scientific Name:

Isurus, Cosmopolitodus or Carcharodon hastalis (genus disputed)

### Genus:

Isurus, Cosmopolitodus or Carcharodon (Disputed)

### Conservation Status:

Extinct

***Isurus hastalis***, ***Cosmopolitodus hastalis*** or ***Carcharodon hastalis***, the **Broad-Tooth White shark**, is an extinct white shark that lived from the Miocene epoch to the Pleistocene epoch. Its teeth can reach lengths up to 3.5 in (7.5 cm) and are found worldwide, especially in Miocene and Pliocene marine deposits. It is believed to be an ancestor to the great white shark (*Carcharodon carcharias*), a hypothesis supported by the transitional species *Carcharodon hubbelli*, and most likely would have been one of the top predators in its ecosystem; preying upon small whales and other mammals. Fossils of marine mammals such as dolphins and desmostylians bearing its tell-tale bite marks indicate this to be true.

A study by Ehret et al. in 2012 has suggested that *I. hastalis* belongs to the genus *Carcharodon*, rather than *Isurus*. Fossil evidence suggests *Carcharodon* is derived from *Carcharodon (Cosmopolitodus) hastalis*. Specimens from the Piasco Formation show an evolutionary mosaic of characters between *C. hastalis* and *C. carcharias*.

## Description

*C. hastalis* teeth can grow up to 3.5 inches in length, suggesting a very large shark (potentially up to 30 feet). Their bodies are probably very similar to that of modern great whites, or a transition between them and ancient makos. They are also believed to have a cosmopolitan distribution, with teeth being found worldwide. The species is also divided into two forms based on tooth morphology; broad-tooth and narrow-tooth, each with a unique evolutionary line.

The broad form is characterized by broad-shaped teeth often described as being identical to that of modern great whites besides the lack of serrations. Fossil evidence shows that the broad-form is the direct ancestor of the genus *Carcharodon*, and specimens from the Pisco Formation in Peru show an evolutionary mosaic between them.

This form is also commonly labeled by its species synonym *xiphodon* to reflect its difference from the narrow-form, although this is scientifically invalid.

The narrow form has more slender teeth than its broad-form counterpart, which resemble broadened *Isurus oxyrhinchus* teeth. Unlike the broad-form, the narrow form is believed to be the ancestor of two extinct sharks, the hooked-tooth "mako" (*Cosmopolitodus/Isurus planus*), and the serrated "mako" (*Carcharomodus escheri*)

## Taxonomic History and Dispute

The taxonomy of *I. hastalis*, especially the status of its genus, has a messy history and has long been subjected to debate. The initial scientific names was first given as *Oxyrhina hastalis* and *Oxyrhina xiphodon* for the narrow and broad-form variations respectively by Swiss naturalist Louis Agassiz in his 1843 paper *Recherches sur les poissons fossiles*, although some indications show that he had coined the taxon as early as 1838. Throughout the early and mid 20th century, different genera and species of other lamniformes would be clumped into the two taxons as the genus *Oxyrhina* began to be used as a wastebasket taxon. Sharks previously identified as variations of *Oxyrhina hastalis* or *Oxyrhina xiphodon* included two species of extinct ancestral makos *Isurus desori* and *Isurus retroflexus*, the serrated mako (*Carcharomodus escheri*), and the false-toothed mako (*Parotodus benedenii*). Eventually, later studies would begin to show that much of the sharks within the genus and two species are distinct from each other, discarding the genus *Oxyrhina* altogether and creating another issue on what new taxons the sharks should be placed in. As of now, the genus is still uncertain and debated.

Shortly after the discarding of the genus *Oxyrhina*, a review by Holec et al. (1995) placed the species *hastalis* and *xiphodon* as a species of mako under the genus *Isurus*, citing the similarities between the teeth of the two and that of modern mako sharks. In 2001, a study by Ward & Bonavi (2001) considered *Isurus hastalis* and *Isurus xiphodon* as conspecific and declared the latter a *nomen dubium*. Although this conclusion is widely accepted, some scientists disagree, with a study by Whitenack and Gottfried (2010) demonstrating geometrically morphological differences between *I. hastalis* and *I. xiphodon*. The scientific name *Isurus hastalis* is considered as the "traditional view" in the debate regarding the shark's taxonomy.

Ward & Bonavi (2001) reexamined the teeth of *I. hastalis* and noted a strong morphological similarity between it and the extant great white shark. The study concluded that *I. hastalis* is a direct ancestor of the great whiteand is more related to it than other makos. This moved the species into the genus *Cosmopolitodus*, which was a move that was first proposed by Glikman (1964) but was long rejected beforehand. Later discoveries supports Ward & Bonavi's conclusion of its ancestry to the great white. An analysis of a newly discovered *Carcharodon hubbelli* by Ehret et al. (2012) further cemented the theory of *C. hastalis* ancestry to the great white, however also proposed that *Cosmopolitodus* should be moved to *Carcharodon*, remarking that the two genera were separated solely due to the lack of serrations in *C. hastalis* and the lack of lateral cusplets in *C. carcharias* and pointing out examples of Late Miocene *C. hastalis* teeth showing basal serrations. Cione et al. (2012) also noted a possibility of moving all species in the genus *Cosmopolitodus* into the genus *Carcharodon* to avoid a possible paraphyly that would occur if one of them, which was traditionally identified as *C. xiphodon*, is a putative sister species of *C. carcharias*. However, the study also noted that this would only be done if the putative sister species is conspecific with *C. hastalis*. The study concluded that there is indeed a putative sister species of *C. carcharias* distinct from *C. hastalis* and proposed the taxon *Carcharodon plicatilis* for it, resolving the paraphyly issue.

## Paleobiology

### Hunting Behavior

*I. hastalis* was a confirmed hunter of marine mammals. Trace fossils in the form of tooth marks on the bones of a Pliocene dolphin of the species *Astadelphis gastaldii* reveal that *I. hastalis* attacked its prey from below and behind, much as the modern great white shark does. The deepest bite marks on the dolphin's ribs indicate the shark aimed for the abdomen of its prey to inflict a fatal bite quickly and incapacitate its prey, and that when the dolphin was attacked a second time, it was bitten near the dorsal fin, suggesting that the dolphin rolled over while injured. The size of the bites indicates further that the shark responsible was estimated to be 4 m (13 ft) long.

## Extinction

The extinction of *C. hastalis* indicates that this shark died out shortly before the appearance of the modern great white, *C. carcharias*. This may be a result of competition favoring the latter; the serrated teeth of the modern great white may have given it an edge over its larger cousin in regards to efficiency when feeding. However, this is merely speculation, and the true nature of *C. hastalis*' extinction is unknown.

1. ↑ https://www.floridamuseum.ufl.edu/florida-vertebrate-fossils/species/carcharodon-hastalis/
2. ↑ https://www.fossilguy.com/gallery/vert/fish-shark/giant-white/index.htm