The Turtle Family Emydidae: A Systematic Review

The family Emydidae represents a large and diverse group of turtles, encompassing 50-60 extant species. This comprehensive review delves into the systematic classification, evolutionary history, and defining characteristics of these fascinating reptiles, providing a clear overview for students. Understanding their systematic relationships is crucial for their conservation, as many species face threats to their survival. This article synthesizes and updates studies that define the diversity and classification within Emydidae, tracing their taxonomic journey.

The Turtle Family Emydidae: An Introduction to Their Taxonomy and Diversity

Emydid turtles are primarily aquatic or semi-aquatic species, characterized by a well-developed bony shell and prominent dorsal rib heads. Their carapace features a cervical scute, five vertebrals, four pleurals, and twelve pairs of marginals, with the posterior marginal scutes not extending to contact the suprapygal bone. A broad bridge typically connects the carapace and plastron, which itself is covered by six pairs of ventral scutes (gular, humeral, pectoral, abdominal, femoral, anal). Notably, mesoplastral bones are absent, and there's no raised bony ridge on the visceral surface of the entoplastron. Their karyotype consists of 50 chromosomes.

Our current understanding of the family Emydidae (within the order Testudines, suborder Cryptodira) is relatively stable, recognizing approximately 52 extant species across 12 genera. These genera include: Actinemys, Chrysemys, Clemmys, Deirochelys, Emydoidea, Emys, Glyptemys, Graptemys, Malaclemys, Pseudemys, Terrapene, and Trachemys. While most are native to North America, Trachemys extends into South America and the West Indies, and Emys spans parts of Europe, northern Africa, and western Asia. These turtles are believed to have originated in the Eocene, 42–56 million years ago.

Early Taxonomic Recognition and Challenges in Emydid Classification

Early efforts in turtle taxonomy, particularly in the 18th and 19th centuries, often lacked detailed documentation for family and genus assignments. Carl Linnaeus's "Systema Naturae" (1758) classified all 11 recognized turtle species into the single composite genus Testudo. This broad classification persisted for many decades. The description of type species for modern emydid genera began with Linnaeus, who documented Testudo orbicularis (now Emys) and Testudo carolina (now Terrapene).

Key figures in early emydid classification include:

• August Friedrich Schweigger (1812, 1814): Credited with the first truly scientific account of turtles, he partitioned Linnaeus's Testudo into six genera, including Emys. His genus Emys encompassed modern emydids, Old World geoemydids, and New World kinosternids, sometimes mixing species based on external morphological characters or the presence of a movable plastron (a trait now known to have evolved multiple times independently).
• John Le Conte (1830): Rejected contemporary revisions, returning all emydids to the composite genus Testudo, except for trionychids (softshells).
• Dumé Ril & Bibron (1835): Pioneered using analytical methods beyond external morphology, incorporating skeletal material and soft anatomy. They separated turtles into two major groups (pleurodires and cryptodires) and placed emydids within "Elodites ou Tortues Paludines" (freshwater and land turtles), a very large group including most known cryptodires.
• John Edward Holbrook (1838): Classified emydids into two families: Chersites (mostly terrestrial, including Cistuda - now Terrapene and Emydoidea) and Elodites (mostly aquatic, including Emys). He aimed to avoid unnatural groupings based solely on hinged plastrons but still placed some solid-plastron taxa into Emys.
• Louis Agassiz (1857): In his "Contributions to the Natural History of The United States of America," he recognized the family Emydoidae, comprising over 60 species of emydids and geoemydids. He proposed five subfamilies, many of which align surprisingly well with modern monophyletic clades despite his opposition to evolutionary theory.

Nomenclatural History and the Modern Restricted Family Emydidae

The authorship of the family name "Emydidae" has been a subject of long-standing controversy. After reviewing various proposals and adhering to the International Code of Zoological Nomenclature (ICZN), Constantine Samuel Rafinesque (1815) is now credited with the authorship. His use of "Emidania" as a subfamily name, derived from his genus Emyda (which itself was derived from Emys Duméril, 1805), established priority. The ICZN's ruling conserving Emys Duméril, 1805, as the type-genus for the family Emydidae cemented Rafinesque's claim.

The concept of a modern, restricted family Emydidae began to take shape with Samuel B. McDowell's influential, albeit controversial, osteological revision in 1964. He partitioned the broader Testudinidae into subfamilies:

• Batagurinae: Comprising Old World emydids and Rhinoclemmys.
• Emydinae (sensu stricto): Including only New World emydids (Chrysemys, Clemmys, Deirochelys, Emydoidea, Malaclemys, Terrapene) and Emys from the Old World. This defined the Emydidae as we largely recognize it today.

McDowell's distinction between these subfamilies relied on several osteological characters:

1. The angular bone of the lower jaw contacts Meckel's cartilage.
2. The basioccipital bone of the cranium lacks strong lateral tuberosities.
3. The joint between the fifth and sixth cervical vertebrae is double, with a pair of condyles.
4. The posterior pair of marginal scutes (M12) on the carapace do not contact the suprapygal bone.

Subsequent studies using biochemical (karyotypes, protein electrophoresis, immunological analyses) and molecular (DNA) data largely supported McDowell's partition, validating the separation of New World Clemmys from Old World batagurines.

