This is Mazon Monday post #224. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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In keeping with Shark Week on the Discovery Channel, we have a Mazon Creek shark... Holmacanthus keithi for this week's Mazon Monday.
Figure 132.1, a reconstruction of Holmacanthus keithi with the mottled coloration of some present-day shark species; by Rob Sula. Scale bar = 10 mm.
Holmacanthus keithi was named for Keith Holm, who found the holotype in Pit 11 in the early 1990s. H. keithi was described by David Bardack and Ranier Zangerl in Chapter 18 of the "Richardson's Guide to the Fossil Fauna of Mazon Creek". Both Bardack (1932-2019) and Zangerl (1912-2004) worked on fossil fish together for many years, naming more than a few Mazon Creek species.
From the "Richardson's Guide to the Fossil Fauna of Mazon Creek".
Figure 18.16. Holmacanthus keithi n. gen. n. sp., holotype FMNH PF 14309; modified camera lucida drawing. Scale bar = 10 mm. Abbreviations: Af, anal fin; C, chondrocranium; Df1, first dorsal fin; Df2, second dorsal fin; E, eye; Fsp, fin spine; H, hyomandibular; MC, Meckel's cartilage; Pf, pelvic fin; PQ, palatoquadrate; SC, scapulocoracoid.
Holmacanthus keithi new genus and species
Diagnosis. Probable hybodontoid (Zangerl, 1981) with two dorsal fins, the first triangular, the second about twice as long and half as high. Each is preceded by short, barely emergent fin spines in this juvenile specimen. Spines vertically ridged. Short based, distally elongate anal fin. Palatoquadrate deep with elongate otic process.
Holotype. FMNH PE 14309, one half of part and counterpart, complete specimen of young individual, Pit 11.
Etymology. Generic name derived from the last name of the collector, Keith Holm, and "acanthus," common name component of sharks. Specific name derived from given name of the collector.
Description. This species known at present from a single specimen, measures 5.7 cm in length from the tip of the snout to the end of the tail. The individual is viewed from a three- quarter dorsal perspective anteriorly and from a lateral aspect more posteriorly. Depth at the first dorsal fin is 1.3 cm. The head is bluntly rounded, and from its anterior tip to the rear of the scapulocoracoid measures one-third of the length of the fish. The two eyes are seen as prominent, slightly ovate, very darkly pigmented structures in about their natural position relative to the chondrocranium. Anterior to the eyes and around as well as below them and more posteriorly are less distinctive and less dark shadows representing the chondrocranium, which still has not calcified. Obtusely rounded at the anterior end, this structure narrows between the eyes and then expands broadly behind them. Paired dark bodies behind the eyes but within the chondrocranium probably represent the otic capsules. The chondrocranium appears to be fairly deep as shown by continuous pigment patterns below and around the eyes. Posterior to the braincase several dark, somewhat ill-defined structures may be parts of the branchial arches. The mandibular arch (palatoquadrate and Meckel's cartilage) is distinguished by irregularly shaped polygonal remains indicating an early stage of cartilage calcification. The palatoquadrate has a deep, anteroposteriorly extended otic region and probably tapers anterior to this process more gradually than in ctenacanthid sharks. But the anterior end of this arch is not well preserved. The palatoquadrate is closely applied to the Meckel's cartilage, which seems most distinct where these cartilages articulate. The location of the mandibular arch, even without well-preserved anterior ends, suggests that the mouth was sub-terminal in position. Some faint impressions along the anterior end of the Meckel's cartilage suggest the early development of dentition. An elongate, rather flat, calcified bar extends from near the posterior end of the chondrocranium across the upper part of the palatoquadrate. This is probably the slightly displaced hyomandibular. Behind the chondrocranium and palatoquadrate, a large, dark, gently curved bar represents the scapulocoracoid. At the bend of this structure, a few indistinct, dark stains presumably are basipterygia of the pectoral fin.
There are two dorsal fins. The one in front is gently triangular in outline; the one behind is half its height but twice its length. Each is preceded by a fin spine, the anterior spine is shorter than the posterior one. Apparently, only the small, slightly falcate tips of these spines emerged from the muscle in which they were embedded. No supporting basil cartilages are evident. The caudal fin is asymmetric. The anal fin, narrow and about twice the length of its base, is nearly in contact with the caudal fin. Of the paired fins, the pectoral is not preserved. The pelvic fin arises below and near the end of the second dorsal fin. Dermal denticles are possibly indicated by impressions along the body surface, but these have, at this stage of development, no distinctive feature and are about the same size as the graininess of the matrix surface.
The abdominal cavity contains several fragments some of which may be bony, but none is identifiable. These remnants do demonstrate that Holmacanthus even at this small size was actively feeding.
Discussion. The new elasmobranch, Holmacanthus, adds another species to the growing list of Mazon Creek chondrichthyans. Although small in size and providing limited diagnostic features, it clearly differs from other chondrichthyans such as Polysentor, Similihariotta, Jimpohlia, and the elasmobranch Bandringa from the Mazon Creek biota in overall shape, fin characteristics, and head features as far as these may be examined among these genera. A fifth Mazon Creek elasmobranch, Dabasacanthus, has, broadly speaking, a similar body shape, but its dorsal fins and spines are much larger with coarser longitudinal costae, Also, in contrast to Holmacanthus, the otic portion of the palatoquadrate in Dabasacanthus is much lower and shorter. Few young individuals of Pennsylvanian chondrichthyans have been described. Preserved elasmobranchs from localities of Pennsylvanian age (Maples and Schultze, 1988) such as the Kinney Quarry in New Mexico or Hamilton Quarry in Kansas, and the Indiana black shale location (Zangerl, 1981) are represented by larger, postlarval or juvenile stages with distinctive dentitions, fin spines, and often chondrocranial features commonly used for comparing elasmobranchs.
H. keithi appears on page 132 in Jack Wittry's "The Fossil Fauna of Mazon Creek".
Holmacanthus keithi Bardack, 1997b
Holmacanthus keithi is a juvenile shark with a poorly calcified skeleton. Just preceding its fins are barely emerging spines. It has two dorsal fins; the first is triangular. The second, located directly behind the first, is twice as long and half as tall. The bluntly rounded head is roughly about one-third the length of the body. The well-marked eyes are closely spaced and oval.
Even as juveniles, H. keithi may have been active predators. What appear to be bony fragments have been found in the gut area of some specimens.
Specimens
Field Museum FMNH PE 14309, one half of the specimen.
The other half is displayed at "Dave's Down to Earth Rock Shop". This photo came from a very informative Fossil Forum post by Ralph Jewell.
