| Heteropetalus | |
|---|---|
 | |
| Well-preserved specimen ofH. elegantulus | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Holocephali |
| Order: | †Debeeriiformes |
| Family: | †Debeeriidae |
| Genus: | †Heteropetalus Lund, 1977 |
| Species: | †H. elegantulus |
| Binomial name | |
| †Heteropetalus elegantulus Lund, 1977 | |
Heteropetalus is a genus ofchondrichthyan from theMississippian ageBear Gulch Limestone ofMontana, United States. One species,H. elegantulus, is known.[1] Known fossils ofHeteropetalus are exceptionally well preserved, displaying details of the soft tissue anatomy such as the eyes and anopercular cover over the gills.[2]Heteropetalus is a member of thefamilyDebeeriidae and theorder Debeeriiformes.[3]
Heteropetalus elegantulus was named in 1977 bypaleontologist Richard Lund, based on several fossil specimens collected from theSerpukhovian-stageBear Gulch Limestone ofFergus County, Montana. Theholotype (specimen based on which the species is named) is housed at theUniversity of Montana Museum of Paleontology, and is designatedMV 2778.[1]
The mouth was positioned subterminally (opened at the front of the snout) and may have had a form oflips, although these were not supported bylabial cartilages (skeletal elements in the lips of sharks). While thepalatoquadrates (upper jaws) were initially interpreted ashyostylic and mobile in the genus' description,[1] further research has shown them to have instead been autodiastylic, meaning they are attached tightly to theskull and are immobile.[2][3] Behind the skull were fourgill arches, which were covered by a softoperculum.[1][2][4] The animal had a scaleless,fusiform body and adiphycercal tail fin. Afin spine protruded from the front of the single long dorsal fin that ran down the length of the body. The species grew to 12 centimeters (4.7 in) intotal length.[1]
Heteropetalus wassexually dimorphic, and the males had larger, modifieddorsal fins which supported hook-like organs.[1]
When first described, the genus was identified as a member of theorderPetalodontiformes. Later research by the paleontologist Rainer Zangerl instead found a tentative relation betweenHeteropetalus andDesmiodus, and the two were classified together in the order Desmiodontiformes.[5][6] The genusDesmiodus and the order Desmiodontiformes have subsequently fallen out of use, however.[6] The most recent research by Richard Lund and Eileen Grogan has placedHeteropetalus in the orderDebeeriiformes and the familyDebeeriidae, due to similarities with the genusDebeerius.[3][7]
The hook-like structure on the dorsal fin of maleHeteropetalus likely functioned as aclasper, and was used to grab females during mating.[1]
Like many otherbenthic fish known from the Bear Gulch Limestone, specimens ofHeteropetalus were preserved by loose, finesediments rapidly burying them duringmudslides. TheH. elegantulus buried by the mud then died byasphyxiation, as indicated by theblood vessels in their fossilized gills, and were preserved in exceptional detail.[2][4]
