Albertonectes

Albertonectes; Temporal range: Late Cretaceous (Campanian), 73.5 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
	Holotype skeleton (TMP 2007.011.0001)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	†Sauropterygia
Order:	†Plesiosauria
Family:	†Elasmosauridae
Subfamily:	†Elasmosaurinae
Genus:	†Albertonectes; Kubo et al., 2012
Type species
	†Albertonectes vanderveldei; Kubo et al., 2012

Albertonectes (meaning 'Alberta swimmer') is a extinct genus of large plesiosaurs that lived in North America during the Campanian stage of the Late Cretaceous period, at around 73.5 million years ago. The only known species is A. vanderveldei, described in 2012 from an almost complete postcranial skeleton discovered in 2007 near Lethbridge, in the province of Alberta, Canada. Although currently only documented from the holotype, a possible second specimen was discovered in 2015 in the same locality.

With a maximum length estimated at 12.1 metres (40 ft), Albertonectes is the longest known elasmosaurid. Like the other representatives of this group, it has a streamlined body with paddle-like limbs, a short tail, a small head, and an extremely long neck. The neck alone is estimated at around 7 meters (23 ft) long, making Albertonectes one of the animals with the longest known necks. It is also the vertebrate with the greatest number of cervical vertebrae identified to date, 76, ahead of its close relative Elasmosaurus.

Discovery and naming

The first known specimen of Albertonectes was discovered in 2007 near the St. Mary River, near the town of Lethbridge, in southern Alberta, Canada. It was discovered unexpectedly by an excavator during ammolite mining carried out by the company Korite International Ltd. At the initial moment of the discovery, the excavator accidentally broke part of the fossils coming from the middle of the animal's trunk, including an unknown quantity of gastroliths (stomach stones). A large part of these fossils have since been noted as lost, and due to the unique character of the specimen as well as its excellent preservation, it was subsequently decided to leave it as is and not to remove any elements found there.^[1]^[2] This specimen, since housed at the Royal Tyrrell Museum of Palaeontology and numbered as TMP 2007.011.0001, consists of an almost complete and articulated skeleton of a large plesiosaur, but the skull is missing. More precisely, it includes 132 vertebrae from the atlas-axis complex to fused tip of the tail vertebrae, a complete pectoral but an incomplete pelvic girdle, almost complete forelimbs and hindlimbs, disarticulated ribs, an abdominal rib, and at least 97 associated gastroliths. The skeleton also preserves two teeth coming from the lamniform shark Squalicorax, the few traces of grooves present on the left coracoid indicating that the specimen would have been scavenged after its death. Stratigraphically, the skeleton was exhumed in an area dating from the middle of the late Campanian of the Bearpaw Formation, a period located around 73.5 million years ago. In 2012, Japanese paleontologist Tai Kubo and his Canadian colleagues Mark T. Mitchell and Donald M. Henderson described the specimen as the holotype of a new genus and species of elasmosaurid named Albertonectes vanderveldei. The genus name Albertonectes comes from Alberta, in reference to the province where the animal was discovered, and from the Ancient Greek word νηκτός (nêktós, "swimmer"). The specific name is named in honor of Rene Vandervelde, the founder of the Korite International, the gem-mining company that discovered the holotype. Albertonectes represents the first significantly complete plesiosaur to be discovered in the Bearpaw Formation, as well as the second elasmosaurid to be identified in the same formation after Terminonatator in 2003.^[1]

On November 9, 2015, it was reported to the Royal Tyrrell Museum that some articulated vertebrae were discovered in conditions similar to those previously mentioned, still near Lethbridge. Subsequently, more vertebrae were discovered by Korite workers, but due to the particularly difficult conditions in winter, the recovery of the fossils had to be done quickly. The fossils recovered consist of a partial skeleton preserving a vertebral column running from the base of the neck to the base of the tail, followed by a few articulated and disarticulated ribs. The discovery was made official the following year by Canadian paleontologist Darren H. Tanke via a summary, in which he referred this specimen as cf. Albertonectes.^[3]

Description

Size comparison

Life reconstruction

Albertonectes has the longest neck of any elasmosaur ever discovered, which reached up to 7 metres (23 ft). The holotype, missing the skull, measures at 11.2 metres (37 ft) from the atlas-axis complex to the tip of the tail, suggesting a total body length of 11.6 metres (38 ft) with the skull.^[1] In 2024, Henderson estimated its total body length up to 12.1 metres (40 ft) and its total body mass around 4.8 metric tons (5.3 short tons).^[2]

Albertonectes is also unique among other elasmosaurids in having 76 neck vertebrae, a record number among elasmosaurids (and vertebrates as a whole).^[4] Callawayasaurus with a similar count of 56 lacks the dumbbell-shaped articular faces that are present on the vertebrae of Vegasaurus. Additional traits rarely seen in other elasmosaurids include: a tapered front-side projection on the pubis that extends to the side beyond the acetabulum, a longitudinal ridge on the side of most neck vertebrae up to cervical 69, a clavicular arch that is wider than the adjacent front edge of the scapula, the lack of pectoral and pelvic bar, a tip of the tail that is made of seven fused tail vertebrae, and a slender humerus with a width-to-length ratio of 0.56, among other traits.

Albertonectes is known from a mature individual, as suggested by the fused neural spines and most cervical ribs to their centra, and by the only partial connection between the trochanter and the capitulum (head) of the femur, seen in TMP 2007.011.0001. Other observation also support an adult age, e.g. rough and wrinkled vertebral surfaces as well as well-developed facets and articulations on its wrist and ankle bones.^[1]

Gastroliths

The single known specimen of Albertonectes preserves gastroliths, 97 of which are exposed and visible. These poked out of the dorsal side of the body between the ribs, as the animal had come to lie down on the seabed on its back some time after death, the gastroliths moving into the present position through gravity as the body decayed. The largest of the visible gastroliths weighed an estimated 1.13 kg. Gastroliths were generally either disc-shaped or spherical; the former would've come from beaches and the latter from river mouths. All were composed of chert, as is the case with several other species of plesiosaurs. It is hypothesized that chert stones are specifically sought out by plesiosaurs as gastroliths for their hardness and durability.^[2]

Classification

Kubo et al. (2012) tested the phylogenetic position of Albertonectes using a highly modified version of Sato (2002)'s data-set, however its position wasn't stable and many elasmosaurids formed rather large polytomies.^[1] This analysis was modified by O’Gorman et al. (2015), and the cladogram below follows their results, showing only the relationships within Elasmosauridae.^[5]