Albertonectes

Albertonectes is an extinct genus of elasmosaurid plesiosaur that lived during the Upper Campanian stage of the Late Cretaceous period, approximately 72 million years ago, in the marine environments of what is now western North America.^[1] Known from a single, nearly complete postcranial skeleton (TMP 2007.011.0001) lacking a skull, it represents one of the most well-preserved elasmosaurid specimens and is distinguished by possessing the longest neck among all known plesiosaurs, with 76 cervical vertebrae measuring about 7 meters in length.^[1] The total body length of the holotype is estimated at 11.2 meters, making it one of the largest elasmosaurs.^[1] The specimen was discovered in the Bearpaw Formation of southern Alberta, Canada, by amateur paleontologists Mike Parks and Robin Cox in 2007, and formally described in 2012 as Albertonectes vanderveldei, honoring the discoverers' friend Levi Vandervelde.^[1] This elasmosaurid exhibits several diagnostic features, including a wide clavicular arch, a lateral longitudinal ridge on the posterior cervical vertebrae, and a slender humerus, placing it within a derived clade of long-necked plesiosaurs from the middle to late Late Cretaceous.^[1] The presence of 97 gastroliths (stomach stones) in the abdominal region, ranging from less than 1 cm to 13.5 cm in diameter, suggests these were ingested to aid digestion or buoyancy control, possibly near coastal areas rich in chert pebbles.^[1] Additionally, the coracoid bone bears tooth marks from scavenging by the shark Squalicorax sp., indicating post-mortem predation shortly after death.^[1] As a piscivorous marine reptile, Albertonectes likely inhabited the Western Interior Seaway, a vast inland sea dividing North America at the time, where it may have foraged for soft-bodied or bottom-dwelling prey using its elongated neck, though the neck's relative stiffness suggests it was not highly flexible for rapid strikes.^[1]^[2] Its discovery highlights the diversity of elasmosaurids in the Late Cretaceous and provides insights into the evolutionary trends toward extreme neck elongation in this group, surpassing even the previously record-holding Elasmosaurus with 71 cervical vertebrae.^[1]^[2]

Discovery and naming

Geological context

The holotype specimen of Albertonectes vanderveldei was discovered in 2007 during open-pit mining operations for gem-quality ammolite (fossilized ammonite shells) conducted by Korite International Ltd., south of Lethbridge in southern Alberta, Canada.^[1] The remains were recovered from the Bearpaw Formation, a predominantly marine shale unit that forms part of the Upper Cretaceous succession in the Western Canadian Sedimentary Basin.^[1] The Bearpaw Formation dates to the upper Campanian stage of the Late Cretaceous, corresponding to an age of approximately 74.3–73.1 million years ago.^[3] This formation records the final major marine incursion of the Western Interior Seaway, a shallow epicontinental sea that periodically flooded the North American interior during the Late Cretaceous.^[4] The depositional environment was characterized by low-energy, open-marine conditions during a transgressive phase, with sediments primarily consisting of dark gray to black shales interbedded with bentonite layers (volcanic ash deposits) and minor thin sandstones, indicative of quiet offshore waters with periodic sediment input from volcanic activity and distant shorelines.^[4]^[5]

Etymology and type material

The genus name Albertonectes is derived from "Alberta," referring to the Canadian province where the holotype was discovered, combined with the Greek word nêktês, meaning "swimmer," in reference to its marine lifestyle. The species name vanderveldei honors the late René Vandervelde, founder of Korite International, the mining company whose operations led to the discovery of the specimen.^[1] The holotype specimen, cataloged as TMP 2007.011.0001, consists of an almost complete postcranial skeleton collected in 2007. It is missing only the skull, the left forelimb, and some distal elements of the hind limbs, preserving 76 vertebrae, a partial pectoral girdle, both pelvic girdles, and most of the limb elements. This specimen was formally described and named in 2012 by Tai Kubo, Mark T. Mitchell, and Donald M. Henderson in the Journal of Vertebrate Paleontology. It is housed at the Royal Tyrrell Museum of Palaeontology in Drumheller, Alberta, Canada.^[1]

