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Paraenhydrocyon and Hesperocyon

Tuesday, July 14, 2026 13:22

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These two extinct taxa,
Paraenhydrocyon and Hesperoocyon grangeri (Fig 1), had several traits (including that overall size difference) that tentatively indicated a mature/immature relationship – except for the teeth. The former and larger taxon had a relatively smaller orbit, smaller teeth and more prominent nuchal crest, as expected in an adult. The latter and smaller taxon had a larger orbit, larger teeth and rounder, softer elements, as seen in other mammal juveniles (Fig 2).

Unfortunately the two extinct taxa were not coeval. The smaller one preceded the larger one by ten million years = the entire Oligocene (33–23 mya). So one is NOT the pup of the other.

Instead, this an example of Cope’s Rule.

Figure 1. Af left: Late Eocene Hesperocyon not to scale. At middle: Hesperocyon to scale. At right Paraenhydrocyon to scale.

Paraenhydrocyon josephi
(Wang 1994; originally Temnocyon wallovianus Cope 1881; Early Miocene; North America, 22mya) was a hypercarnivore nesting with Hesperocyon. Based on the angle of the occiput, the nose drops, the face rises, the eyes point more anteriorly, the canines are sabers.

Hesperocyon gregarius
(Scott 1890, Wang 1994; Late Eocene to Early Oligocene, 38mya, 80cm in length, USNM 437888) was traditionally considered a basal canid or dog.

Ontogeny
Like birds and other archosauromorphs in the LRT, mammal neonates and juveniles have ‘cute’ skull proportions featuring a large orbit housing a large eyeball, a relatively larger cranium and a shorter snout.. These distinct traits represent the first stage in their allometric growth.

If Paraenhydrocyon pups were similar to Hesperocyon adults, this would be a case of ontogeny recapitulating phylogeny.

(Fig 2. Monodelophis domestica grown series = ontogeny. Numbers (e.g. 27) refer to days after birth. In this pouchless taxon, the young ride on the back of the mother. Note the tooth number formula does not change here except for the number of molars. That number starts at two and rises to four. Also note the size of the teeth relative to the skull: larger in the younger specimens.

Allometry in Archosauromorpha
makes phylogenetic analysis more difficult because juveniles have different structures and proportions than adults.

On the other hand in Lepidosauromorpha, allometry is not so prevalent. When hatchlings look like adults, except 8x smaller, as in pterosaurs, this is called Isometry, a different growth strategy indicated by a minimum of change.

Figure 3. Asioryctes is basal to Leptictis in the LRT and retains small, slender canines (orange).

Mammal taxa preceding Didelphis and Monodelphis in the LRT
generally at 4 or more premolars and often 4 molars (see Asioryctes Fig 3). Didelphis and Monodelphis have three premolars and four molars as adults. Loss of the smallest premolar in Asioryctes is suspected, but not yet indicated.

References
Scott 1890 WB. Hesperocyon. The dogs of the American Miocene. Princeton Coll. Bull., 2(2):37–39.
Wang X 1994. Phylogenetic systematics of the Hesperocyoninae. Bulletin of the American Museum of Natural History. 221: 1–207.

wikiParaenhydrocyon
wiki/Hesperocyon
wiki/Recapitulation_theory

Source: https://pterosaurheresies.wordpress.com/2026/07/14/paraenhydrocyon-and-hesperocyon/

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