Header Image: James St. John, Flickr. CC BY 2.0

Come on–you know what I’m all about. If you’ve been following Rex Machina for a while, then you should’ve anticipated another spinosaur-related blog post. Don’t act surpised.

Anyways…let’s talk about spinosaur teeth. The fossil record for this group of animals is chock-full of their chompy bits, and their teeth can yield fascinating insights. For example, comparative studies with other extinct and extant species tell us what the diet of spinosaurids might have looked like. And a new study from Kamonlak Wongko et al. is showing that even the subtlest details in tooth morphology can help shed light on global spinosaur diversity–particularly in Asia.

Aw yeah, let’s get to talkin’ bout some awesome spinosaur teeth! (Photo credit: Sarah Bossenecker, Twitter @tetrameryx)

Spinosaurs in Asia

Though fossil evidence is still scant, we do know that spinosaurids were a rather successful group of theropods. They have been found on almost every major continent, save North America and Antarctica. Paleontologists have identified at least twelve known taxa, with new ones being discovered every year. And within the last 30 years, discoveries in Asia have greatly expanded our understanding of the wider distribution of these enigmatic theropods.

One demonstration of the distribution of spinosaurids across the globe (Credit: PaleoGeekSquared, Wikimedia Commons CC BY-SA 4.0)

In the late 1980s, paleontologists described the first presence of spinosaurids in Asia. A study by Buffetaut & Ingavat named the first Asian taxa, Siamosaurus suteehorni, through tooth material. Later excavation and study in the early 2000s revealed more significant spinosaur remains, and in 2012, scientists named Ichthyovenator laosensis as the “first definitive” Asian spinosaurid. Scientists established the new species based on a number of vertebrae and neural spines, a dorsal rib, and hip bones.

Today, there are reports of other potential spinosaurid species in multiple Cretaceous deposits throughout the continent–particularly Japan, China, and Thailand. While this material has not been officially described yet, the possibility of several other spinosaur taxa in Asia paints a picture of a diverse ecosystem where these theropods might have thrived.

Focusing on Thailand

After Buffetaut & Ingavat published on Siamosaurus, they kept looking at the teeth from both the Khok Kruat formation and Khon Kaen province of Thailand. Looking at this material–in combination with some cervical and dorsal vertebrae–they would later go on to suggest that multiple spinosaur taxa were present in the region. Until recently, no one had fully explored this suggestion.

A map of Early Cretaceous Thailand (Credit: Andrew Racey, ResearchGate)

The Wongko et al. (2019) study intended to look at the available tooth material for Thai spinosaurids and run comparative analyses on them to understand the diversity of taxa in Southeast Asia. From a pool of over 60 specimens from the Phu Wiang Fossil Research Center & Dinosaur Museum in Thailand, Wongko et al. looked at eight from Khok Kruat. They analyzed and identified the tooth features for each specimen; then, the team compared the teeth to other known spinosaurid material. Among the comparison specimens were:

Known Siamosaurus material

A Chinese spinosaurid

Spinosaurus remains from the Kem Kem Beds in Morocco

Suchomimus

Baryonyx

Irritator challengeri

The results of these comparative studies are compelling in their implications for spinosaurid diversity in Asia.

Parsing out the Asian Spinosaurids

The Wongko et al. (2019) study was able to establish a handful of key observations and assessments about spinosaurid taxa in both Thailand and Southeast Asia more generally. First, the research team was able to positively identify all the specimens from Khok Kruat were spinosaurid in origin. Second, they were able to identify the Thai specimens as distinct from most of the other taxa they compared against.

Unsurprisingly, the Thai specimens most closely resembled those of Asian spinosaurines–the subfamily of spinosaurids more closely related to Spinosaurus. The teeth shared features with other spinosaurines such as a wrinkled enamel surface of the crown and crown base, and more lateral (sideways) compression. Because these features were abundant the studied specimens, the researchers concluded they did not belong to the Baryonychine group (those spinosaurids more closely related to Baryonyx). Of the spinosaurines used in the comparison, the studied specimens shared common characteristics with I. challengeri, the Kem Kem spinosaurs, Siamosaurus, and the Chinese spinosaurid material.

This establishment of the studied specimens as spinosaurines is compelling. However, Wongko et al. found something much more intriguing in their study: two distinct spinosaur morphotypes. Surprisingly, the team concluded that at least two spinosaurs are represented in Khok Kruat.

Morphotype I is found only in Khok Kruat, and dates back to the Aptian stage of the Lower Cretaceous–about 125-113 million years ago (Mya). Younger than Morphotype II–which dates to around the Barremian (129-125 Mya)–the latter morphotype is widely distributed through both Khok Kruat and Sao Khua. This was the same region where Buffetaut & Ingavat’s Siamosaurus specimen was collected. In fact, Wongko et al. identified Morphotype II as being from Siamosaurus. However, they could not associate Morphotype I with any currently known taxa.

