Estesesox: a small fish on the early pike lineage

Estesesox foxi was a small freshwater fish on the evolutionary lineage leading toward pikes. It is known from isolated jaw and palatal bones, with vertebrae referred to the genus in later work. The fossils range from western North American Cretaceous deposits into the earliest Paleocene, making the genus a useful record of freshwater fish across the end-Cretaceous extinction. Its evidence belongs among the varied records in the ancient fish catalogue, though it does not preserve a complete body.

Quick facts

A name for fossils once assigned elsewhere

Small dentaries from western North American freshwater deposits were initially associated with Platacodon nanus, a fish named from isolated teeth. In 1992, Mark Wilson, Donald Brinkman and Andrew Neuman re-examined the jaws and palatal bones and described Estesesox foxi as an early member of the pike lineage. The new genus honours palaeontologist Richard Estes; the species name recognises Richard Fox, whose collections included material from the Milk River, Foremost and Oldman formations.

The holotype, UCMP 53709, is a nearly complete left dentary from locality Lull 2 in Wyoming’s Lance Formation. Its rear edge is missing and the underside is damaged, but the tooth-bearing surface and much of the bone remain readable. It is a jaw fossil, not a miniature version of a complete pike skeleton. This distinction matters because familiar modern pikes provide comparisons for particular bones but cannot supply the unknown proportions of Estesesox.

How the jaw and palate identify the fish

The dentary is elongated and has a low coronoid process. Near its symphysis, up to five rows of small teeth are present; the innermost row extends farther back while outer rows shorten. Their C-shaped bases allowed the teeth to move slightly under pressure. Researchers used this feature together with the jaw outline and palatine anatomy to distinguish the fossils from unrelated fishes and to place them near Esocidae.

There is also a short transverse canal for a branch of the trigeminal nerve below the tooth margin. Its position differs from the more posterior, curved arrangement in living Esox. The palatine is broad in some Campanian specimens and carries numerous longitudinal tooth rows directed toward the throat. These details help explain the classification; they do not reveal soft tissues, colour or the exact way the fish captured prey.

Variation through time and growth

Specimens from the Milk River Formation in Alberta provide a late Santonian record. More abundant material comes from Campanian Foremost, Oldman and Dinosaur Park deposits. Maastrichtian fossils are known from the Hell Creek and Lance formations in Montana and Wyoming. The range therefore combines several freshwater systems across a broad region, rather than one locality or a single fossil bed.

Some Maastrichtian dentaries have different numbers of tooth rows at the front. These were once treated as evidence for more than one fish, but a larger series showed a pattern consistent with growth. Likewise, vertebral forms that had appeared taxonomically distinct were interpreted as coming from different parts of one vertebral column. Late Campanian and Maastrichtian palatines still differ in width and tooth-row number, however, and isolated specimens do not establish whether age, growth or lineage change explains that pattern.

A fish across the Cretaceous–Paleogene boundary

Lower Paleocene fossils from the Tullock Member of Montana’s Fort Union Formation include a dentary, palatines and vertebral centra referred to Estesesox. The jaws closely resemble latest Maastrichtian examples. Because these fossils occur in the earliest Paleocene, they provide more specific evidence of persistence across the boundary than an unidentifiable fish bone would. Their fragmented state still prevents a detailed account of population size or ecological recovery.

The teeth are compatible with gripping small aquatic prey, but no gut contents or associated bite marks identify a menu. A streamlined, ambush-hunting fish like a modern pike is a plausible comparison, not a directly preserved behaviour. Even total length remains unmeasured because no complete skeleton is known. The fossils establish a small fish with a pike-like dental and jaw pattern; the remaining body plan should stay visibly provisional.

Explore related evidence in the ancient fish catalogue.