Description

Overall morphology

Albertonectes vanderveldei was a large elasmosaurid plesiosaur, estimated to have reached a total body length of 12.1 meters (40 feet) and a mass of approximately 4.8 metric tons based on volumetric reconstructions.^[6] The animal exhibited a streamlined body plan characteristic of advanced elasmosaurids, adapted for efficient aquatic life in the Late Cretaceous Western Interior Seaway. This included a proportionally small head, inferred from the overall skeletal proportions preserved in the holotype specimen, a markedly elongated neck, a robust and compact torso supported by broad ribs, four well-developed paddle-like limbs serving as primary propulsive structures, and a relatively short tail that contributed minimally to overall length.^[7] The limb girdles and appendages were specialized for subaqueous locomotion, featuring hyperphalangy in the digital rows of the paddles, which increased the surface area for thrust generation. Propodials (humeri and femora) were elongated and robust, providing leverage, while epipodials (radii, ulnae, tibiae, and fibulae) were similarly extended and broadened to facilitate the expansive, wing-like motion typical of plesiosaur swimming. These adaptations underscored the animal's reliance on undulatory and oscillatory limb movements for propulsion in marine environments. The general body proportions highlighted the neck's dominance, comprising about 58% of the total length and underscoring its central role in the taxon’s morphology, with the holotype preserving a record number of cervical vertebrae that further accentuates this feature (detailed in the Axial skeleton section).^[7]^[6]

Axial skeleton

The axial skeleton of Albertonectes vanderveldei is characterized by an exceptionally high total vertebral count, including 76 cervical vertebrae, 22 dorsal vertebrae, 5 sacral vertebrae, and 33 caudal vertebrae. This configuration underscores the extreme elongation typical of elasmosaurid plesiosaurs, with the postcranial skeleton preserved nearly completely in the type specimen (TMP 2007.011.0001).^[1] The cervical vertebrae, numbering 76, represent the greatest number known among all vertebrates, surpassing the 71 cervicals of its close relative Elasmosaurus platyurus and exceeding counts in other elasmosaurids such as Styxosaurus (typically 50–60).^[1] This record-breaking series results in a neck length of approximately 7 meters (23 feet), comprising over half the animal's estimated total body length of 12.1 meters. The cervical centra are notably elongated and shallowly amphicoelous, featuring low neural arches, prominent lateral longitudinal ridges on the posterior elements, and ventral notches; their size increases gradually from anterior to posterior, enhancing overall neck extension without abrupt transitions.^[1] The dorsal vertebrae, totaling 22, exhibit shorter centra compared to the cervicals, with the anterior ones retaining a ventral notch similar to the neck region; neural spines are damaged in the holotype but indicate a relatively short trunk. The 5 sacral vertebrae articulate closely with the ilia, supporting the pelvic girdle in a manner consistent with elasmosaurid adaptations for aquatic locomotion. The 33 caudal vertebrae form a short tail, with the distal elements showing chevron facets and reduced haemal arches, tapering to a fused tip that contributes minimally to propulsion. This vertebral arrangement, particularly the profuse cervical count, implies adaptations for greater neck flexibility and foraging reach in open marine habitats, allowing Albertonectes to access prey at extended distances while maintaining a streamlined body profile.

