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Hexagonaria
Hexagonaria
Hexagonaria
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Hexagonaria
Temporal range: Devonian 416–359.2 Ma
Hexagonaria percarinata
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Cnidaria
Subphylum: Anthozoa
Class: †Rugosa
Order: †Stauriida
Family: †Disphyllidae
Subfamily: †Hexagonariinae
Genus: †Hexagonaria
Gürich, 1896
Species
  • H. anna
  • H. cristata
  • H. fusiformis
  • H. percarinata
  • H. alpenensis
  • H. subcarinata
  • H. attenuate
  • H. potterensis
  • H. profunda
  • H. mirabilis

Hexagonaria is a genus of colonial rugose coral. Fossils are found in rock formations dating to the Devonian period, about 350 million years ago. Specimens of Hexagonaria can be found in most of the rock formations of the Traverse Group in Michigan. Fossils of this genus form Petoskey stones, the state stone of Michigan.[1] They can be seen and found in most Midwestern U.S. states.

Hexagonaria is a common constituent of the coral reefs exposed in Devonian Fossil Gorge below the Coralville Lake spillway[2] and in many exposures of the Coralville Formation in the vicinity of Coralville, Iowa.[3]

Species Identification

[edit]

Based on Erwin C. Stumm [de]'s Corals of the Traverse Group of Michigan Part 13, Hexagonaria, published in 1970.[1] Notably, other species of Hexagonaria occur in rocks outside the Michigan Basin; those other species are not covered by the chart below and may overlap with the species below in size and numbers of septa. Other features diagnose the differences between species; the metrics below merely represent handy features for diagnosis in the Michigan Basin, not comprehensive diagnoses.

Species Corallites (diameter) Septa (count)
H. anna 12–16 mm 32-44
H. cristata 16–20 mm 36-44
H. fusiformis 6–10 mm 34-38
H. percarinata ~10 mm 38-40
H. alpenensis 2–6 mm 26-28
H. subcarinata 8–12 mm 30-34
H. attenuate 8–14 mm 30-38
H. potterensis 4–15 mm 36-38
H. profunda 13–15 mm 38-42
[edit]

References

[edit]

Further reading

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Hexagonaria

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from Grokipedia
Hexagonaria is a genus of extinct colonial rugose corals belonging to the order Rugosa, which thrived during the Middle Devonian period, approximately 393 to 382 million years ago. These corals are distinguished by their massive colonies composed of tightly packed, hexagonal corallites—individual tubes housing polyps—that exhibit prominent septa and tabulae for structural support. Fossils of Hexagonaria are widely distributed in Devonian rock formations across North America, Europe, and Asia, with the most famous examples forming the Petoskey stone from the species H. percarinata, a glacially polished fossil commonly found along Lake Michigan shores and designated as Michigan's state stone. The genus encompasses numerous species, such as H. percarinata, H. anna, and H. fisherae, each adapted to shallow marine environments like reefs and lagoons where colonial growth facilitated and . Rugose corals like Hexagonaria constructed their skeletons from , featuring a three-layered intercorallite that enhanced colony integrity, a trait evident in thin-section analyses of specimens. Their hexagonal patterning, often highlighted through polishing, not only aids in identification but also reflects the evolutionary efficiency of space utilization in colonial forms during the diversification of . Paleontological studies highlight Hexagonaria's role in Devonian ecosystems, where it contributed to reef-building alongside other rugosans and contributed to the peak before the Late Devonian extinction events. Today, Hexagonaria fossils serve as important index fossils for correlating Middle Devonian strata and are prized by collectors for their aesthetic appeal in polished forms.