What Do These Morphotypes Mean?

There are a lot of interesting implications for the two distinct morphotypes identified in the Wongko et al. (2019) study. Most importantly, this research alters our understanding of spinosaurids in Asia. Because we have little to go in terms of spinosaur material–in Asia and elsewhere–we don’t have a clear picture of their distribution and evolution. The fact that these teeth most likely belong to different species in a very close timeframe point to two exciting prospects.

First, it says there may be more diversity in Asian spinosaurids than we currently understand. Second, there’s a distinct possibility these taxa were co-occuring in the same regions and environments. This opens up a world ripe for speculation; if these spinosaur species were living side by side, how did they interact? Would they have differentiated in prey items, or were they in direct competition? Or, as Wongko et al. lightly speculate, does a difference in tooth morphology potentially indicate some level of sexual dimorphism?

A Side Thought

From a personal perspective, I wonder what an extended diversity of spinosaurids in Asia means for the possibility of North American taxa. If they were as successful on the continent as the fossil record appears to be showing, what are the chances their range might have extended across into Alaska and beyond?

When pondering about this possibility, the obvious, potential test case for comparison is therizinosaurids. Up until the early 2000s, they were considered exclusive to Asia. Now we know they actually crossed into Western North America, and likely had a pan-Laurasian distribution. If they could make the journey, who’s to say spinosaurids didn’t either?

The famed Quaternary land-bridge Beringia may have existed in the Cretaceous…could spinosaurs have crossed into North America if that was the case? (Image Credit: US National Park Service)

There are a few environmental factors of Cretaceous Alaska that could have worked in favor of spinosaurs. Places like Denali were fluvial environments with gently sloping floodplains. And, due to the warmer climate, the region would have experienced more mild winters. Add on top of that spinosaur-friendly environments such as marshes and river deltas, and the opportunity might have existed for spinosaurids to make a successful, permanent migration into North America (possibly).

Now, my thoughts on the matter are clouded by limited understanding, so I’m definitely open to feedback if my line of thinking is off-track. And in the event I’m wrong, an even more interesting question presents itself: what stopped spinosaurids from making it to North America, even when the geographic constraints were lifted? Hopefully someone out there is trying to find an answer. For now, all we’ve got to go on is teeth.

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Further Reading

A Swim Test for Spinosaurus: Challenging the Semi-Aquatic Hypothesis

Fragments of Bone: A Partial Spinosaur Fossil Tackles Big Mysteries

Iterations of a Theropod: An Abridged History of Spinosaurus

References

Allain, Ronan & Xaisanavong, Tiengkham & Richir, Philippe & Khentavong, Bounsou. (2012). The first definitive Asian spinosaurid (Dinosauria: Theropoda) from the Early Cretaceous of Laos. Die Naturwissenschaften. 99. 369-77. 10.1007/s00114-012-0911-7.

Arden, Thomas M.s., Catherine G. Klein, Samir Zouhri, and Nicholas R. Longrich. “Aquatic Adaptation in the Skull of Carnivorous Dinosaurs (Theropoda: Spinosauridae) and the Evolution of Aquatic Habits in Spinosaurids.” Cretaceous Research 93 (2019): 275–84. https://doi.org/10.1016/j.cretres.2018.06.013.

Britannica, The Editors of Encyclopaedia. “Aptian Stage.” Encyclopædia Britannica. Encyclopædia Britannica, inc., n.d. https://www.britannica.com/science/Aptian-Stage.

Britannica, The Editors of Encyclopaedia. “Barremian Stage.” Encyclopædia Britannica. Encyclopædia Britannica, inc., n.d. https://www.britannica.com/science/Barremian-Stage.

“Denali’s Cretaceous Climate & Landscape.” National Parks Service. U.S. Department of the Interior, n.d. https://www.nps.gov/dena/learn/nature/cretaceous-climate.htm.

Smith, David K., Lindsay E. Zanno, R. Kent Sanders, Donald D. Deblieux, and James I. Kirkland. “New Information on the Braincase of the North American Therizinosaurian (Theropoda, Maniraptora) Falcarius Utahensis.” Journal of Vertebrate Paleontology 31, no. 2 (2011): 387–404. https://doi.org/10.1080/02724634.2011.549442.

Wongko, Kamonlak & Buffetaut, Eric & Khamha, Suchada & Lauprasert, Komsorn. (2019). Spinosaurid theropod teeth from the Red Beds of the Khok Kruat Formation (Early Cretaceous) in Northeastern Thailand. Tropical Natural History 19(1). 8-20. https://www.researchgate.net/publication/332698408_Spinosaurid_theropod_teeth_from_the_Red_Beds_of_the_Khok_Kruat_Formation_Early_Cretaceous_in_Northeastern_Thailand