Gastroliths

Ninety-seven gastroliths were preserved in the abdominal region of the holotype specimen of Albertonectes vanderveldei, visible in situ within the articulated postcranial skeleton. These stones, primarily composed of chert, ranged in size from small pebbles with diameters less than 1 cm to larger examples up to approximately 14 cm in diameter. The largest individual gastrolith weighed an estimated 1.13 kg.^[8] The gastroliths were clustered in the gut area, suggesting they were ingested intentionally by the animal rather than displaced post-mortem. Their total estimated mass was approximately 9.3 kg, representing less than 0.2% of the animal's body mass.^[8] This arrangement and quantity indicate the stones served functions such as grinding ingested food in the stomach or aiding buoyancy control, particularly useful for a piscivorous diet involving the digestion of fish bones and scales.^[8] The preservation of these gastroliths is notable, as the high degree of articulation in the holotype allows for their direct association with the digestive tract of A. vanderveldei, providing rare insight into plesiosaurian gastric processes.

Classification

Systematic placement

Albertonectes is classified within the kingdom Animalia, phylum Chordata, class Reptilia, order Plesiosauria, family Elasmosauridae, and subfamily Elasmosaurinae.^[9]^[1] The genus is monotypic, containing only the type species Albertonectes vanderveldei, named and described in 2012 based on a nearly complete postcranial skeleton from the Bearpaw Formation in Alberta, Canada.^[1] Upon its initial description, Albertonectes vanderveldei was placed within Elasmosauridae due to shared features such as an extremely elongated neck comprising 76 cervical vertebrae—the highest count known among plesiosaurs—and other postcranial characteristics aligning with late Cretaceous elasmosaurids.^[1] Its assignment to the subfamily Elasmosaurinae is supported by the extreme neck length, the shape of mid-cervical centra (elongated and can-shaped), and vertebral features like lateral longitudinal ridges on the posterior cervicals, which distinguish it from earlier elasmosaurids with shorter necks.^[1]^[10] Members of Elasmosauridae are diagnosed by extreme elongation of the neck (often over 60 cervical vertebrae), a relatively small skull with long, slender, interlocking teeth, and limbs modified into broad, paddle-like flippers for propulsion in marine environments.^[1] These traits reflect adaptations for a piscivorous lifestyle in open ocean settings during the Late Cretaceous.^[9]

Phylogenetic relationships

Albertonectes vanderveldei is positioned as a derived member of the Elasmosauridae, specifically within the subfamily Elasmosaurinae, which has been synonymized with Styxosaurinae in several cladistic analyses of Late Cretaceous plesiosaurs.^[11] This placement reflects its occurrence among long-necked elasmosaurids from the Western Interior Seaway during the Campanian stage, highlighting a late phase of diversification in this group. Early phylogenetic assessments, such as the preliminary analysis in its original description, confirmed its inclusion in a clade of middle to Late Cretaceous elasmosaurids characterized by extreme neck elongation, using a modified dataset derived from prior studies on sauropterygian relationships. Subsequent revisions have refined its interrelationships, identifying Albertonectes as the sister taxon to Elasmosaurus platyurus and part of a more inclusive clade that also encompasses Styxosaurus and Libonectes. This topology emerges from expanded cladistic matrices, including the comprehensive plesiosaur phylogeny of Druckenmiller and Russell (2008), which analyzed 152 postcranial and cranial characters across Plesiosauria, and the faunal turnover study by Benson and Druckenmiller (2014), incorporating 243 characters to resolve elasmosaurid ingroup relationships with improved resolution.^[12] For instance, a 2016 reassessment of Styxosaurus using a modified version of the Benson and Druckenmiller (2014) matrix recovered Styxosaurinae (including Albertonectes, Styxosaurus, Terminonatator, and Elasmosaurus) with 87% bootstrap support, emphasizing its derived status among North American elasmosaurids.^[11] The phylogenetic position of Albertonectes is supported by key autapomorphic and synapomorphic characters, notably its exceptionally high cervical vertebral count of 76, exceeding that of most elasmosaurids and aligning it with the extreme neck elongation seen in its closest relatives, as well as a specific phalangeal formula in the paddles (e.g., elongated propodials and reduced hyperphalangy).^[11] These traits, evaluated in the 20-character analysis of Kubo et al. (2012) and corroborated in broader datasets, underscore its role in the terminal diversification of long-necked plesiosaurs, contributing to the adaptive radiation of elasmosaurids in epicontinental seaways during the final stages of the Cretaceous. This positioning implies that such specialized morphologies persisted until near the end of the Mesozoic, with Albertonectes representing one of the geologically youngest well-known elasmosaurids from the Bearpaw Formation.