Description and Morphology

General Characteristics

Hexagonaria is a genus of extinct colonial rugose corals in the class , phylum . These corals are characterized by interconnected hexagonal corallites, forming colonies where multiple polyps coexisted within a shared skeletal framework. Unlike solitary rugose corals, the colonial habit of Hexagonaria allowed for collective growth and resource sharing among polyps. Colonies of Hexagonaria typically exhibited encrusting or massive forms, with diameters reaching up to 30 cm or more. The basic anatomy consisted of a skeleton secreted by the polyps, featuring septa as radial plates that divided the interior of each corallite. This structure supported the polyps' tentacles for capturing prey in ancient marine environments. Their overall form bears a superficial resemblance to modern scleractinian corals, though the two groups are evolutionarily distinct. Hexagonaria existed exclusively during the Period of the Era, approximately 416 to 359 million years ago.

Corallite Structure

The corallites of Hexagonaria are characteristically hexagonal prisms, typically measuring 2–20 mm in diameter, and are tightly packed in a cerioid, honeycomb-like colonial arrangement that shares common walls between adjacent units. This polygonal geometry provides structural efficiency in the compact colonies, distinguishing the genus from other rugose corals with more rounded or irregular corallite shapes. Internally, the corallites feature thin, radial numbering 20–44 per corallite, which are often long and continuous from but short enough not to fully meet at the center, forming a stellate that enhances skeletal support without a continuous axial fusion. These may exhibit wavy or carinate (ridged) margins, contributing to the genus's diagnostic trabecular microstructure. Horizontal tabellae, present as flat to slightly arched partitions, divide the corallite interior into compartments, aiding in compartmentalization and providing additional stability during growth. An axial , or central pillar, occurs in some specimens, offering enhanced stability in the corallite axis, though it is absent in others where the remain discontinuous. Wall thickness varies, with a thin external epitheca enclosing the structure, while dissepiments—small, globose to elongate vesicles—fill the peripheral spaces between corallites, forming multiple columns that are flat or inclined toward the axis.

Taxonomy and Classification

History of Classification

The genus Hexagonaria was originally described by Georg Gürich in 1896, based on specimens from deposits in the Holy Cross Mountains of Poland, where he erected it to accommodate colonial rugose corals previously assigned to names such as Lithodendron and Cyathophyllum hexagonum. Earlier descriptions of similar fossils from , including Michigan's strata, appeared in Alexander Winchell's 1863 report on the state's geological survey, which documented colonial corals from the Marshall and Huron groups without formal generic assignment. Significant advancements in classification came in the mid-20th century, particularly through Erwin C. Stumm's detailed 1970 on the corals of Michigan's Traverse Group, which clarified species-level distinctions within Hexagonaria and emphasized morphological variations in corallite arrangement and septal development. Stumm's work built on prior revisions, such as those distinguishing Hexagonaria from related genera through its characteristic hexagonal corallite patterns and carinate . Taxonomic revisions in the late placed Hexagonaria within the family Disphyllidae and the subfamily Hexagonariinae (established by Bulvanker in 1958), reflecting its shared traits with other colonial rugosans like dissepiment development and colony integration. Debates persisted regarding synonymy with genera such as Billingsastraea, particularly over wall structure and carina morphology, with some early studies proposing merger based on overlapping features in North American faunas, though later analyses upheld separation. Today, is widely accepted as a valid within the extinct class , with phylogenetic studies employing cladistic methods to explore its relationships to other corals, highlighting evolutionary trends in coloniality and septal complexity across paleogeographic realms.

