New Jurassic mammalian fossil discovered with an unusual tooth replacement pattern
A recent study published in Papers in Palaeontology describes the discovery and identification of a new species of pinheirodontid multituberculate based on the preserved hemimandible from the Upper Jurassic (Tithonian) Freixial Formation in the Ulsa quarry, Portugal.
Cambelodon torreensis was part of the multituberculata order, an incredibly long-lived and successful mammalian lineage, explains Victor Carvalho, lead author of the study.
“It’s important to recognize that Multituberculata was an exceptionally successful mammalian order, persisting for around 100 million years and spreading across several continents, including Europe, Asia, Oceania, and North America. During this extensive timespan, they underwent remarkable morphological diversification, especially in their dentition, an adaptation well-documented in the fossil record,” says Carvalho.
“In Europe, Portugal boasts the richest multituberculate fossil record, primarily from the renowned Guimarota coal mine. These fossils provide compelling evidence that multituberculates were evolving rapidly and continuously, showcasing their adaptive flexibility over time.”
However, the exact evolutionary history and phylogenetic positioning of multituberculates and specifically pinheirodontids are contested. Despite multituberculata having an extensive fossil record in the supercontinent Laurasia, until recently, pinheirodontidae have only been known from a few isolated teeth.
In 2021, the Ulsa quarry, located in the Freixial Formation in the village of São Pedro da Cadeira, was discovered by Graça Ramalheiro.
Dated to the upper Tithonian, the quarry was found to contain multiple macro- and microfossils.
In 2021, a block was removed from the quarry and sent to the Sociedade de História Natural (SHN) laboratory for further analysis. Within the block was specimen SHN.830, a right hemimandible with parts of the incisors and premolars still preserved. The mandible had belonged to a juvenile, still in the process of replacing its deciduous teeth (baby teeth) with definitive teeth.
Despite the block containing several other fossil remains, none have been positively identified as belonging to C. torreensis so far, explains Carvalho. “Fortunately, we have recovered several postcranial bones, including articulated vertebrae. So far, evidence points to the presence of multiple multituberculate species at various ontogenetic stages within the bonebed, alongside remains of dryolestids.
“As such, the identification of this material requires careful analysis and cannot yet be confidently assigned to Cambelodon. If any of these bones are confirmed to belong to multituberculates, it would represent a significant contribution to our understanding of the postcranial anatomy of these basal multituberculates, especially considering that fewer than half a dozen multituberculate postcranial elements have been recovered from the Guimarota mine.”
The Portuguese coal mine, Guimarota, contains the most diverse multituberculate fossil record, with over 10 different multituberculate species represented. Of these, six species have premolar teeth, enabling their comparison to specimen SHN.830.
It was found that although SHN.830’s teeth had various similarities to other multituberculate families, including paulchoffatiids, plagiaulacids, allodontids, and pinheirodontids, none of the above orders were a direct match with SHN.830. Instead, SHN.830 showcased various unique morphological characteristics, including large triangular lobes, five projections on both p3 and p4 teeth (premolar 3 and 4), and nine basal cusps on p4.
It was determined that specimen SHN.830 was a new species, C. torreensis.
As the C. torreensis specimen was a juvenile with teeth still erupting, the eruption pattern could be determined.
All extant mammals share the same eruption pattern, a sequential anteroposterior replacement pattern in which the front teeth fall out first, followed one by one by each other tooth.
Unlike most multituberculates, which had a posteroanterior replacement pattern, two multituberculate species, Kielanodon hopsoni and Rugosodon eurasiaticus, had a non-sequential posteroanterior replacement pattern. This meant that they replaced their teeth from back to front, skipping a tooth at a time.
It was determined that C. torreensis also exhibited a non-sequential posteroanterior replacement pattern.
“This pattern is exceptionally rare, which makes it challenging to study in depth,” explains Carvalho.
“However, based on current research, particularly from the dental replacement patterns seen in euharamiyidans, an early offshoot of the multituberculate lineage, it appears that posteroanterior, non-sequential tooth replacement is more likely a derived trait rather than a plesiomorphic (ancestral) condition among basal multituberculates.
“The absence of this pattern in Late Cretaceous and Early Paleogene multituberculates may indicate that it was a transient trait, one that either lacked long-term adaptive value or became obsolete under shifting environmental or selective pressures.”
It was previously believed that this pattern only existed in Paulchoffatiidae. However, C. torreensis belongs to the Pinheirodontidae, which extends the temporal range into the Tithonian multituberculates.
“Future directions for our research include a comprehensive review of the Pinheirodontidae family, as new material has become available, offering additional insights, particularly into their tooth replacement patterns. We also plan to continue the study of the Ulsa Quarry, where numerous unidentified fossils from a variety of groups have the potential to significantly enhance its paleobiological characterization,” says Carvalho.
“Additionally, a broader paleobiological reconstruction of the Freixial Formation is now possible, following the discovery of another microfossil-rich bonebed in older deposits, which notably includes multituberculate remains as well as new, more productive palynological layers.”