Paleobiology

Diet and feeding

Albertonectes vanderveldei was primarily piscivorous, with a diet inferred to consist mainly of small fish, based on the characteristic anatomy of elasmosaurids, which includes a small head relative to body size and an exceptionally long neck adapted for pursuing agile prey in open water. The absence of the skull in the known specimen limits direct evidence, but elasmosaurid dentition typically features slender, conical teeth suited for piercing and grasping soft-bodied prey like fish, rather than crushing harder items.^[7] The presence of 97 gastroliths—polished chert stones recovered from the abdominal region of the holotype—further supports this, as such stones have been hypothesized to aid in mechanically grinding ingested fish bones and scales within the stomach, though their exact function remains debated and may also include buoyancy control.^[1]^[13] The gastroliths imply a feeding strategy focused on soft-bodied marine organisms, such as fish or squid, excluding diets heavy in hard-shelled mollusks or crustaceans that would require different digestive adaptations. Stomach contents from related elasmosaurids confirm this niche, with preserved fish remains (e.g., Enchodus and other small teleosts) indicating opportunistic predation on schooling fish in the water column.^[14] Albertonectes likely employed an ambush or pursuit tactic, maintaining a straight neck during steady swimming and using primarily ventral flexion—given the neck's relative stiffness—to position the head for prey capture, leveraging its 76 cervical vertebrae for extended reach without repositioning the body. In the shallow inland sea of the Bearpaw Formation, Albertonectes filled a top marine predatory role, specializing in a narrow piscivorous niche amid diverse vertebrate assemblages. This position suggests potential dietary overlap and competition with large mosasaurs like Tylosaurus, which shared similar elemental signatures indicative of fish- or squid-based diets in the same environment.

Locomotion and habitat

Albertonectes vanderveldei, like other elasmosaurid plesiosaurs, propelled itself through the water primarily using its four paddle-like flippers, which functioned as hydrofoils to generate thrust through oscillatory movements. The foreflippers were larger than the hind ones and served as the primary propulsors, while the hind flippers contributed significantly to overall efficiency, producing up to 60% more thrust when synchronized with the foreflippers in a coordinated swimming motion.^[15] The tail, though short, provided secondary propulsion through undulatory motions and aided in steering during travel.^[16] The exceptionally long neck, composed of 76 cervical vertebrae, offered flexibility primarily in ventral bending, enabling maneuvering in aquatic environments and potentially assisting in subtle adjustments for depth changes or orienting the head to detect prey via visual cues from below.^[17] This flexibility was complemented by the ability to stiffen the neck during straight-line swimming, integrating it into the overall streamlined body for efficient locomotion.^[17] Albertonectes inhabited the shallow to mid-depth waters of the Western Interior Seaway during the upper Campanian stage of the Late Cretaceous, a period marked by marine transgression into central North America that formed the Bearpaw Sea.^[4] The Bearpaw Formation, where its fossils were discovered, records deposition in shallow marine conditions adjacent to a deltaic system.^[18] Key adaptations for fully aquatic life included lightweight, low-density bones exhibiting osteoporotic-like microanatomy, which reduced overall body weight to enhance buoyancy control without compromising structural integrity.^[19] Additionally, the presence of 97 chert gastroliths in the abdominal region likely served as ballast to fine-tune buoyancy and position in the water column, complementing lung deflation for diving, though their role is debated and may also include aiding digestion.^[20]^[13] As a fully marine species with no evidence of terrestrial interactions, Albertonectes probably led a pelagic lifestyle, migrating to follow seasonal prey distributions within the seaway.^[17]