Recognized Species

The genus encompasses several valid species from Middle deposits, predominantly in , distinguished by variations in corallite morphology and septal development. A prominent North American species is H. percarinata, widely recognized as the fossil forming the , while others such as H. anna, H. cristata, H. fusiformis, H. fisherae, H. alpenensis, H. subcarinata, H. attenuata, H. potterensis, H. profunda, and H. mirabilis occur in associated formations. These species exhibit cerioid colonial growth with hexagonal corallites, and identification relies on metrics like corallite diameter and septal count, alongside features such as carinae and width. Hexagonaria percarinata (Sloss, 1939; emended Stumm, 1970), the dominant species in , features mature corallites averaging 10 mm in diameter with 30–34 , marked by prominent yardarm carinae and a false inner wall formed by crowded dissepiments; its epithet derives from the intensely keeled ("percarinata") septa resembling perforated ridges. It is common in the Traverse Group, particularly the Gravel Point Formation, and some earlier names like Prismatophyllum percarinatum are junior synonyms. Hexagonaria anna (Whitfield, 1882; emended Stumm, 1948) has corallites 9–16 mm in diameter (average 10 mm) and 36–40 septa of equal length extending halfway to two-thirds toward the axis, with narrow peripheral platforms and incomplete tabulae; synonyms include Stylastraea anna and Prismatophyllum annum. It occurs in the Dundee and Columbus limestones of southeastern Michigan and northwestern Ohio. Hexagonaria cristata is characterized by larger corallites (16–20 mm diameter) and 36–44 septa, with strongly developed zigzag carinae; it is found in Traverse Group equivalents in Michigan. Hexagonaria fusiformis exhibits smaller corallites (6–10 mm diameter) and 34–38 , with fusiform colony shapes and attenuated ; distributed in Middle strata of the . Hexagonaria fisherae is known from the H. fisherae group, featuring yardarm-carinate forms common in certain facies. Hexagonaria alpenensis, H. subcarinata, H. attenuata, H. potterensis, H. profunda, and H. mirabilis are less common but valid, varying in corallite size (8–15 mm) and septal counts (32–42), often with regional endemism in Michigan's Traverse Group; for instance, H. subcarinata shows subcincta-like thin and aulate structures, while H. goldfussi (Goldfuss, 1826) serves as a junior synonym for aspects of the type species H. hexagona in some classifications.
SpeciesCorallite Diameter (mm)Septa Count
H. percarinata~1030–34
H. anna9–1636–40
H. cristata16–2036–44
H. fusiformis6–1034–38
H. fisherae8–1232–36
H. alpenensis8–1232–36
H. subcarinata10–1434–38
H. attenuata7–1132–40
H. potterensis9–1336–42
H. profunda12–1538–42
H. mirabilis10–1434–40
These metrics provide key identification criteria, with overall ranges of 6–20 mm diameter and 26–44 septa across the genus; H. percarinata dominates Middle Devonian outcrops in Michigan, while others like H. anna extend to Ohio and Indiana.

Paleobiology and Ecology

Growth and Reproduction

Hexagonaria colonies expanded through asexual budding, in which new polyps developed from parent corallites, forming interconnected structures within a shared calcium carbonate skeleton. This process allowed for gradual colony growth, with corallites maintaining close physical contact characteristic of cerioid colonial rugose corals. Annual growth bands, visible in some fossil specimens, reflect seasonal variations in growth, with dissepiment patterns showing alternating dense and sparse layers. Colony development in Hexagonaria typically began with an initial encrusting stage on suitable substrates, transitioning to more massive, hemispherical forms as continued. Growth rates, inferred from banding patterns in rugose corals, are estimated at approximately 10 mm per year, indicating relatively slow skeletal accretion compared to some modern scleractinian corals. Reproduction in Hexagonaria occurred through both asexual and means, akin to other colonial rugose corals. involved offset , where daughter corallites formed peripherally or intermurally from existing ones, contributing to maintenance and expansion. is inferred from the group's biology, with gametes released into the water column for , producing larvae that settled to initiate new ; direct evidence for this is morphological rather than explicit. sites often aligned with corallite walls, briefly referencing the hexagonal morphology that facilitated efficient packing.