Two Major Evolutionary Lineages within Emydidae

The Emydidae family (sensu Gaffney 1984) is broadly divided into two major phylogenetic lines based on habitat adaptations:

1. Subfamily Emydinae: Primarily semi-terrestrial forms. This group includes genera such as Actinemys, Clemmys, Emydoidea, Emys, Glyptemys, and Terrapene. Synapomorphies (shared derived characters) defining this subfamily include the palatine bone being excluded from the triturating surface of the jaw, a larger posterior palatine foramen, and a unique myoglobin electromorph (pI = 6.9).
2. Subfamily Deirochelyinae: Mostly aquatic forms. This subfamily comprises genera like Chrysemys, Deirochelys, Graptemys, Malaclemys, Pseudemys, and Trachemys. Its defining synapomorphies include the humeropectoral sulcus of the plastron excluded from the entoplastron, jugal bone contacting the palatine, reduced or absent foramen caroticopharyngeale, sexual size dimorphism (females larger), spade-shaped plica media of the penis, lack of musk glands on the shell (except sometimes in Malaclemys), reduced rostral pores, plastral scutes sloughed in response to growth, a unique myoglobin electromorph (pI = 6.8), and titillation courtship behavior.

"Crown" ages for these subfamilies are estimated to extend back to the Oligocene, with Deirochelyinae dating to 31 Ma and Emydinae to 29 Ma. While generally accepted, some generic relationships, such as those within Pseudemys, Trachemys, and Terrapene, still require further resolution.

The Relationship Between Deirochelys and Emydoidea within Emydidae

Initially, Emydoidea (Blanding's Turtle) and Deirochelys (Chicken Turtle) were thought to be closely related, often grouped due to similar specialized elongate cervical vertebrae and skull structures. This perspective was supported by observations from Baur (1889) and McDowell (1964), who noted cranial similarities and did not attach much significance to differences in shell kinesis.

However, later studies provided strong evidence for their separate evolutionary paths:

• Musk Glands (Waagen, 1972): Deirochelys lacks musk glands, a trait shared with more aquatic emydids, while Emydoidea possesses one pair in the axillary region, similar to other semi-terrestrial emydids like Clemmys, Emys, and Terrapene.
• Shell Kinesis and Scapula (Bramble, 1974): Emys, Emydoidea, and Terrapene—all hinged-plastron emydids—share a specialized, segmented scapula with a unique suprascapula. This allows them to close their shell differently from Old World box turtles. This shared derived condition (synapomorphy) indicated Emydoidea belongs with the semi-terrestrial lineage, not the aquatic line of Deirochelys.
• Convergent Evolution (Jackson, 1978b): The cranial and neck similarities between Deirochelys and Emydoidea were reinterpreted as convergent adaptations for "pharyngeal" or "gape and suck" feeding to capture fast-swimming prey. This feeding strategy has evolved independently in distantly related turtle families, highlighting it as a homoplasy rather than a sign of close kinship.
• Molecular Data (Fair, 1982; Seidel & Adkins, 1989): Molecular analyses, including serum protein cross-reactions and myoglobin variation, showed Emydoidea and Emys to be more similar to each other and to Clemmys, distinct from Deirochelys. Deirochelys shared its myoglobin form with aquatic emydines, while Emydoidea shared its form with Emys, Clemmys, and Terrapene.

These findings, supported by fossil records and subsequent nucleotide sequencing, firmly place Deirochelys in the aquatic Deirochelyinae subfamily and Emydoidea in the semi-terrestrial Emydinae subfamily. This demonstrates how diverse systematic techniques refine our understanding of evolutionary relationships.

Frequently Asked Questions About the Emydidae Turtle Family

What are the main distinguishing features of the Emydidae turtle family?

The Emydidae family is primarily characterized by small to moderately large aquatic or semi-aquatic species with a robust bony shell. Key features include a carapace with a cervical scute, five vertebrals, four pleurals, and twelve pairs of marginals (with the posterior pair not contacting the suprapygal bone). They have a broad bridge connecting the carapace and plastron, which has six pairs of ventral scutes and lacks mesoplastral bones. Their karyotype consistently features 50 chromosomes.

How many species and genera are recognized within Emydidae today?

Currently, the Emydidae family is recognized to comprise approximately 52 extant species, classified into 12 distinct genera. These genera include Actinemys, Chrysemys, Clemmys, Deirochelys, Emydoidea, Emys, Glyptemys, Graptemys, Malaclemys, Pseudemys, Terrapene, and Trachemys.

Where are Emydidae turtles primarily found?

Emydid turtles are mostly native to North America. However, the genus Trachemys extends its range into South America and the West Indies, while the genus Emys is found across parts of Europe, western Asia, and northern Africa.

What are the two major evolutionary lineages within the Emydidae family?

The Emydidae family is divided into two major subfamilies based on their habitat adaptations and shared characteristics: the semi-terrestrial Emydinae (including genera like Emys and Terrapene) and the more aquatic Deirochelyinae (including genera like Chrysemys and Trachemys). Each subfamily is defined by specific skeletal and molecular character states.

Why is understanding the systematic relationships of Emydidae important?

Understanding the systematic relationships and developing a stable taxonomy for Emydidae turtles is crucial for their conservation and future survival. Many species within this family are threatened due to habitat loss and human exploitation. A clear classification helps in defining diversity, identifying endangered populations, and developing effective protection strategies, providing a foundation for future investigations and student research in this field.