Habitat and Paleoecology

Hexagonaria colonies inhabited shallow, warm marine waters of the Devonian seas, typically at depths ranging from 0 to 50 meters, within tropical reef systems and lagoons situated on carbonate platforms. These environments were characterized by clear, sunlit conditions near the equator, fostering robust growth in stable, well-oxygenated settings with normal marine salinity. As primary reef builders, Hexagonaria played a crucial role in constructing framework structures within bioherms, often forming dense aggregations that contributed to stability and expansion. They coexisted and interacted closely with stromatoporoids, which dominated reef frameworks alongside rugose corals, as well as brachiopods, , trilobites, cephalopods, and other corals, creating diverse benthic communities. The presence of symbiotic relationships with photosynthetic in Hexagonaria is debated; while earlier inferences suggested possible mutualism for nutrient provision, a 2024 study using coral-bound isotopes found no evidence of photosymbiosis in cerioid colonial rugose corals like Hexagonaria, unlike in tabulate and some dendroid forms. In these reef ecosystems, Hexagonaria competed for space and resources with tabulate corals, both groups vying for optimal positions in the photic zone. The genus was adapted to oxygenated conditions but showed vulnerability during episodic anoxic events in the Late Devonian, contributing to broader declines in rugose coral assemblages. Evidence from skeletal repairs in related rugose corals indicates exposure to physical damage, potentially from predators, though specific interactions remain understudied. Colony growth forms, such as branching or encrusting morphologies, facilitated integration into these dynamic communities.

Fossil Record and Distribution

Geological Age and Stratigraphy

Hexagonaria, a genus of colonial rugose corals, is primarily known from the Middle to Late Devonian period, spanning the Givetian and stages approximately 390 to 372 million years ago. This temporal range reflects its evolutionary peak during a time of diverse reef-building activity in shallow marine environments, with the earliest appearances in the Givetian and persistence into the . Stratigraphically, Hexagonaria fossils are abundant in carbonate-dominated sequences, particularly and dolomite beds that represent platform and reefal . Key North American formations include the Traverse Group in and the Bell in , where well-preserved colonies occur in biostromal and biohermal layers. Equivalent strata in , such as those in the Dinant and Namur basins of , contain similar coral-bearing carbonates and shales, facilitating transatlantic correlations. The plays a significant role in biozonation, with species assemblages used to correlate stratigraphic layers across basins due to their stratigraphic utility from the Couvinian substage of the Givetian through the upper . For instance, Hexagonaria percarinata is characteristic of Middle horizons. Hexagonaria experienced a decline during Late Devonian biotic crises, with major turnovers at the Middle-Upper Devonian boundary and the end of Frasnian substage 1, leading to its full extinction by the end-Frasnian Kellwasser event. This extinction pattern aligns with broader rugose coral losses amid global environmental perturbations.

Global Occurrence

Hexagonaria fossils are primarily distributed across North America, Europe, and North Africa, reflecting the widespread Devonian reef environments in which these colonial rugose corals thrived. In North America, the genus is most abundant in the Midwestern United States, particularly within the Traverse Group formations of Michigan, where specimens are commonly exposed along Lake Michigan and Lake Huron shorelines near Petoskey and in quarries around Alpena. Notable discoveries include Hexagonaria alpenensis in the Alpena Limestone, a key reefal unit yielding well-preserved colonial forms. Further south, in Iowa, Hexagonaria forms a significant component of coral reefs at the Devonian Fossil Gorge near Coralville Lake, where erosion from a 1993 flood revealed extensive bedding planes rich in these fossils. In Ohio, specimens occur in the Columbus Limestone of central regions, such as roadcuts in Dublin, often preserved in carbonate matrices. In , Hexagonaria exhibits a broad distribution tied to platform carbonates, with key sites in the Dinant and Namur basins of , including the Philippeville region and abandoned quarries at Engis and Neuville. records, particularly in the Federal Republic of (FRG) stratigraphic equivalents, document species like H. quadrigemina in Givetian strata, while English occurrences are noted in outcrops, though less extensively documented than continental sites. Asian finds, including in various formations, extend the genus's range eastward, with reports from unspecified localities contributing to its cosmopolitan paleogeography. North African occurrences center on Morocco's reefs, where diverse Hexagonaria species are recovered from regions like the Hamar Laghdad area and in the , often in commercial fossil yielding polished specimens. Preservation typically involves silicification, as seen in Michigan's Petoskey stones where replaces original , or calcitization in hosts elsewhere, enhancing visibility in pebbles and quarry exposures. Collection efforts began in the with early discoveries from Midwestern quarries, such as those in Michigan's Rockport area, where laborers unearthed large colonies during limestone extraction. Modern collections benefit from natural erosion events, like floods exposing sites in , and ongoing quarrying in , facilitating access to abundant, well-preserved material.

Cultural and Scientific Significance

Relation to Petoskey Stone

The is a type of silicified derived from the colonial rugose Hexagonaria percarinata and closely related species, commonly collected as polished pebbles from the beaches of , particularly in the northern Lower Peninsula region. These stones represent the preserved skeletal structures of ancient colonies that formed extensive reefs in shallow seas. During , the original skeleton of H. percarinata undergoes and replacement, where silica-rich fluids infiltrate the porous , gradually substituting the with , a variety of . This process, occurring over millions of years within the sedimentary layers of the Traverse Group, particularly the Gravel Point Formation, transforms the fragile into durable, patterned nodules resistant to . The resulting stones exhibit a mottled brown-gray coloration due to impurities and varying silica densities, with the hexagonal corallite patterns—reflecting the six-sided tubular structure of the coral polyps—becoming prominently visible only after or . In 1965, the was officially designated as Michigan's state stone through Public Act 89, signed into law by Governor George Romney on June 28 of that year, recognizing its geological significance and cultural ties to the region's Native American heritage, including its naming after the leader Chief Petosegay. In 2025, it marked the 60th anniversary of this designation. Originating from Middle deposits approximately 385 million years old, these fossils provide a tangible link to ancient marine ecosystems while serving as identifiable markers for amateur geologists seeking them along glacial drift lines in areas like Petoskey and Charlevoix.

Paleontological and Economic Value

Hexagonaria fossils hold significant paleontological value as index fossils for biostratigraphic in the Middle period, particularly within zonation schemes that facilitate precise dating and regional mapping of sedimentary layers. Species such as Hexagonaria profunda define biofacies zones that reflect environmental variations, enabling to reconstruct depositional settings across and beyond. Their abundance in specific stratigraphic units, like the Delaware Limestone, underscores their utility in confirming ages for fossil-bearing rocks. In terms of evolutionary and ecological insights, Hexagonaria exemplifies the rapid diversification of rugose corals during the Givetian stage of the Middle , with morphological adaptations like yardarm carinae indicating responses to changing marine conditions. These colonial forms contributed to reef ecosystems, forming part of biohermal structures in shallow, tropical seas where they interacted with stromatoporoids and other reef builders. Studies utilizing Hexagonaria distributions have informed paleoecological reconstructions, revealing sensitivity to facies shifts that parallel broader climate and sea-level fluctuations in the . Additionally, analyses of rugose coral assemblages, including Hexagonaria, illuminate patterns during the Late (Frasnian-Famennian) event, highlighting declines in colonial forms amid global biotic crises. Economically, Hexagonaria specimens support a in polished fossils, with material sourced from Devonian deposits in traded internationally for decorative and educational purposes. In , where Hexagonaria percarinata forms the state's official stone known as , the activity drives tourism through beach hunting along shores, bolstering local economies in areas like Petoskey and Charlevoix. This rockhounding attracts visitors year-round, contributing to jewelry sales and seasonal revenue in communities. Conservation efforts focus on mitigating overcollection threats, as unregulated gathering could deplete accessible outcrops. Michigan regulations limit individuals to 25 pounds of rocks or fossils annually from state-owned lands, including bottomlands, with enforcement including confiscations of excess material. These measures protect sites vital for both and public enjoyment, while broader preservation addresses and development pressures on exposures.

References

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